NOAA TECHNICAL MEMORANDUM NWS CR-102 POSTPRINT VOLUME NATIONAL WEATHER SERVICE AVIATION WORKSHOP Kansas City, Missouri December 10-13, 1991 Scientific Services Division Central Region Headquarters Kansas City, Missouri MARCH 1992 U.S. DEPARTMENT OF COMMERCE National Oceanic and Atmospheric Administration National Weather Service NOAA TECHNICAL MEMORANDA National Weather Service, Central Region Suiaseries The National Weather Service Central Region (CR) subseries provides an informal medium for the documentation and quick dissemination of results not appropriate, or not yet ready, for formal publication. The series is used to report on work in progress, to describe technical procedures and practices, or to relate progress to a limited audience. These Technical Memoranda report on investigations devoted primarily to regional and local problems of interest mainly to regional personnel, and hence will not be widely distributed. Papers 1 through 15 are in the former series, ESSA Technical Memoranda, Central Region Technical Memoranda (CRTM); Papers 16 through 36 are in the former series, ESSA Technical Memoranda, Weather Bureau Technical Memoranda (WBTM). Beginning with Paper 37, the papers are part of the series, NOAA Technical Memoranda NWS. Papers that have a PB or COM number are available from the National Technical Information Service, U. S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22151. Order by accession number shown in parenthesis at the end of each entry. Prices vary for all paper copies. Microfiche are $4.50. All other papers are available from the National Weather Service Central Region, Scientific Services, Room 1836, 601 East 12th Street, Kansas City, MO 64106. ESSA Technical Memoranda Precipitation Probability Forecast Verification Summary Nov. 1965 - Mar. 1966, SSD Staff, WBCRH, May 1966. A Study of Summer Showers Over the Colorado Mountains. William G. Sullivan, Jr., and James 0. Severson, June 1966. Areal Shower Distribution - Mountain Versus Valley Coverage. William G. Sullivan, Jr., and James 0. Severson, June 1966. Heavy Rains in Colorado June 16 and 17, 1965. SSD Staff, WBCRH, July 1966. The Plum Fire. William G. Sullivan, Jr., August 1966. Precipitation Probability Forecast Verification Summary Nov. 1965 - July 1966. SSD Staff, WBCRH, September 1966. Effect of Diurnal Weather Variations on Soybean Harvest Efficiency. Leonard F. Hand, October 1966. Climatic Frequency of Precipitation at Central Region Stations. SSD Staff, WBCRH, November 1966. Heavy Snow or Glazing. Harry W. Waldheuser, December 1966. Detection of a Weak Front by WSR-57 Radar. G. W. Polensky, December 1966. Public Probability Forecasts. SSD Staff, WBCRH, January 1967. Heavy Snow Forecasting in the Central United States (an Interim Report). SSD Staff, January 1967. Diurnal Surface Geostrophic Wind Variations Over the Great Plains. Wayne E. Sangster, March 1967. Forecasting Probability of Summertime Precipitation at Denver. Wm. G. Sullivan, Jr., and James 0. Severson, March 1967. Improving Precipitation Probability Forecasts Using the Central Region Verification Printout. Lawrence A. Hughes, May 1967. Small-Scale Circulations Associated with Radiational Cooling. Jack R. Cooley, June 1967. Probability Verification Results (6-month and 18-month). Lawrence A. Hughes, June 1967. On the Use and Misuse of the Brier Verification Score. Lawrence A. Hughes, August 1967 (PB 175 771). Probability Verification Results (24 months). Lawrence A. Hughes, February 1968. Radar Prediction of the Topeka Tornado. Norman E. Prosser, April 1968. Wind Waves on the Great Lakes. Lawrence A. Hughes, May 1968. Seasonal Aspects of Probability Forecasts: 1. Summer. Lawrence A. Hughes, June 1968 (PB 185 733). Seasonal Aspects of Probability Forecasts: 2. Fall. Lawrence A. Hughes, September 1968 (PB 185 734). The Importance of Areal Coverage in Precipitation Probability Forecasting. John T. Curran and Lawrence A. Hughes, September 1968. Meteorological Conditions as Related to Air Pollution, Chicago, Illinois, April 12-13, 1963. Charles H. Swan, October 1968. Seasonal Aspects of Probability Forecasts: 3. Winter. Lawrence A. Hughes, December 1968 (PB 185 735). Seasonal Aspects of Probability Forecasts: 4. Spring. Lawrence A. Hughes, February 1969 (PB 185 736). Minimum Temperature Forecasting During Possible Frost Periods at Agricultural Weather Stations in Western Michigan. Marshall E. Soderberg, March 1969. An Aid for Tornado Warnings. Harry W. Waldheuser and Lawrence A. Hughes, April 1969. An Aid in Forecasting Significant Lake Snows. H. J. Rothrock, November 1969. A Forecast Aid for Boulder Winds. Wayne E. Sangster, February 1970. An Objective Method for Estimating the Probability of Severe Thunderstorms. Clarence L. David, February 1970. Kentucky Air-Soil Temperature Climatology. Clyde B. Lee, February 1970. Effective Use of Non-Structural Methods in Water Management. Verne Alexander, March 1970. A Note on the Categorical Verification of Probability Forecasts. Lawrence A. Hughes and Wayne E. Sangster, August 1970. A Comparison of Observed and Calculated Urban Mixing Depths. Donald E. Wuerch, August 1970. NOAA Technical Memoranda NWS Forecasting Maximum and Minimum Surface Temperatures at Topeka, Kansas, Using Guidance from the PE Numerical Prediction Model (FOUS). Morris S. Webb, Jr., November 1970 (COM 71 00118). Snow Forecasting for Southeastern Wisconsin. Rheinhart W. Harms, November 1970 (COM 71-00019). A Synoptic Climatology of Blizzards on the North-Central Plains of the United States. Robert E. Black, February 1971 (COM 71-00369). Forecasting the Spring 1969 Midwest Snovmelt Floods. Herman F. Mondschein, February 1971 (COM 71-00489). The Temperature Cycle of Lake Michigan 1. (Spring and Sumer). Lawrence A. Hughes, April 1971 (COM 71-00545). Dust Devil Meteorology. Jack R. Cooley, May 1971 (COM 71-00628). Summer Shower Probability in Colorado as Related to Altitude. Alois G. Topil. May 1971 (COM 71-00712). An Investigation of the Resultant Transport Wind Within the Urban Complex. Donald E. Wuerch, June 1971 (COM 71- 00766). The Relationship of Some Cirrus Formations to Severe Local Storms. William E. Williams, July 1971 (COM 71- 00844). The Temperature Cycle of Lake Michigan 2. (Fall and Winter). Lawrence A. Hughes, September 1971 (COM 71-01039). (Continued on Back Cover) CRTM 1 CRTM 2 CRTM 3 CRTM 4 CRTM 5 CRTM 6 CRTM 7 CRTM 8 CRTM 9 CRTM 10 CRTM 11 CRTM 12 CRTM 13 CRTM 14 CRTM 15 WBTM CR 16 WBTM CR 17 WBTM CR 18 WBTM CR 19 WBTM CR 20 WBTM CR 21 WBTM CR 22 WBTM CR 23 WBTM CR 24 WBTM CR 25 UBTM CR 26 WBTM CR 27 WBTM CR 28 WBTM CR 29 WBTM CR 30 WBTM CR 31 WBTM CR 32 WBTM CR 33 WBTM CR 34 WBTM CR 35 WBTM CR 36 NWS CR 37 NWS CR 38 NWS CR 39 NWS CR 40 NWS CR 41 NWS CR 42 NWS CR 43 NWS CR 44 NWS CR 45 NWS CR 46 NOAA TECHNICAL MEMORANDUM NWS CR-102 POSTPRINT VOLUME NATIONAL WEATHER SERVICE AVIATION WORKSHOP Kansas City, Missouri December 10-13, 1991 Scientific Services Division Central Region Headquarters Kansas City, Missouri MARCH 1992 UNITED STATES DEPARTMENT OF COMMERCE Barbara Franklin Secretary National Oceanic and Atmospheric Administration John A. Knauss Under Secretary National Weather Service Elbert W. Friday, Jr. Assistant Administrator Foreword The Organizing Committee of the Aviation Workshop consisted of Joseph T. Schaefer (NWS Central Region) 1 , James T. Skeen (NWS Office of Meteorology) 2 , Kenneth R. Rizzo (NWS Central Region) 3 , Gary Schmeling (NWS Central Region), M. Douglas Mathews (National Severe Storms Forecast Center, National Aviation Weather Advisory Unit), Richard P. McNulty (NWS Training Center), and Beverly D. Lambert (NWS Central Region). While many individuals provided assistance at the Workshop itself, special recognition belongs to William Henry (NWS Training Center, Retired). The onerous task of compiling, processing, and producing this document fell upon the most able shoulders of Beverly Lambert. 1 2 Now affiliated with Now affiliated with Washington, D.C. NWS Training Center, Kansas City, Missouri, the National Transportation Safety Board, Now affiliated with National Weather Service Forecast Office, Milwaukee/Dousman, Wisconsin. TABLE OF CONTENTS Page No. Program for Aviation Workshop P-1 Session One: Aviation User Requirements Part I 1.1 "Aeronautical Meteorology: A Global View" by 1 Charles H. Sprinkle (Aviation Services Branch, NWS Branch, NWS Office of Meteorology) and Ken MacLeod (World Meteorological Organization) 1.2 "The Requirements Process for Aviation Weather" by 7 Rick Heuwinkel (Federal Aviation Administration) 1.3 "The Center Weather Service Unit" by John L. White 8 (CWSU Memphis, TN) 1.4 "The Effects of Weather on Delays in the National 12 Airspace System" by Steve Henderson (Central Flow Weather Service Unit) 1.5 "The Regional Airline's Unique Requirements" by Richard 23 McAdoo (Henson Aviation/USAir Express) Session Two: Aviation User Requirements Part II 2.1 "General Aviation Requirements for Weather Services" by 30 Steven Brown (Aircraft Owners and Pilots Association) 2.2 "The Professional Pilot's Perspective on Weather" by 31 Tim Miner (American Airlines) 2.3 "The Impact of Terminal Forecasts on Fuel Loading 38 Planning" by Jeff Hubright (Delta Airlines) 2.4 "The FAA Weather R & D Activities" by Arthur L. Hansen 41 (Weather Research Program, Federal Aviation Administration) Session Three: Fog and Stratus Forecasting 3.1 "Development and Dissipation of Fog and Stratus" by 42 Lynn L. LeBlanc (Northeast Louisiana Univ.) 3.2 "Sea Fog and Stratus: A Major Aviation Hazard in the 43 Northern Gulf of Mexico" by G. Alan Johnson and Jeffrey Graschel (WSFO New Orleans, LA) 3.3 "The Effects of Summertime Stratus at San Francisco 52 International Airport on the Nationwide Flow of Commercial Airline Traffic" by Walt Strach (CWSU Fremont, CA) 3.4 "An Objective Forecasting Aid for Summertime Low Clouds 63 During San Francisco International Airport's Evening 'Rush'" by Henry Lau (WSFO San Francisco, CA) Session Four: Forecasting Mesoscale En-route Weather 4.1 "NMC's Monitoring and Aviation Branch: Organization, 86 Products, and Forecasting Techniques" by Vince McDermott (Monitoring and Aviation Branch, National Meteorological Center) 4.2 "Development and Application of an Icing Prediction 91 Equation" by David W. Bernhardt and Michael R. McCarter (WSO Springfield, IL) 4.3 "Forecasting Airborne Volcanic Ash in Alaska" by 104 Lee Kelley (WSFO Anchorage, AK) 4.4 "Pilots' Understanding of Low-Level Wind Shear 105 Terminology" by Robert L. Jackson (WSFO Seattle, WA) 4.5 "Observations and Conclusions on Low Level Turbulence in 111 the Central United States" by Steve A. Amburn (WSO Tulsa, OK) Session Five: Special Aviation Related Services 5.1 "Weather Forecast for Soaring Contests" by Dan Gudgel 118 (CWSU Bakersfield, CA) and Larry E. Burch (NWS Western Region) 5.2 "Hot Air Balloon Pilot Weather Briefings" by Walt De Voe 123 (WSO St. Cloud, MN) 5.3 "The Importance of Pilot Reports in Weather Service 127 Operations" by Richard E. Arkell (WSFO Charleston, WV) 5.4 "Pilot Weather Briefings...Things to Consider and Steps 139 to Follow" by Bernard Esposito and Vincente Carreras (WSFO Miami, FL) 5.5 "Aviation Weather Briefing Service Training for the 146 Future" by Larry G. Sharron (Transport Canada Training Institute) Session Six: Present Terminal Forecast Procedures 6.1 "National Weather Service Terminal Forecasts and Federal 150 Regulations" by Joe Pedigo (WSFO St Louis, MO) 6.2 "EWINS (Enhanced Weather Information System)" by 170 Randy Baker (United Parcel Service, Louisville, KY) 6.3 "Enhanced FT" by Lynn Maximuk (NWS Central Region, 176 Transition Program Manager) Session Seven: Doppler Radar and Downbursts 7.1 "Aviation Hazard Identification Using Doppler Radar" by 181 Michael Eilts (National Severe Storms Laboratory) 7.2 "Application of the WSR-88D Combined Moment Product in 194 Aviation Nowcasting" by Lee C. Anderson (WSFO Des Moines, IA) and Douglas Green (Operations Training Facility, NEXRAD Operational Support Facility) 7.3 "The Prediction of Pulse-Type Thunderstorm Gusts Using 199 Vertically Integrated Liquid Water Content (VIL) and the Cloud Top Penetrative Downdraft Mechanism" by Stacy R. Stewart (Federal Aviation Administration Academy) 7.4 "An Examination of Downbursts in the Eastern Great Plains 208 Associated with a Very Warm Mid-Level Environment" by Stephen F. Byrd (WSFO Omaha, NE) 7.5 "Toward a Climatology of South Texas Downbursts" by 220 Nezette N. Rydell and Judson W. Ladd (WSFO San Antonio, TX) Session Eight: Profilers, MDCRS, and LDS 8.1 "Using Profiler Data in Aviation Forecasting" by 225 Eric Thaler (WSFO Denver, CO) 8.2 "Some Considerations on a Density Current Nose and Low 233 Level Jet in Case Study" by Jim Johnson (WSO Dodge City, KS) 8.3 "Use of Wind Profiler Data in Aviation Forecasting at the 245 National Aviation Weather Advisory Unit" by Richard J. Williams and Franklin D. Woods (National Aviation Weather Advisory Unit, National Severe Storms Forecast Center) 8.4 "The Meteorological Data Collection and Reporting System 251 (MDCRS): System Overview and Benefits" by Ralph Petersen and Clifford Dey (Development Division, National iii Meteorological Center), and Ronald Martin, Ronnie Londot and George Ligler (Aeronautical Radio, Inc.) 8.5 "Improved Weather Reconnaissance System" by R. Gale 256 Carter (US Air Force Reserves) 8.6 "Use of Real Time Lightning Location Data at the 259 National Aviation Advisory Unit" by William E. Carle (National Aviation Weather Advisory Unit, National Severe Storms Forecast Center) Session Nine: ASOS 9.1 "On the Effect of Automated Surface Observations on 263 National Weather Service Forecast Products" by Richard P. McNulty (National Weather Service Training Center) 9.2 "The Impact of the Automatic Weather Observing System 267 (AWOS) on the Terminal Forecast Program at Hayden and Gunnison, Colorado and Jackson, Wyoming" by Ken Rizzo (WSFO Milwaukee, WI) and Charles Bejin (WSFO Cheyenne, WY) 9.3 "Automated Surface Observations: A Major Change for 271 Aviation Operations" by Phil Clark (WSFO Omaha, NE) 9.4 "Improved Aviation-Oriented Observational Products" by 277 Andrew D. Stern, Raymond H. Brady III, and Patrick D. Moore (WSFO Washington, DC) and Gary Carter (NWS Eastern Region, Scientific Services Division) Session Ten: Using Computer Generated Products and Satellite Data 10.1 "A Rapid Update Analysis and Prediction Cycle at NMC 287 for Aviation Forecasting" by Thomas Schlatter (Forecast System Laboratory, Environment Research Laboratories) 10.2 "Thunderstorm Forecasting Using Gridded Model Output 299 and the FAA's Meteorologist Weather Processor (MWP)" by Thomas M. Hicks and James R. Ott (CWSU Ft. Worth, TX) 10.3 "On the Possibility of Using the MRF Normal Modes for 306 Short Range High Altitude Turbulence Forecasting" by Valerie J. Thompson (WSFO Washington D.C.) 10.4 "Verification of Canada's Regional Finite Element (RFE) 316 Model's Aviation Package" by Claude Cote (Newfoundland Weather Centre, AES, Gander Newfoundland) 10.5 "Applications of GOES Satellite Data in the Analysis of 328 Non-Convective Aviation Weather" by Gary Ellrod (National Environmental Satellite, Data, and Information Service) iv 10.6 "Forecasting for Aviation Weather Hazards in the Western 336 North Pacific" by Thomas S. Yoshida (WSO Guam, Pacific) Session Eleven: The Future of Aviation Weather 11.1 "Advances in Meso-Scale Modeling at NMC" by John Ward 337 (Development Division, National Meteorological Center) 11.2 "A Microcomputer-Based Climatological Information 338 System for Terminal Forecast Prediction" by G. B. Jelly (Canadian Forces Forecast Centre, Trenton Ontario) 11.3 "STRATUS: A Prototype Expert System for Low Cloud 341 Forecasting" by Dennis Jacob (Quebec Weather Centre, AES, Montreal Quebec), Michael C. Desmarais, Frances de Verteuil, and Peter Zwack (Centre de Research Informatique de Montreal) 11.4 "What is a Neural Network (and Can They be Useful to 349 Weather Forecasters)?" by Donald W. McCann (National Aviation Weather Advisory Unit, National Severe Storms Forecast Center) 11.5 "Development of Graphic Aviation Weather Forecasting 357 Techniques and Products Within Canada" by Brad Shannon (Prairie Weather Centre, AES, Winnipeg, Manitoba) 11.6 "Current and Future Capabilities in Forecasting the 363 Trajectories, Transport and Dispersion of Volcanic Ash Clouds at the Canadian Meteorological Center" by Real D'Amours (Canadian Meteorological Center, Montreal Quebec) 11.7 "The Aviation Program at NOAA's Forecast Systems 373 Laboratory" by Michael J. Kraus (Forecast System Laboratory, Environmental Research Laboratories) Two hour laboratories to be given simultaneously on Tuesday, Wednesday, and Thursday afternoons: LI "Terminal Forecast - One Systematic Approach" by 378 Steven A. Amburn (WSFO Tulsa, OK) L2 "Downburst Forecasting" by Judd Ladd (WSFO San 379 Antonio, TX) L3.1 "Icing" by Ron Olson (National Aviation Weather 389 Advisory Unit, National Severe Storms Forecast Center) L3.2 "Turbulence" by Mike Streib (National Aviation Weather 390 Advisory Unit, National Severe Storms Forecast Center) v Digitized by the Internet Archive in 2018 with funding from University of Illinois Urbana-Champaign Alternates https://archive.org/details/postprintvolumenOOnati PROGRAM FOR THE AVIATION WORKSHOP December 10-13, 1991 Session One: Aviation User Requirements Part I -- 8:30-10:15 a.m., Tuesday, December 10, 1991 Chair - James Travers, NWS Aviation Services Branch Welcome - Richard Augulis, Director, NWS Central Region 1.1 "Aeronautical Meteorology: A Global View" by Charles H. Sprinkle (Aviation Services Branch, NWS Office of Meteorology) and Ken MacLeod (World Meteorological Organization) [Presented by Dorothy Haldeman, NWS Aviation Services Branch] 1.2 "The Requirements Process for Aviation Weather" by Rick Heuwinkel (Federal Aviation Administration) [Presented by Myron Clark, Federal Aviation Administration] 1.3 "The Center Weather Service Unit" by John L. White (CWSU Memphis, TN) 1.4 "The Effects of Weather on Delays in the National Airspace System" by Steve Henderson (Central Flow Weather Service Unit) 1.5 "The Regional Airline's Unique Requirements" by Richard McAdoo (Henson Aviation/USAir Express) Session Two: Aviation User Requirements Part II -- 10:45 a.m.-12:15 p.m., Tuesday, December 10, 1991 Chair - Tom Heffner, NWS Pacific Region 2.1 "General Aviation Requirements for Weather Services" by Steven Brown (Aircraft Owners and Pilots Association) 2.2 "The Professional Pilot's Perspective on Weather" by Tim Miner (American Airlines) 2.3 "The Impact of Terminal Forecasts on Fuel Loading Planning" by Jeff Hubright (Delta Airlines) 2.4 "The FAA Weather R & D Activities" by Arthur L. Hansen (Weather Research Program, Federal Aviation Administration) P-1 Session Three: Fog and Stratus Forecasting -- 1:15-2:45 p.m., Tuesday, December 10, 1991 Chair - Dale Eubanks, NWS Alaska Region 3.1 "Development and Dissipation of Fog and Stratus" by Lynn L. LeBlanc (Northeast Louisiana Univ.) 3.2 "Sea Fog and Stratus: A Major Aviation Hazard in the Northern Gulf of Mexico" by G. Alan Johnson and Jeffrey Graschel (WSFO New Orleans, LA) 3.3 "The Effects of Summertime Stratus at San Francisco International Airport on the Nationwide Flow of Commercial Airline Traffic" by Walt Strach (CWSU Fremont, CA) 3.4 "An Objective Forecasting Aid for Summertime Low Clouds During San Francisco International Airport's Evening 'Rush'" by Henry Lau (WSFO San Francisco, CA) Concurrent Laboratory Sessins 3:15 p.m. ... Icebreaker - 5:30-7:30 p.m. Session Four: Forecasting Mesoscale En-route Weather -- 8:30-10:00 a.m., Wednesday, December 11, 1991 Chair - Armando Garza, NWS Southern Region 4.1 "NMC's Monitoring and Aviation Branch: Organization, Products, and Forecasting Techniques" by Vince McDermott (Monitoring and Aviation Branch, National Meteorological Center) 4.2 "Development and Application of an Icing Prediction Equation" by David W. Bernhardt and Michael R. McCarter (WS0 Springfield, IL) 4.3 "Forecasting Airborne Volcanic Ash in Alaska" by Lee Kelley (WSFO Anchorage, AK) 4.4 "Pilots' Understanding of Low-Level Wind Shear Terminology" by Robert L. Jackson (WSFO Seattle, WA) 4.5 "Observations and Conclusions on Low Level Turbulence in the Central United States" by Steve A. Amburn (WS0 Tulsa, OK) Session Five: Special Aviation Related Services -- 10:30 a.m.-12:00 p.m., Wednesday, December 11, 1991 Chair - Lans Rothfusz, NWS Southern Region 5.1 "Weather Forecast for Soaring Contests" by Dan Gudgel (CWSU Bakersfield, CA) and Larry E. Burch (NWS Western Region) P-2 5.2 "Hot Air Balloon Pilot Weather Briefings" by Walt De Voe (WSO St. Cloud, MN) 5.3 "The Importance of Pilot Reports in Weather Service Operations" by Richard E. Arkell (WSFO Charleston, WV) 5.4 "Pilot Weather Briefings...Things to Consider and Steps to Follow" by Bernard Esposito and Vincente Carreras (WSFO Miami, FL) 5.5 "Aviation Weather Briefing Service Training for the Future" by Larry G. Sharron (Transport Canada Training Institute) Keynote Address - Robert C. Landis, Assistant Administrator for Weather Services -- 1:00-2:00 p.m., Wednesday, December 11, 1991 Session Six: Present Terminal Forecast Procedures -- 2:15-3:15 p.m., Wednesday, December 11, 1991 Chair - Walter Rodgers, MIC, CWSU Palmdale, California 6.1 "National Weather Service Terminal Forecasts and Federal Regulations" by Joe Pedigo (WSFO St Louis, MO) 6.2 "EWINS (Enhanced Weather Information System)" by Randy Baker (United Parcel Service, Louisville, KY) 6.3 "Enhanced FT" by Lynn Maximuk (NWS Central Region, Transition Program Manager) Concurrent Laboratory Sessions 3:30 p.m. ... Session Seven: Doppler Radar and Downbursts -- 8:30-10:00 a.m., Thursday, December 12, 1991 Chair - Sylvia Graff, NWS Eastern Region 7.1 "Aviation Hazard Identification Using Doppler Radar" by Michael Eilts (National Severe Storms Laboratory) 7.2 "Application of the WSR-88D Combined Moment Product in Aviation Nowcasting" by Lee C. Anderson (WSFO Des Moines, IA) and Douglas Green (Operations Training Facility, NEXRAD Operational Support Facility) 7.3 "The Prediction of Pulse-Type Thunderstorm Gusts Using Vertically Integrated Liquid Water Content (VIL) and the Cloud Top Penetrative Downdraft Mechanism" by Stacy R. Stewart (Federal Aviation Administration Academy) 7.4 "An Examination of Downbursts in the Eastern Great Plains Associated with a Very Warm Mid-Level Environment" by Stephen F. Byrd (WSFO Omaha, NE) P-3 7.5 "Toward a Climatology of South Texas Downbursts" by Nezette N. Rydell and Judson W. Ladd (WSFO San Antonio, TX) Session Eight: Profilers, MDCRS, and LDS -- 10:30 a.m.-12:30 p.m., Thursday, December 12, 1991 Chair - James Skeen, NWS Aviation Services Branch 8.1 "Using Profiler Data in Aviation Forecasting" by Eric Thaler (WSFO Denver, CO) 8.2 "Some Considerations on a Density Current Nose and Low Level Jet in Case Study" by Jim Johnson (WSO Dodge City, KS) 8.3 "Use of Wind Profiler Data in Aviation Forecasting at the National Aviation Weather Advisory Unit" by Richard J. Williams and Franklin D. Woods (National Aviation Weather Advisory Unit, National Severe Storms Forecast Center) 8.4 "The Meteorological Data Collection and Reporting System (MDCRS): System Overview and Benefits" by Ralph Petersen and Clifford Dey (Development Division, National Meteorological Center), and Ronald Martin, Ronnie Londot and George Ligler (Aeronautical Radio, Inc.) 8.5 "Improved Weather Reconnaissance System" by R. Gale Carter (US Air Force Reserves) 8.6 "Use of Real Time Lightning Location Data at the National Aviation Advisory Unit" by William E. Carle (National Aviation Weather Advisory Unit, National Severe Storms Forecast Center) Session Nine: ASOS -- 1:30-2:45 p.m., Thursday, December 12, 1991 Chair - Larry Burch, NWS Western Region 9.1 "On the Effect of Automated Surface Observations on National Weather Service Forecast Products" by Richard P. McNulty (National Weather Service Training Center) 9.2 "The Impact of the Automatic Weather Observing System (AWOS) on the Terminal Forecast Program at Hayden and Gunnison, Colorado and Jackson, Wyoming" by Ken Rizzo (WSFO Milwaukee, WI) and Charles Bejin (WSFO Cheyenne, WY) 9.3 "Automated Surface Observations: A Major Change for Aviation Opera¬ tions" by Phil Clark (WSFO Omaha, NE) 9.4 "Improved Aviation-Oriented Observational Products" by Andrew D. Stern, Raymond H. Brady III, and Patrick D. Moore (WSFO Washington, DC) and Gary Carter (NWS Eastern Region, Scientific Services Division) P-4 Concurrent Laboratory Sessions 3:00 p.m. ... Session Ten: Using Computer Generated Products and Satellite Data -- 8:30-10:15 a.m., Friday, December 13, 1991 Chair - Ken Haydu, National Hurricane Center 10.1 "A Rapid Update Analysis and Prediction Cycle at NMC for Aviation Forecasting" by Thomas Schlatter (Forecast System Laboratory, Environment Research Laboratories) 10.2 "Thunderstorm Forecasting Using Gridded Model Output and the FAA's Meteorologist Weather Processor (MWP)" by Thomas M. Hicks and James R. Ott (CWSU Ft. Worth, TX) 10.3 "On the Possibility of Using the MRF Normal Modes for Short Range High Altitude Turbulence Forecasting" by Valerie J. Thompson (WSFO Washington D.C.) 10.4 "Verification of Canada's Regional Finite Element (RFE) Model's Aviation Package" by Claude Cote (Newfoundland Weather Centre, AES, Gander Newfoundland) 10.5 "Applications of GOES Satellite Data in the Analysis of Non- Convective Aviation Weather" by Gary Ellrod (National Environmental Satellite, Data, and Information Service) 10.6 "Forecasting for Aviation Weather Hazards in the Western North Pacific" by Thomas S. Yoshida (WS0 Guam, Pacific) Session Eleven: The Future of Aviation Weather -- 10:45a.m.-1:00 p.m., Friday, December 13, 1991 Chair - Lee Harrison, FAA Academy 11.1 "Advances in Meso-Scale Modeling at NMC" by John Ward (Development Division, National Meteorological Center) 11.2 "A Microcomputer-Based Climatological Information System for Terminal Forecast Prediction" by G. B. Jelly (Canadian Forces Forecast Centre, Trenton Ontario) 11.3 "STRATUS: A Prototype Expert System for Low Cloud Forecasting" by Dennis Jacob (Quebec Weather Centre, AES, Montreal Quebec), Michael C. Desmarais, Frances de Verteuil, and Peter Zwack (Centre de Research Informatique de Montreal) 11.4 "What is a Neural Network (and Can They be Useful to Weather Forecasters)?" by Donald W. McCann (National Aviation Weather Advisory Unit, National Severe Storms Forecast Center) P-5 11.5 "Development of Graphic Aviation Weather Forecasting Techniques and Products Within Canada" by Brad Shannon (Prairie Weather Centre, AES, Winnipeg, Manitoba) 11.6 "Current and Future Capabilities in Forecasting the Trajectories, Transport and Dispersion of Volcanic Ash Clouds at the Canadian Meteorological Center" by Real D'Amours (Canadian Meteorological Center, Montreal Quebec) [Presented by Gilles Desautels, Atmospheric Environment Service] 11.7 "The Aviation Program at NOAA's Forecast Systems Laboratory" by Michael J. Kraus (Forecast System Laboratory, Environmental Research Laboratories) Two hour laboratories to be given simultaneously on Tuesday, Wednesday, and Thursday afternoons: LI "Terminal Forecast - One Systematic Approach" by Steven A. Amburn (WSFO Tulsa, OK) L2 "Downburst Forecasting" by Judd Ladd (WSFO San Antonio, TX) L3.1 "Icing" by Ron Olson (National Aviation Weather Advisory Unit, National Severe Storms Forecast Center) L3.2 "Turbulence" by Mike Streib (National Aviation Weather Advisory Unit, National Severe Storms Forecast Center) P-6 Session 1.1 AERONAUTICAL METEOROLOGY: A GLOBAL VIEW Charles H. Sprinkle President, Commission For Aeronautical Meteorology World Meteorological Organization Kenneth J. Macleod Chief, Aeronautical Meteorology World Meteorological Organization 1. INTRODUCTION No other industry is more sen¬ sitive to weather than the aeronau¬ tical industry. In spite of our im¬ proved ability to observe and fore¬ cast the weather to a greater degree of accuracy than ever before, ad¬ verse meteorological conditions continue to severely impact the operational safety and efficiency, as well as the system's capacity. The impact of severe weather and Instrument Meteorological Condi¬ tions (IMC) reaches far beyond the commercial aircraft in the sky. It reaches aircraft and crew schedul¬ ing, airport management, cost con¬ trol, and passenger convenience. For the smaller general aviation fleet, it may mean canceling the flight which may have been for busi¬ ness or recreational purposes. The impact of weather on avia¬ tion has been recognized since the turn of the century when aviation was starting. On the 17th of Decem¬ ber 1903, the first successful flights by an engine-powered air¬ craft took place in North America. Take-off weight was approximately 380 kilograms. The longest flight lasted 59 seconds and the flight distance was 355 meters. After four flights, a wind gust overturned the aircraft and caused some damage. On that historic day, not only was the possibility of flight on the heavi- er-than-air principle demonstrated, but also the necessity for meteoro¬ logical assistance to such an under¬ taking. This meteorological assis¬ tance has grown in tandem with the expansion of aviation. It encom¬ passes not only safety issues which remain the prime consideration but also the economy and efficiency of air operations. This paper traces the historical involvement of meteo¬ rology, with both national and in¬ ternational aviation, outlines the present contributions and, with air travel expected to double in the next decade, attempts to foresee future trends. 2. HISTORICAL OVERVIEW The First World War provided the spur that aviation needed to develop and because of the impera¬ tives involved in war-time opera¬ tions, came to depend more and more on advice from meteorologists. Some of the first national meteorological services were indeed expressly cre¬ ated to meet this growing demand for services. Following the war, the momentum continued. Commercial air transport start¬ ed on 8 February 1919 with the first public Paris to London flight -- two days later, the Paris to Brussels link was established. On the 25th of August that same year, operating airlines created the International Session 1.1 Air Transport Association (IATA), and the forerunner of the Interna¬ tional Civil Aviation Organization (ICAO) was organized in Paris as an intergovernmental organization. In the decade following the First World War, regular air trans¬ port networks developed in Europe and North America. It was recog¬ nized as an important contributing factor to the world economy. Meteo¬ rology also developed rapidly and because air transport, particularly in Europe, was primarily an interna¬ tional activity, the need for inter¬ national cooperation and coordina¬ tion in the carrying out and ex¬ change of meteorological observa¬ tions was agreed to be essential. As the conflict 25 years earlier spurred meteorology, the Second World War greatly expanded aircraft capabilities and meteorological knowledge. The intergovernmental World Meteorological Organization (WMO), founded in 1950, is one of the spe¬ cialized agencies of the United Na¬ tions. One of the arms of the WMO is the Commission for Aeronautical Meteorology (CAeM) which assists WMO in carrying out the purposes of the Organization with respect to aero¬ nautical meteorology. There are now 311 experts from 120 Member coun¬ tries of WMO in the CAeM, which usually meets every four years. The Commission carries out its work aimed at satisfying operational aviation requirements by a system of working groups and rapporteurs. 3. THE AERONAUTICAL WEATHER SYSTEM The aviation weather forecast, warning, and information system serving the aviation community today is made up of three separate and distinct processes. While separate and distinct, each step is closely linked, and the failure of any one causes the entire system to fail. These processes are: 1) observing and detecting; 2) warning and fore¬ cast formulation; and 3) dissemina¬ tion of the forecast, warning, and/or information. 3.1 Observing/Detecting Before a forecast and warning program can be developed, present weather must be determined. In gathering this data, a wide vari¬ ety of stations and observing sys¬ tems is called upon. In the USA, for example, surface weather con¬ ditions are observed at more than 1,000 land stations. New sensor and computer technology will continue to revolutionize the taking of surface weather observations (including doppler radar technology), upper air (including automated aircraft re¬ porting systems and atmospheric profilers) and earth observations from satellite. 3.1.1 World Weather Watch (WWW) and Global Observing System (GOS) The WWW is a global system for the collection, analysis, and distribution of weather and other environmental information. It is an integrated system composed of na¬ tional facilities and services owned and operated by individual countries which are Members of WMO. The oper¬ ation of the WWW is based on the fundamental concept that each of the 160 Member countries of WMO under¬ takes, according to its means, in meeting certain responsibilities in 2 Session 1.1 BASIC SYNOPTIC NETWORK STATIONS WITH COMPLETE OBSERVATIONAL PROGRAM Total Aerodromes Percent Region I (Africa) 449 246 55 Region II* (Asia) 722 186 26 Region III (South America) 185 150 81 Region IV (North and Central America) 144 54 38 Region V (South-West Pacific) 230 112 49 Region VI* (Europe) 3775 1161 31 TOTAL 5505 1909 35 *except Members which do not specify observing stations the agreed global scheme so that all (iii) The GDPS , a network i countries may benefit from the con- world and regional computerized solidated efforts. It is a unique processing centers. achievement in international cooper¬ ation; in no other field of human endeavor, and particularly in sci¬ ence and technology is there, or has there ever been, such a truly world¬ wide operational system to which virtually every country in the world contributes for the common good every day of every year. WMO's WWW coordinates opera¬ tional meteorological activities and planning on a global basis and has three basic components: (i) The GOS comprising facili¬ ties on land, at sea, in the air, and in outer space for the observa¬ tion and measurement of meteorologi¬ cal elements, (ii) The GTS, a world-wide telecommunication system for the rapid exchange of observational information as well as analyzed and processed information, including forecasts, which are produced by, The GOS consists of over 9,500 meteorological land stations and some 7,000 merchant ships making standard observations throughout the world. About 900 of the land sta¬ tions and 30 ships make upper-air soundings at least once a day to obtain data on pressure, tempera¬ ture, humidity, and winds up to heights of 30 km. These are comple¬ mented by observations from about 3,000 commercial aircraft and satel¬ lite observations giving global coverage of the Earth's cloud cover, vertical temperature and humidity profiles, sea surface and land tem¬ peratures, and snow and ice cover. The 1980s saw the introduction of some 350 automated or partially automated weather stations on land, 100 moored buoys or other fixed platforms serving as automatic ma¬ rine stations, and several hundred buoys, of which about 200 are pres¬ ently active, drifting with the ocean currents, and some 600 ground- based weather radars. The GOS has developed into a composite system with no single observing component 3 Session 1.1 or measuring technique able to pro¬ vide the total data set required. It now produces, every day, approxi¬ mately eight million characters of alphanumeric observational data, which are distributed regionally or globally. The provision of meteorological support to aviation depends to a very high degree upon these WWW Basic Systems. The accuracy of forecasts for aviation, like all other meteorological forecasts de¬ pends upon the quality of the obser¬ vational network. Aviation require¬ ments form a strong inducement for the upkeep and improvement of the WMO Basic Systems. Observing sta¬ tions at aerodromes generate fre¬ quent and reliable surface observa¬ tions. The importance of aviation to the GOS is particularly striking when looking at the aerodromes' share of the total amount of synop¬ tic observations which are regularly available on a worldwide basis. 3.2 Forecasting The meteorologist is inte¬ grally involved in the forecast and warning process today and will con¬ tinue to be in the future. Planned technology implementations and asso¬ ciated operational changes will pro¬ vide the forecaster with the tools and opportunity to improve aeronau¬ tical meteorological services. 3.2.1 World Area Forecast System (WAFS) The WAFS, designed specif¬ ically for aviation was adopted in 1982 by both WMO and ICAO. It con¬ sists of two World Area Forecast Centers (WAFCs), situated in London and Washington, which prepare global forecasts of upper air wind and temperatures, tropopause heights and maximum wind speed, direction, and height. These forecasts in 5- x 2.5-degree grid format are prepared twice daily by the WAFCs and are valid for 12, 18, 24, and 30 hours after the time (0000 and 1200 UTC) of the synoptic data on which the forecasts are based. The forecasts are prepared for flight levels 50, 100, 180, 240, 300, 340, 390, and 450 and also for flight levels 500 and 600 when and where these are required. These products, in digital form, from the WAFCs, are issued to 15 Regional Area Forecast Centers (RAFCs) situated in Moscow, New Delhi, Tokyo, Melbourne, Cairo, Dakar, Nairobi, Las Palmas, Frank¬ furt, London, Paris, Washington, Wellington, Brasilia, and Buenos Aires. On the basis of the data received, these RAFCs prepare and transmit to meteorological authori¬ ties in their service area, upper wind and temperature charts for FL 340 and other agreed levels. They also prepare significant weather charts issued four times a day for fixed valid times of 0000, 0600, 1200, and 1800 UTC for their respec¬ tive areas of responsibility. 3.2.2 National Capabilities/ Responsibilities The WAFS is designed to serve the enroute phase of flight (above FL 100). National aeronauti¬ cal meteorological services will have to continue to play a signifi¬ cant role in the future, even after the implementation of the WAFS. For example, National services will continue to provide forecast and warning services for aerodromes. 4 Session 1.1 3.3 Dissemination 3.3.1 WAFS At the joint WMO/ICAO meeting in 1982, in Montreal, at which the WAFS was adopted, there was agreement that during the ini¬ tial phase of the WAFS, and until the improved ICAO Aeronautical Fixed Telecommunications Network was able to play its part, the WMO GTS would be the suitable means of fulfilling, in general, the requirements for communications from WAFCs to RAFCs, between WAFCs and between RAFCs, and to some extent, also from RAFCs to users. This has, in fact, been the case in the intervening years since 1982. In the final phase of WAFS, there will be only two WAFCs prepar¬ ing and disseminating both signifi¬ cant weather and upper wind and temperature forecasts. There are essentially two on-going efforts underway in WAFS to attain the final phase. The first is the dissemination of data via satellite broadcast system(s). It was envisioned that small, receive-only stations would fulfill international aeronautical WAFS data dissemination require¬ ments. The United States' part in dissemination is to provide for two satellite uplinks, one for the Ca¬ ribbean/South American area and the other for the Asian/Pacific area. The second ongoing effort con¬ cerns the automation of the signifi¬ cant weather elements objectively by computer. The final phase will not be attained until development of the capability by the WAFCs of producing significant weather forecasts by computer. When this is realized there would no longer be a need for RAFCs. However, since this realiza¬ tion is in the future, RAFCs would continue to prepare and to the ex¬ tend possible, transmit their sig¬ nificant weather forecast charts to a WAFC for satellite broadcast. The national meteorological services of individual States/Members would continue to receive and process forecast infor¬ mation directly from a WAFC via satellite communications. 3.3.2 National Responsibilities The WAFS will provide for the dissemination of global products to a single authorized representa¬ tive in a Member State, which will then have to disseminate that in¬ formation within that State. Also, arrangements will have to be made to distribute those products prepared to serve flights below FL 100. 4. FUTURE TRENDS The operational meteorologists who, from day to day, tries to con¬ tribute to the safety and efficiency of aviation, seems to be faced with an ever increasing range of require¬ ments, from wind and temperature information for a particular runway on the aerodrome to global upper air data for centralized flight plan¬ ning. Forecasts of flight condi¬ tions are required for light air¬ craft engaged in activities in the lower range of the boundary layer as well as for supersonic operations in the stratosphere. Nearly every new problem area in aviation has its meteorological aspects. 5 Although the advances in avia¬ tion technology will continue to make flying less weather-sensitive, meteorological information will remain essential for air transport operations. Because of the high operating costs of modern airliners optimum use must be made of the meteorological data and forecasting accuracy available. The future air navigation system will be based on the establishment of reliable data links between aircraft and ground systems. 5. CONCLUSIONS Scientific and technological advances are emerging that will enable us to make significant im¬ provements to aeronautical informa¬ tion. Developing technologies seem to indicate that the detection and warning phases of our trilogy are somewhat more in hand as far as planning, development, and implemen tation are concerned than is the final dissemination of that vital information to the end user, the enroute pilot. The use of these technologies by the meteorologists, communicators, and processors to provide pertinent and succinct in¬ formation to the pilot is a complex task that points to an ever increas ing role of education and training in order to take full advantage of the evolving aeronautical weather system. Finally, and back to a Global point of view, the WAFS is very significant since it allows for the dissemination of state-of-the- science global model output data to potentially every meteorological service in the world. Session 1.2 THE REQUIREMENTS PROCESS FOR AVIATION WEATHER Rick Heuwinkel Federal Aviation Administration Washington, D.C. MANUSCRIPT WAS NOT AVAILABLE AT THE TIME OF PUBLICATION 7 Session 1.3 A BRIEF REVIEW OF THE CENTER WEATHER SERVICE UNIT'S REAL TIME WEATHER SUPPORT TO THE AIR ROUTE TRAFFIC CONTROL CENTER John L. White Center Weather Service Unit Memphis, Tennessee We cannot look at the role of the Center Weather Service Unit (CWSU) without first looking at the Air Route Traffic Control Center (ARTCC). The ARTCC is housed in a large multi-floored building. The buildings at all 21 ARTCCs are basi¬ cally the same with the exception of a few sites. Each center has a vast complex of varied communications being fed into it. Almost all of the radar and radio communications used by the air traffic controllers are ingested from remote sites, often several states in distance. Around 600 people work in an ARTCC. Two to three hundred of these people (depending upon the site) are air traffic controllers. The rest are electronic technicians, communication specialists, mainte¬ nance and administrative people. There are also four CWSU meteo¬ rologists on the ARTCC staff. We are National Weather Service employ¬ ees and are considered a vital part of the center's operations. I should also point out that our sala¬ ries are reimbursed by the FAA. The Memphis ARTCC airspace runs from Tulsa, Oklahoma, on the west to beyond Nashville, Tennessee, on the east. The northern extent of the area is from just south of Evans¬ ville, Indiana, and the southern boundary is near Hattiesburgh, Mis¬ sissippi . The large airspace that a cen¬ ter controls is broken up into smaller areas called sectors (much like counties in a state). This allows a one to three person team to control the air traffic in this sector. The airspace is further divided into low, high and ultra high sectors to facilitate the safe and efficient flow of air traffic. The routes that the planes fly are called airways and have a dis¬ tinctive nomenclature. These routes are much like the freeways of the sky and run from one radio beacon to the next. This also means that controllers think in terms of sec¬ tors and airways, not states and cities. The CWSU meteorologist must also learn this same convention to be able to effectively brief ARTCC personnel on weather situations. The mission of the CWSU is to promote the safe and efficient flow of air traffic by providing accurate and timely weather information and forecasts to the air route traffic control center, towers and flight service stations. I should point out that 85-95% of the air traffic delays in the southeast are weather related. We cannot change the weather but being prepared for it can greatly improve operational efficiencies. The prime area of responsibili¬ ty for the CWSU meteorologist is the multi-state air space of the ARTCC. However, we must also watch the weather at all of the major airports across the country since a delay at 8 Session 1.3 any airport may eventually affects the flow of traffic in our airspace. A delay at Chicago will often mean a delay at Memphis because it is fre¬ quently the same aircraft, or a connecting one, that is scheduled to ultimately arrive here. Departing flights must also be delayed if the acceptance rate at the destination airport is reduced. It is safer and much more cost effective to hold an aircraft on the ground rather than delay it in the air in an area that is experiencing bad weather. Some of the busiest days at the center are when the weather in our area is good. If there is a strong line of thunderstorms running from north of Detroit, Michigan, to southern Illinois much of the east to west air traffic from each coast is forced south across the Memphis area. This requires additional air traffic controller staffing and is frequently one of our main forecast¬ ing concerns. I mentioned earlier that the CWSU staff members are National Weather Service employees. The CWSU receives its administrative support from the National Weather Service. This support is provided by the WSFO in the state that the ARTCC is lo¬ cated in. Operationally the CWSU falls under the FAA ARTCC staff. This means that the FAA tells us what to do and the WSFO MIC-AM writes our efficiency reports on how well we did our job. This can, of course, theoretically present some adminis¬ trative problems but usually does not. The other problem with this arrangement is that there is some question as to what each agency should provide to the CWSU in the way of support. NWS says that we are working for the FAA and they should provide everything needed. The FAA on the other hand states that since they are reimbursing our salaries plus more than a 20% over¬ head fee that the NWS should provide almost all administrative and train¬ ing functions. Austere budgets make the issue more acute. This is especially true when the CWSU MICs need an adminis¬ trative computer or a Professional Development Weather system is needed for training the CWSU staff. The hours of operation at a CWSU are usually from 5:00 or 6:00 a.m. to 9:00 or 10:00 p.m. (depend¬ ing on the site and centers require¬ ments) . Communications at a CWSU are composed of standard FTS and commer¬ cial phone lines to the CWSU office and operational area. The CWSU also has access to the FAA's dedicated hotline system. This gives the CWSU dedicated hotlines to the control¬ lers in the center, most Towers and Flight Service Stations in the cen¬ ters airspace and to the meteorolo¬ gist at surrounding centers. Many centers also have access to the local WSFO on this system. Care should be taken when talking on these systems since all of the hot¬ lines and some of the other lines are recorded. Radio protocol should be used and one should never say anything that should not be recorded or slip out over the airways if the line is a multi-use line and may be on a speaker phone near a control¬ lers position where open microphones are common. The interface for exchanging information between the CWSU and the user in the ARTCC is the Traffic 9 Session 1.3 Management Unit Weather Coordinator. The Traffic Management Unit (TMU) is the section that is responsible for the coordinated, safe and efficient flow of air traffic within the en¬ tire center's airspace. The person in this section assigned meteorolog¬ ical issues is designated as the Weather Coordinator. They have direct access to the Air Traffic Control System Communications Center in Washington, D.C. by hotline and computer for required coordination. The CWSU works closely with the TMU Weather Coordinator and helps make operational decisions affecting the safe and efficient flow of air traffic. The Weather Coordinator is also responsible for passing and receiving weather information to and from the air traffic controllers and their supervisors. The CWSU participates in stand up briefings several times a day. The weather presentation is routine¬ ly a critical part of the briefing. These briefings are held just prior to the major traffic rushes each day. Additional briefings are held in bad weather situations as needed. The meteorologist also prepares written briefings several times a day. This written briefing is being replaced by graphic weather briefing loops on the Meteorologist Weather Processor Briefing Terminals (the new interactive color computer sys¬ tem installed in each center). The Briefing Terminals are installed in the air traffic controllers work areas. These terminals provide excellent color graphics and are and exceptional briefing tool. The CWSU also provides support in the form of written products. The Center Weather Advisory (CWA) and Meteorological Impact Statement (MIS) are non scheduled bulletins created by the CWSU. These two products are issued as required and sent out on the Service A line as well as being distributed internally on the FAA communication system. The FAA system transmits the CWA and MIS to the controller positions as well as towers and approach control facilities. The CWA is a 1-2 hour nowcast and closely equates to SIGMET/ Convective SIGMET criteria, but is usually issued for a smaller areal extent. It may also be used to enhance an existing SIGMET/ Convective SIGMET when needed to meet local operational requirements. The MIS is a 4-12 hour forecast and is used as more of a planning forecast for the FAA. The MIS cri¬ teria equates to much the same cri¬ teria as that used in an AIRMET but is again tailored to the centers needs. The CWSU also provides critical weather support for aircraft emer¬ gencies, tailored airway (route) forecast and tailored terminal fore¬ casts. We provide short range and extended forecasts to help schedule planned equipment outages for main¬ tenance. We also provide valuable input for ARTCC staffing functions. ARTCC supervisors must adjust their man¬ ning to meet the air traffic loads and adverse weather definitely im¬ pacts these workloads. Additional controller staffing is frequently based on the CWSU's weather fore¬ cast. The success of the Memphis CWSU has largely been due to the location we have on the operational floor. 10 Session 1.3 The Area Manager, Traffic Management Unit, System Maintenance Supervisor and CWSU are located in a command console area. This puts these vital operational functions in a central area and greatly facilitates coordi¬ nation efforts. This location is beneficial to the CWSU. We can continually keep up with the operational and mainte¬ nance functions of the ARTCC and can easily provide the meteorological input needed to help accomplish the centers mission. The installation of the Meteo¬ rologist Weather Processor (MWP) has been invaluable to the CWSU. The MWP will eventually replace the DIFAX, Harris Laser FAX (GOES) and The Service A/B System. The Remote Terminal to AFOS (RTA) will be main¬ tained, however, and will still give us access to the Memphis WSFO AFOS. MWP is a leased system funded by the FAA for use by the CWSUs. The contractor is Harris Inc., and they provide all the services and supplies for the system. The Harris offices in Melbourne, Florida, re¬ ceive the NWS Family of Services and GOES data. This information is relayed via satellite to the MWP receiver at each ARTCC. A dual micro-computer system processes the data and relays it to the work¬ station at the CWSU position. The workstation consists of two color CRTs, a keyboard and an opti¬ cal mouse. The system also has a color printer for graphics and a dot matrix printer for alphanumerics. MWP is an interactive system and is capable of creating and modi¬ fying weather maps, soundings, cross sections and a host of other essen¬ tial products. Many of these prod¬ ucts can be generated automatically by the system and are available as needed. This is especially benefi¬ cial when there is no meteorologist on duty at night or when the CWSU meteorologist arrives in the morning to open up the sta¬ tion. The ability of MWP to create a mosaic of NWS radar data from sites in and around the ARTCCs airspace is invaluable. The ability to run the radar mosaic in a continuous loop makes it an outstanding tool for forecasting as well as briefing. The satellite looping feature has also expanded our capabilities for forecasting. Overlaying radar data and surface/upper air data over the satellite has further expanded our insight into how weather systems interact and how these systems af¬ fect air traffic. The graphic edit¬ ing capabilities of MWP have allowed us to send accurate, timely and professional looking weather prod¬ ucts to the MWP Briefing Terminals located in the areas where the con¬ trollers work. This provides the CWSU meteorologist a new avenue of passing vital weather information on to the user in an easily read for¬ mat. This concludes our brief look at the operations of the CWSU. I hope it was enlightening and I would like to encourage everyone to make arrangements to visit an ARTCC and see the CWSU in operation. This would be of particular benefit to the NWS aviation forecasters. I am sure that it would be interesting to see how your Terminal Forecasts are used and how they affect the avia¬ tion industry. 11 Session 1.4 THE EFFECTS OF WEATHER ON DELAYS IN THE NATIONAL AIRSPACE SYSTEM Steve Henderson Central Flow Weather Service Unit Washington, D.C. 1. INTRODUCTION Some 70% of the delays to com¬ mercial air traffic in the U.S. are due to weather. The mission of the Air Traffic Control System Command Center (ATCSCC) is to minimize de¬ lays while maintaining the safe, efficient, and expeditious use of the National Airspace System (NAS). In support of this mission, the Central Flow Weather Service Unit (CFWSU), provides meteorological advice to the Air Traffic Control¬ lers who make decisions regarding the management of the flow of air traffic. An airport's ability to accom¬ modate arriving and departing air¬ craft depends on several factors. The primary factors are number, length, and configuration of run¬ ways; and also the type of approach in use (visual or instrument). Weather determines runway configura¬ tion and whether instrument or visu¬ al approaches are in use. Airlines create their schedules based on public demand and marketing techniques. As a result there are peak demand times when the number of aircraft scheduled to land at a given airport nears or exceeds the capacity of that airport. When demand exceeds capacity, airborne holding of aircraft results. In order to decrease or eliminate air¬ borne holding, ATCSCC assigns vary¬ ing amounts of delay to the depar¬ ture time of an individual aircraft which is scheduled to arrive during a time when the demand exceeds the landing capacity of the destination airport. Holding on the ground is generally preferable to airborne holding since it reduces air traffic congestion and reduces costs to the airline. Hourly airborne holding costs are approximately $10,000 per airliner while ground holding costs are approximately $1800 per airlin¬ er. The five airports which experi¬ ence the greatest number of delays are: 0RD 18%, LGA 10%, EWR 9%, SF0 8%, and JFK 7%. The focus of the ATCSCC, and consequently also that of the CFWSU, is largely directed toward the busiest twenty two air¬ ports in the country (Figure 1). 2. FACTORS AFFECTING THE CAPACITY OF AN AIRPORT All major airports have multi¬ ple runways and thus, several types of landing configurations. Weather is the factor which determines which configuration is to be used. Each configuration has its unique landing capacity. At Boston (Figure 2), with visual approaches, a northeast wind will allow for an acceptance rate of 62 aircraft per hour. How¬ ever, a northwest wind requires the use of a different runway configura¬ tion which has an acceptance rate of only 30 aircraft per hour. On a recent Friday at Boston, for exam¬ ple, there were 11 hours during which the demand exceeded 30 air¬ craft per hour. Ceilings and visibility can have a significant impact on an 12 Session 1.4 13 Session 1.4 AIRPORT DIAGRAM AL-58 (FAA) BOSTON.MASSACHUSETTS AIRPORT DIAGRAM ^ boston.massachusetts BOSTON/GENERAL EDWARD LAWRENCE LOGAN INTL (BOS) FIGURE 2 14 Session 1.4 airport's capacity to land aircraft. Airports with parallel runways less than a mile apart are especially impacted. For example, at Seattle (Figure 3) as two aircraft turn onto final approach of the parallel run¬ ways if clouds or visibility prevent them from seeing each other, then instrument approaches must be used instead of visual approaches. The acceptance rate for visual approach¬ es is 55, while it is 34 for instru¬ ment approaches. Analogous situa¬ tions exist at San Francisco, Den¬ ver, Atlanta, and Charlotte. When runways are wet, the use of crossing runways is prohibited. This, of course, reduces the accep¬ tance rate of an airport. This is a factor at Chicago's O'Hare (Figure 4) airport where with visual ap¬ proaches and dry runways the accep¬ tance rate is 80-85. When runways are wet under visual conditions the acceptance rate is reduced to 60-62. Noise abatement procedures at some airports designate a preferred runway configuration. However, some weather conditions prevent the use of the preferred configuration. Charlotte, North Carolina, and Wash¬ ington, D.C.'s National airports are two examples of airports with weath¬ er sensitive noise abatement proce¬ dures. At Charlotte there is a preferred direction for landing and departing on the north-south orient¬ ed runways due to a school which is located near the end of the runways. At Washington's National Airport, arriving aircraft follow the twist¬ ing Potomac River if weather condi¬ tions permit. Each runway configu¬ ration can have different arrival capacities. New York City airports, JFK, LGA, and EWR are also affected by noise abatement procedures. Due to the proximity of the three New York City area airports, arriving and departing traffic re¬ quires a high degree of coordinated integration. Any weather that af¬ fects the traffic at one airport can have a domino type affect on traffic at the other two airports even though those airports themselves may not be directly affected by the weather. A good example is the seabreeze front that sometimes re¬ quires that the runway configuration at JFK (Figure 5) be changed while the other two airport configurations remain unchanged. Changing the runway configuration at JFK causes its traffic to conflict with traffic at LGA and EWR. 3. THE IMPACT OF WEATHER ON TERMINAL OPERATIONS If the traffic contains air¬ craft with different landing ap¬ proach speeds, then additional spac¬ ing is necessary to keep the faster aircraft from overtaking the slower aircraft. This decreases the number of aircraft that can land in a given hour. Some airports, such as Boston and St. Louis (Figures 2 and 6), have a short runway which is primar¬ ily used for slower flying aircraft. By separating the slower from the faster aircraft, the landing capaci¬ ty is increased. If the crosswind component becomes too great to use the short runway, the slower traffic must be mixed with the faster traf¬ fic which is using a more favorable runway. This will, of course, re¬ duce the acceptance rate of the airport. Air traffic rules specify the horizontal distance that is to be maintained between aircraft as they land. As the headwind component of 15 91038 AIRPORT DIAGRAM 214 Al-582 (FAA) Session 1.4 SEATTLE-TACOMA INTL(SEA) SEATTLE. WASHINGTON AIRPORT DIAGRAM FIGURE 3 SEATTLE. WASHINGTON SEATTLE-TACOMA INTL(SEA) 16 Session 1.4 91150 AIRPORT DIAGRAM 86 AL-166 (FAA) CHICAGO-CHARE INTI (ORD) CHICAGO. ILLINOIS' AIRPORT DIAGRAM FIGURE 4 17 CHICAGO. ILLINOIS CHICAGO-O'HARE inti (ORD) Session 1.4 91038 AIRPORT DIAGRAM 190 NEW YORK/JOHN F. KENNEDY INTIUFK) Al-610 (FAA) NEW YORK. NEW YORK ATIS 128.725 (NE) 117.7 (SW) 115.4 KENNEDY TOWER 119.1 258.3 GNDCON 121.9 348.6 CLNC DEL 135.05 AIRPORT DIAGRAM FIGURE 5 NEW YORK. NEW YORK 18 Session 1.4 90207 AIRPORT DIAGRAM 248 AL-360 (EAA) ST. LOUIS/LAMBERT-ST. LOUIS INTL (STL) ST. LOUIS. MISSOURI AIRPORT DIAGRAM fi gure 6 ST. LOUIS. MISSOURI ST. LOUIS/LAMBERT-ST. LOUIS INTL (STL) 19 Session 1.4 the wind increases, an aircraft's groundspeed decreases. When ground- speed decreases, fewer aircraft can land in a given time interval. This effect is especially significant at airports like Atlanta where the downwind leg and final approach are parallel. Most reduced acceptance rates are due to the presence of clouds or reduced visibility that preclude the use of visual approaches. This is a factor at all airports, though all are not affected the same way. At Atlanta (Figure 7), for example, clouds with bases as high as 4500 feet and/or a visibility as high as 5 miles will prevent visual ap¬ proaches. At St. Louis visual ap¬ proaches can be made, with ceilings as low as 1500 feet and a visibility as low as 3 miles. Some landing patterns are such that the ceiling and/or visibility 4 to 6 miles from the airport is just as significant as that at the airport itself. San Francisco and Atlanta are two such airports. Thunderstorms are probably the most difficult factor to deal will in trying to adjust the arrival demand for an airport. The greatest disruption occurs when a thunder¬ storm is over or just off the end of the runway. Generally, all depar¬ ture and arrival operations stop until the thunderstorm dissipates or moves several miles away. The im¬ pact is generally not dependent on the intensity of the thunderstorm. All thunderstorms are treated the same. Therefore, the duration of a thunderstorm is more significant than its intensity. Consequently, a slow moving thunderstorm will cause more and longer delays than a faster moving severe thunderstorm. Other than the terminal itself, there are other locations in the vicinity of a terminal where the presence of a thunderstorm will cause significant disruption and delays. There are predefined arriv¬ al fixes and departure gates through which air traffic normally passes. When one or more of these fixes or gates are blocked by a thunderstorm, aircraft request deviations. In the terminal environment, the room for deviating from the normal traffic patterns is very limited. When deviations are required, the accep¬ tance rate is reduced and sometimes arrivals are stopped entirely for a period. Some airports have a Severe Weather Avoidance Program (SWAP). The SWAP specifies alternate ap¬ proach and departure patterns and routes which can be used when thun¬ derstorms impact the normal opera¬ tions. Although the SWAP specifies alternate routes, these usually cannot handle as much traffic as the normal gates and routes. It is usually not possible to preplan exactly when and which alternate routes will be utilized to minimize the disruption due to thunderstorms. When thunderstorms can be forecasted to occur within a specific time frame, ATCSCC may delay the depar¬ ture time for aircraft destined for the impacted airport. Some northern airports which seldom experience delays during the warmer part of the year may have significant delays in winter due to snow. Other than the obvious neces¬ sity of keeping the runways cleared of snow; taxiways and gate areas must also be plowed. When plowing operations are underway, acceptance rates are reduced. Proper plowing is more involved than street plow¬ ing. Care must be taken not to 20 Session 1.4 AIRPORT DIAGRAM 34 ATIANTA/THE WILLIAM B. HARTSFIELO ATLANTA INTI <4 AL-26 (FAA) ATLANTA. G£< AXIS AJUt 4 D» IS . ATLANTA TO 1 19_5 348.6 Rwyi 8L-26R and 8f 119.1 348.6 Rwy* 9L-27R and 96 GND ( 121.9 348.6 Rv»y* 8L-26R and 86 121.75 348.6 Rwy. 9L-27R and 96 US HOLD / QNC 12 AIRPORT DIAGRAM FIGURE 7 ATLANTA, G€ORGlA ATLANTA/THE WILLIAM B. HARTSFIELO ATLANTA INTl(ATL) 21 Session 1.4 obscure runway and taxiway lights. Also, snow cannot be allowed to pile up beside the runways and taxiways to a depth that becomes an obstruc¬ tion to aircraft wings. Snow removal operations are, of course, sensitive to snowfall rates and also to strong winds and drift¬ ing snow. Departing aircraft may encounter additional delays due to the need for deicing and/or snow removal. 4. THE IMPACT OF WEATHER ON ENROUTE OPERATIONS The primary cause of weather related delays to the enroute phase of flight is due to thunderstorms. Although icing and turbulence cause altitude deviations, these usually do not cause significant delays. The ATCSCC coordinates and imple¬ ments the rerouting necessitated by thunderstorms. Rerouting is imple¬ mented for long lines of thunder¬ storms and large areas of numerous thunderstorms. The enroute delays due to thunderstorms can be in ex¬ cess of an hour for the longer solid lines of thunderstorms. Even short lines or clusters of thunderstorms can cause significant deviations and delays when they affect parallel airways that are close together. Across northern Illinois there are three nearly parallel major airways which can be impacted by a line of thunderstorms as short as 50 miles. 5. SUMMARY Weather is the major factor affecting the landing capacity of an airport. At the busier airports, the air traffic scheduled to arrive sometimes exceeds the airport's capacity. Most often this occurs when the acceptance rate of the airport is reduced due to weather conditions. The ATCSCC requires precise and timely weather informa¬ tion in order to make the air traf¬ fic management decisions which mini¬ mize the delay of aircraft destined for airports where the demand ex¬ ceeds capacity. 22 Session 1.5 THE REGIONAL AIRLINE'S UNIQUE REQUIREMENTS Captain Richard McAdoo Director of Flying Henson Aviation/USAir Express I would first like to thank the National Weather Service for giving the Regional Airline Association the opportunity to address this work¬ shop. We feel it is most important that all facets of our industry be knowledgeable of the strengths and weaknesses of each of the users and suppliers, so that we can operate in the National Aerospace System safe¬ ly, and efficiently. The letter we received inviting us to participate stated the work¬ shop's purpose was to "...improve NWS service to the aviation communi¬ ty through the application of tech¬ nology and advanced forecasting techniques....dedicated to user requirements,_" This morning I will address user requirements as they pertain to the regional/ commuter activities in our nation. By the nature of the presentation, I will address issues where we, as short haul carriers, need better, expanded support. I will be asking for changes in policy, philosophy, and financial priorities. This in no-way reflects on the current per¬ formance of the National Weather Service. The issues are real, as is the need for your support -- they are safety related. In verifying the validity of a carrier's analysis of weather condi¬ tions and data collection, the Inspector's Handbook (8400.10) states: The purpose...is to prevent unsafe flight operations." What I will try to show over the next quar¬ ter hour is how the regional air carriers are far more capable of operating in the National Aerospace System than what appears to be the government's understanding, and how regional carriers are limited in serving the public by antiquated rules, interpretations, and in some cases a hesitation by the service to change to meet the changing techno¬ logical world in which we live and work. This morning I represent the Regional Airline Association. An organization representing 75 region¬ al/commuter airlines which carry 97% of the passengers flying in the regional, short haul market. The carriers which comprise the member¬ ship of the RAA fly everything from Cherokee 6's and Cessna 172s to British Aerospace 146s and Fairchild F-28s. The carrier for whom I work flies 38 deHavilland Dash 7 and Dash 8 aircraft, and will carry over 2 million passengers in 1991. The capability of the high performance turbo-prop aircraft the regional/ commuter carriers fly is far superi¬ or to that which the "major air¬ lines" flew just a few years ago. The regional carriers are capable and in fact are qualified, to oper¬ ate category II ILS approaches. The state of the art on-board computers fully support every segment of the revenue flight. To use a phrase currently in the advertising media, this is not the commuter your daddy used to fly. Why am I spending a few pre¬ cious minutes explaining where the regional airline is located on the technological time line? I want us 23 Session 1.5 all to have the same foundation upon which to build while I discuss the association's evaluation of the National Weather Service. Our aircraft and crews are the best in the business. But, no mat¬ ter how good they are, they have to have support. In the regional air¬ line marketplace we usually begin and end our flights within the same weather mass or system. Simplisti- cally said, if I experience a weath¬ er delay outbound, I will probably experience a weather delay inbound. The next question we ask ourselves, does this weather also affect one or both of our alternates. Usually the answer to that is yes. But, just because the depar¬ ture, destination and alternates are affected by the weather, does that mean all the forecasts are the same, and should run concurrently the same? I say not so. The forecasts for HPN, ISP, JFK, and LGA should not be identical -- they are each influenced differently by the water, the factories, and the winds. Most regional aircraft will fly between 8 and 16 legs per day. That, by the way, is probably higher usage than the KCI Express bus that brought us in from the airport. Needless to say, but I will anyway, that type of commitment with the equipment requires on-time perfor¬ mance all day, every day. We cannot afford to experience delays due to conservative weather forecasting. I will talk more about that in a min¬ ute. Henson Aviation/USAir Express dispatches 38 aircraft from nearly 40 cities over 320 times per day -- 320 flights east of a line between Pensacola, Florida, to Huntsville, Alabama, to Cleveland. Even a mod¬ erately insignificant weather system can have tremendous influence on our operation. Most of the regional carriers have a hub and spoke operation with a major carrier. On bad weather days I believe we run a spoke and rim operation, but that is not our plan. This type operation brings the regional aircraft back to a major terminal virtually every other leg. If the weather is forecast below minimums at either the major air¬ field, or the feeder field, the carrier looses 2 legs, and passen¬ gers are poorly served, inconveni¬ enced in our National Aerospace System. Again here, too conserva¬ tive a forecast, or a forecast with marginal or below minimums weather over an extended period of time is unsatisfactory. This conservative approach, under the umbrella or safety, fails to meet either the requirements of the carriers or of the traveling public. A lot of what I will talk about is related to risk taking -- risk taking, but not at the expense of safety. I identified 7 areas where increased support by the National Weather Service should be a high priority in that they are for the Regional Airline Association. The list is not all inclusive, but it is a starting point for putting what needs to be fixed, in the fix-it bin. 1. The Forecast 2. Contract Forecasting Ser¬ vices 3. Enhanced Weather Informa¬ tion Services 4. Winds Aloft 24 Session 1.5 5. RVR Reporting in the Hour¬ ly Sequence 6. AWOS/ASOS 7. Turbulence and Icing Re¬ ports 1. THE FORECAST Along with "conditional words", an extended forecast without revi¬ sion and with an unusually long validity period can and does limit the regional carrier's operations. For example: FT AMD 1 261508 1508Z C8 OVC 11/2R-F 1810 OCNL C20 OVC 5F CHC C5 OVC 1/4TRW. 23Z 21 SCT C45 BKN 5H OCNL C21 BKN 3 FH -- ETC Lets assume this was Philadel¬ phia, for instance. Now, unless you are operating under the exemption, the carrier cannot file to PHL as a destination -- chance of 1/4 mile visibility. But how else does this forecast close down operations from Harrisburg, Allentown, Baltimore, Washington, and Kennedy? Is it realistic to believe that there is a chance of 1/4 mile in TRW from 10:00 a.m. local all the way through to 6:00 p.m. such that it would not be possible for aircraft to land for 8 hours? If this forecast is not re¬ vised, a carrier without the exemp¬ tion will be virtually shut down the entire revenue day. Of course magi¬ cally, at 6:00 p.m. the chance of 1/4 mile disappears, and an occa¬ sional appears at 3 miles. From an operator's point of view, this type forecasting indi¬ cates either a misunderstanding or the regional carriers's needs, or a lack of concern on the part or the duty forecaster in examining and determining when the visibility should be acceptable for most users. This is not a unique example as far as the forecast is concerned, but with the unique route structure of a regional carrier, such a fore¬ cast would have a major impact in the carrier's ability to serve the public -- a charter the National Weather Service also holds. In this forecast, it is actual¬ ly the conditional words "chance of" that shuts down the airport. Every forecast should be, in the best judgement of the forecast¬ er, honest, forthright, and reason¬ able. There is no doubt that some forecasts have conditional words added to compensate for liability if something out of the ordinary oc¬ curs. I have strong reservations in use of conditional words since con¬ ditional words, according to the inspector’^ handbook, are binding. We, the carriers, have to dis¬ patch taking into account the worst possible weather scenario. The forecasters give me the worst possi¬ ble weather conditions as if they will exist for the entire forecast period. This whole process is inef¬ ficient, costly, and certainly not in the best interest of the travel¬ ing public or the air carriers. The carrier should only be bound by the main body of the fore¬ cast, and then be charged with the responsibility to evaluate the con¬ ditional phrases and determine their impact on safety and the operation. 2. CONTRACT FORECASTING ✓ Carriers, given the forecast weather, closely watch the trends in 25 Session 1.5 the real world. When those trends indicate that the forecast should be revised, such requests have been made. I think it is important for this group to know, that carriers have begun giving up in their ef¬ forts to ask for assistance from the NWS. For years we have asked, when the actual weather conditions were not developing as the forecast, for an amendment to the forecast. For years we were told it would be looked at. For years we were given the argument that what we were expe¬ riencing was an anomaly (for 5 hours), or just plain shut out. In our case, and in the case of an increasing number of regional/ commuter carriers, we have decided it is no longer worth the aggrava¬ tion and effort to get a reasonable response, and switched to a contract forecaster. The contract forecaster gives us bi-hourly forecasts, and rarely utilizes conditional phrases or words. Yes the contract forecaster was expensive in the beginning, but when I can continue to run revenue producing flights, the costs become negligible. We have switched all our forecast data to a contractor, and the cost per airport has been significantly reduced, and the qual¬ ity is excellent. As a note, the contract fore¬ casting has consistently been more optimistic in the forecast condi¬ tions, and we have not missed a flight into a destination based on the contract forecast. Please do not read more into this statement. All this last com¬ ment means, or should mean, is that the NWS forecasts are too conserva¬ tive, too restrictive, and could be considerably more realistic, and still absolutely safe. To help put this into perspective, an airline is guaranteed never to have an aircraft accident if it never flies an air¬ craft -- but what is an airline for? 3. ENHANCED WEATHER INFORMATION SYSTEMS -- EWINS Along this same vein or thought, the Regional Airline Asso¬ ciation is aware of 2 regional car¬ riers that are either already quali¬ fied in the EWINS program, or will be shortly. An EWINS dispatcher is really looking at the proposed fore¬ cast, the real world, and determin¬ ing if they are in sync. Why should a carrier be required to revise a forecast, if the weather service fully supports its mission? EWINS is an alternative to the contract forecaster. I believe Mr. Randy Baker from United Parcel will be addressing this workshop later on today. 4. WINDS ALOFT Throughout all of the Henson Airlines route structure, there are only 20 (possibly 21) stations pro¬ viding data for winds aloft. Addi¬ tionally, the winds aloft program provides for winds at 12,000, 18,000, 24,000 and 30,000. The 6,000 foot spacing fails to give enough data for the dispatcher and the pilot to choose the best alti¬ tude for the route of flight. The high performance turbo-prop aircraft is measurably more effi¬ cient the higher it flies. The dif¬ ference of 2,000 or 4,000 feet makes a considerable difference in the amount of fuel consumed. But, it is difficult to properly flight plan taking into account winds and tem¬ perature when the data comes to us in 6,000 foot increments, and then 26 Session 1.5 only sporadically along the route structure. 5. RVR ON HOURLY SEQUENCE REPORTS RVR or VR is reported on the hourly sequence reports for the primary instrument runway. There appears to be no flexibility in that program. I am sure part of that is in the software for the 10 minute averaging that is reported, but still, the reported VR may not be applicable, nor useful. Two weeks ago those of us on the East Coast had the opportunity to live through several days of just basic miserable weather -- for and low visibility. The category III runway at JFK was not available due to construction activity. We were all using other runways, but the VR on the sequence reports was for a closed runway -- now I ask, does that make sense. It turns out that in this case, the NWS was told not to transmit the VR for runway 4 until further no¬ tice. It is interesting to note, no one knows who told the NWS not to transmit -- neither the FAA nor the NWS -- and no one knew how to get the VR turned on. More salient to our situation here today though, is, that VR for alternate runways should be made available on the sequence reports. Regional/commuter aircraft are mak¬ ing every effort to avoid the prima¬ ry, heavy jet runway, but the small¬ er carriers must receive support from the system. Dispatchers and pilots should be able to receive hard copy trans¬ mission of the VR on the runways they will be using. As I mentioned before, any delay while telephone calls are made to destination air¬ ports does disservice to the public for that leg, and usually for many legs thereafter. And, even for the heavy jets, if the prevailing winds dictate using other than the primary runway, the VR for a runway not being used at all is both a waste of time and precious dollars in transmission. 6. AWOS VERSES ASOS That short title sounds like the beginning of a battle, which I understand it may be. The question is, should it be? I think it is important to leave here believing, maybe even knowing, that the Automated Weather Observation Systems and the Automat¬ ed Surface Observation Systems can be complimentary. Of the airfields served by Henson Aviation, 10% are AWOS equipped. Without the automated systems, some of which have been in place for several years now, those communities might not have aircraft service to our great nation. Obviously there has been a tremendous amount of testimony con¬ cerning the switch-over. I am sure all the arguments have been made. Unfortunately, I have not heard all the answers. I do not want to throw the baby out with the bath water. AWOS is alive and well. Maybe it is not all you would like it to be, but it is considerably better than look¬ ing for a part-time weather observ¬ er, full time airport employee to give you his or her best guess when you're outer marker inbound. The cost of AWOS is about a third of that for ASOS. Would not 27 Session 1.5 cost be a major factor while we are trying to balance a budget? My last comments on this relate to the availability and installation of the equipment. It is the under¬ standing of the Regional Airline Association that the ASOS units will be installed first in medium to high density airports, with the smaller community airports to follow. With the cost per unit in excess of $100,000 per unit, how long will it take to get out to the community and county airports? And will there be any funding left? The regional/commuter airlines were started to carry passengers from small rural airports into the metropolitan airports. The small community passengers keep the re¬ gional carriers, as well as the major carriers financially alive. Without the automated system, the smaller airports will loose 121 carrier service. Is that serving the public? I don't think so. There are AW0S units available now. They work well in Alaska, and Florida. They increase safety for the flying public, and they finan¬ cial are affordable to most non¬ metropolitan airports. The phasing out of AW0S must be readdressed in the light of safety, reasonableness, common sense, and finance. 7. TURBULENCE AND ICING REPORTS I only have a one liner here, -- please return those reports to the Area Forecasts. Sigmets and Airmets are fine, but from a region¬ al carrier point of view, they bet¬ ter serve the user in the FAs. THIS IS THE WRAP-UP The National Weather Service grew up with the airline industry. The technology kept pace with the advancing capabilities of the jet aircraft. Unfortunately, the NWS left behind a void when the high performance commuter aircraft took to the skies. The commuter aircraft of today serve a necessary and useful purpose to the public. With the advent of deregulation, the major carriers left the small towns and concentrat¬ ed on metropolitan areas, and long haul. The regional carriers sprung up to carry those passengers needing access to our nation and the world. NWS does not always appear to see it that way. There are 3 strata of aircraft now operating in the National Aero¬ space System -- the high performance jets, the high performance turbo¬ props, and the general aviation piston driven aircraft. Lets make sure we are supporting all users of the system. Lets not approach the forecast¬ ing business with the worst awful ism we can think of. Lets give the user the basic information and realize, if the weather is not ideal, the carrier has the responsibility, and will exercise that responsibility, to operate its aircraft safely. Get rid of the conditional phrases, or at least make them rea¬ sonable and justifiable. It is my understanding the regulations on forecasting and user requirements date back to 1936. I don't remember 1936, but a lot has happened since then -- a lot has happened since I started flying in 28 Session 1.5 1966. Lets update our rules and regulations to 1991. The technology is here now, lets use it. Lastly, concerning automated weather reporting equipment. Air¬ lines constantly address the most economic means for accomplishing a task. Lets make sure we are getting the best product for our dollar. The user here is not only the regional/commuter carrier, but also the passenger. The passenger pays the price, no matter how it is bur¬ ied initially. Thanks for the opportunity to address this workshop. I hope I have sparked an interest to look at our system, and examine how it sup¬ ports all the users, and insure all the users are receiving the best product. 29 Session 2.1 GENERAL AVIATION REQUIREMENTS FOR WEATHER SERVICES Steven Brown Aircraft Owners and Pilots Association MANUSCRIPT WAS NOT AVAILABLE AT THE TIME OF PUBLICATION 30 Session 2.2 THE PROFESSIONAL PILOT'S PERSPECTIVE ON WEATHER Timothy H. Miner 1 American Airlines, Inc. Allied Pilots Association There is a gap developing be¬ tween the professional pilot and the aviation weather community. This gap is an increasing unawareness of the needs of the aviator and the capabilities of the weather pro¬ gram. It is important that the weather community understand the nature of this gap because ultimate¬ ly aviation safety will suffer if it continues to grow. 1. The Role of Aviation Weather for the Pilot Figure 1 shows a "model" of the pilot's perspective of aviation weather. By examining each element of the model, one can understand some of elements that make up the weather-aviator gap. The model focuses around the point when the environment causes the aviator to have to make a weather-related deci¬ sion. The outcome can be a safe course of action or an unsafe ac¬ tion. The decision to fly or not to fly, the best route of flight to avoid a perceived hazard, or how to get away from a hazard that is al¬ ready encountered are some of the THE PILOT TECHNOLOGY MISSION PERCEPTION FICURE 1 A SIMPLE VIEW OF PILOT WEATHER INTERACTION more important decisions that the weather can force on a pilot. How the pilot handles the environmental situation will depend on three types of knowledge. The first is how much weather "theory" the pilot learned during basic and advanced training. The second type of knowledge is a situational awareness of what is going on in the atmosphere right now--a weather briefing prior to flying or an update during the flight. Finally, there is a knowl¬ edge based on experience using the theory and briefing in a similar situation--this is judgement that 1 The author is a flight-officer check airman on the Boeing 727 air¬ craft. He is weather consultant to the American Airlines Flight Department and to the National Safety Committee of the Allied Pilots Association. He is a Staff Weather Officer (Reserve) to the 171st Air Refueling Wing, Pennsylva¬ nia Air National Guard, and currently serves on the AMS National Committee for Aviation, Range, and Aerospace Forecasting. This work is supported by the AVMET Foundation, a national non-profit organization of pilot-meteorologist- educators, where the author is Executive Director. Opinions expressed in this paper are strictly those of the author. 31 Session 2.2 only comes from prior experience or from discussing the situation in detail with an experienced aviator. The knowledge base is then changed based on two different attitude-filters to arrive at the environmental situation where the airman has to make the safety-related decision. If one looks at each component of the mod¬ el , then some of the causes for the gap between pilots and weathermen become obvious. 1.1 Pilot Weather Training There have been many papers recently published that describe the current trend to decrease the amount of aviation weather training for pilots ( Aviation Weather... , 1986; Massey, 1989; Miner and McCoy, 1989). This paper summarizes some of the main points. The military, where the bulk of airline new hires come from, may not be a good role model for weather training. Currently the United States Air Force pilot-candidate receives 16 hours of weather in¬ struction, eight of which are pre¬ sented in question and answer format on a computer. The subject is "taught" by a young instructor pilot--many times the newest. To its credit the Air Force does re¬ quire some annual "refresher" train¬ ing (several hours in a classroom) on meteorology. The Navy and Army spend about 30 hours of classroom time teaching weather but neither require follow-on training. Civilian oriented weather training is both a success and po¬ tentially a failure. Many universi¬ ties have already established avia¬ tion departments and majors in avia¬ tion science, along with offering flight training. Most of these programs offer one or two college courses in meteorology either through their own department or in cooperation with local atmospheric science departments. Pilots coming from this type of training seem to be the most "weather wise". At the other end of the spectrum is the pilot trained by a local fixed-base operator, a flight school located at a local airport. Student pilots are guided by a single instructor and are at the whim of that instructor's strengths and weaknesses. All too often, weather is a weakness. In 1989 this author asked for inputs on weather training from the readership of a national popular flying publi¬ cation. One letter is very inter¬ esting. "...The knowledge require¬ ment in the weather theory area is appropriate for pilot training. My sense of the requirement is taken from practical expe¬ rience and the information needed to pass FAA written exams.... The knowledge requirement for reading weather reports, fore¬ casts, and charts exceeds the reality of the opera¬ tional environment. In over 2,000 hours of flying I never had to read a weather report, forecast, or chart...My point is that getting weather in¬ formation in an under¬ standable format is not a problem. Therefore, read¬ ing weather reports, fore¬ casts and charts need not be a testable event on written tests. Instead, training and testing should emphasize decision¬ making..." (Miner,1990) 32 Session 2.2 This letter came from an FBO instru¬ ment and ground school instructor pilot. While he is correct that for his level of flying a Flight Service Station briefing is sufficient, his students who go off to become pilots for regional, national, and interna¬ tional flagship airlines will be missing a critical skill--providing a self-briefing for weather. This pilot also states that his guide to how much weather theory is necessary is based on the FAA written examina¬ tions. In 1986 the FAA's Aviation Weather Task Force reported that the FAA's testing program was inade¬ quate. It is probably common knowl¬ edge that a pilot can take all four written examinations for pilots, can miss everyone of the weather ques¬ tions, and still pass each test to eventually become a captain of an international jet. The task force's report also stated that the weather material tested was in sore need of revision. Guidance from the FAA Federal Aviation Regulations (FARs) is also lacking. To earn a commercial pilot certificate a pilot must pass a test on "Meteorology, including the char¬ acteristics of air masses and fronts, elements of weather fore¬ casting, and the procurement and use of aeronautical weather reports and forecasts" (FAR 61.125). The re¬ quirements for the Airline Transport Pilot certificate (the PhD of flying in the United States) state that a test must be passed on seven meteo¬ rological topics. They include "(c) The general system of weather col¬ lection and dissemination; (d) Wea¬ ther maps, weather forecasting, and weather sequence abbreviations sym¬ bols, and nomenclature; (e) Elemen¬ tary meteorology, including knowl¬ edge of cyclones as associated with fronts; (f) Cloud forms; (g) Nation¬ al Weather Service Federal Meteoro¬ logical Handbook No. 1, as amended; (h) Weather conditions, including icing conditions and upper-air winds, that affect aeronautical activities;... (j) Information from airplane weather observations and meteorological data reported from observations made by pilots on air carrier flights; (and) (k) The in¬ fluence of terrain on meteorological conditions and developments, and their relation to air carrier flight operations" (FAR 61.153). The ATP requirements come only AFTER 1500 hours of flying time--the pilot with the most experience has the greatest requirements for weather knowledge. So what do the air carriers do to increase the weather knowledge of their new-hire pilots? Very lit¬ tle. Training costs money and most carriers assume that the pilots they hire already have the skills they need. In the initial new-hire six to eight week training programs only one to three hours is spent on wea¬ ther training--usually on how to read company weather reports. Weather training for profes¬ sional pilots is weak at best. IF YOU ARE GOING TO GIVE A PILOT WEATHER INFORMATION, THEN KEEP IT SIMPLE. AVOID METEORO¬ LOGICAL TERMINOLOGY (I.E. BAR0- CLINICITY, ADVECTION, ETC.) 1.2 Pilot Weather Briefings The second base of knowledge a pilot must have to make a safe flight decision relating to weather is an understanding of the current weather situation in which he or she is flying. Unfortunately this is the area that has the least consis¬ tency between airlines. 33 Session 2.2 Most of the professional pilots in the United States have no direct access to meteorologists from their own company. This is direct con¬ trast to the military, where most professional pilots come from and receive their initial training. Only the four largest passenger¬ carrying companies and the night- freight operations even have company meteorology staffs. In between the meteorologist--which would probably be the NWS--and most professional pilots is the dispatcher, an indi¬ vidual responsible for accomplishing the route selection, flight plan¬ ning, and flight following. Dis¬ patchers receive about the same amount of training as a pilot--the dispatcher's test and the ATP cer¬ tificate test have the same question pool. Most dispatchers are located at a central facility so that the pilot must usually provide a weather "self-brief" before every flight. All airlines provide textual information to their pilots as part of the flight documentation pro¬ cess. Most textual information includes a synoptic description, which consistently generates com¬ ments from pilots like, "How about spelling out the words all the way, guys!" Other textual information includes SAs and FTs for the take¬ off, landing and alternate airports, and significant weather hazard de¬ scriptions for terminals, thunder¬ storms, and clear air turbulence. Wind information usually comes out in printed flight plans but only for the filed route and altitude. While most pilots receive tex¬ tual weather briefings and have a dispatcher as a point of contact, very few have access to aviation weather graphic products that can provide the "big picture". In the fall of 1991, the Board of Directors of the Airline Pilots Association (ALPA) approved the recommendation of its Aviation Weather Committee to formally recognize the need for graphic weather displays to provide adequate and safe weather briefings. It is appropriate to show how the three largest companies provide graphic weather information to its pilots. American Airlines uses an Alden C-5000 Graphic Weather Display in each of its major hubs (15 air¬ ports). This system uploads by satellite a dozen NWS graphic prod¬ ucts and two satellite images for display on a color CRT or printed on paper. It also carries two color weather maps created by AA meteorol¬ ogists. To support the South Ameri¬ can operations, the Miami hub ob¬ tained a Jeppesen-Dataplan terminal--which had a staff of mete¬ orologists behind it--to provide Significant Weather Prognostic Charts below Peru. AA provides no weather graphics to any of the re¬ maining 150 airports that are not major hubs. Delta Airlines uses the Kavourous Vista system to provide custom graphics--made by Kavourous meteorologists-- and satellite imag¬ es to its pilots at Atlanta. In 1992 several other hub airports will also have Vista systems, but again most of the airports have no graphic support. United Airlines does pro¬ vide graphic weather pictures to all 105 airports in the continental United States. This airline has a three-year contract with Jeppesen- Dataplan--with its own meteorology staff--to provide a two-panel weath¬ er map on an 8 1/2 by 11 sheet of paper which contains radar data, frontal locations, significant haz¬ ards at flight level and weather depiction information. This sheet is faxed to each station from one to four times a day depending on the level of activity at that airport. 34 Session 2.2 In two out of three cases the NWS products are not used by major in¬ ternational airlines for pilot briefings. RECOGNIZE THAT MOST PRO¬ FESSIONAL PILOTS NEVER TALKED TO A METEOROLOGIST NOR SAW GRAPHIC WEATHER INFORMATION BEFORE FLYING. Because of the lack of weather graphics, pilots have had to look for alternate sources of informa¬ tion. Massey (1989) cited one sur¬ vey that showed the most common weather graphic used by professional pilots in the United States is the back page of the USA Today . When asked what single improvement could be made to their company's weather briefing program, over 40 percent of the professional pilots polled said they would like to have the Weather Channel permanently running in their flight planning rooms. The appeal of this weather source is that it is REAL TIME--a quality that pilots crave. MOST PROFESSIONAL PILOTS HAVE A HEALTHY SKEPTICISM ABOUT WEATHER INFORMATION THAT GROWS THE FARTHER AWAY THE INFORMATION IS FROM REAL-TIME. Pilots are also looking for concise forecasts. The more fudging and conditional a forecast is, the less credibility it has to the us¬ er. To be well received, a forecast must show the pilot as precisely as possible forecasted weather conditions--especially when terminal forecasts are made. 1.3 Pilot Experience Finally, a pilot comes to wea¬ ther decision point with a set of aviation experiences which allow the pilot to use the weather theory and the weather briefing in a safe man¬ ner. The experience level depends on the pilot's flight time and the opportunity to deal with weather. The average level of experience within the professional pilot corps is decreasing. Because of the FAR that states that airline pilots must retire after reaching 60 years of age, there is a finite level of experience that professional pilots reach. American Airlines and Delta Airlines have been hiring pilots for the last three to four years. As American has grown the average expe¬ rience level of its new captains has decreased to about seven to eight years. United Airlines is on the verge of a major hiring effort due to increasing number of retire¬ ments. As some airlines have ceased operations, the younger pilots are being hired but rarely do the older pilots return to flying. Flying experience levels continue to de¬ crease which create captains with less and less experience to make weather-related decisions. 2. Weather Attitudes That Change Pilots Perspectives There is more to the equation. The professional pilot arrives at a weather-decision point with a knowl¬ edge of weather theory, a knowledge of current weather events, and an experience level. The same pilot can make different decisions on different days because of two "atti¬ tude filters" that can directly influence how weather is perceived. 2.1 Technology Perception The first filter is the pilot's perception of the technology that is at his or her disposal. The tech- 35 Session 2.2 nology is usually the type of air¬ craft under his or her command. Icing is an excellent example. These conditions are a terror to a single-engine Cessna. Icing is a concern to the pilot of a light twin. But, icing is usually only a bore--only one more switch to turn on and off--to the pilot of a jet¬ liner. As technology "increases" the need to worry about the weather decreases. Unfortunately, there is a small percentage of "jet-jockeys" that have become convinced that their technological marvels are immune to weather's hazards. They are the one's that can be heard saying, "...WHY THIS BABY WILL GO THROUGH ANYTHING..." This unprofessional attitude has the potential to lead to injury and equipment damage. Even more common is the subtle filter that aviation decision-makers have that adding more and more tech¬ nology to aircraft (i.e. radars, stormscopes, etc.) produces safer aviation. What is dangerous is that many times there is little or no time spent training the pilot to use this technology. There are many documented cases of pilots just turning off equipment because they were not trained to use it proper¬ ly. This is the biggest reason for the overall decrease in pilot weath¬ er education--aircraft have gotten bigger and fly higher. 2.2 Mission Perception The final attitude that influ¬ ences just how a pilot responds in a weather-decision situation is that pilot's perception of the mission. Examples of this are the importance of the flight and how fast it needs to be completed. Some pilots might take shortcuts through areas of forecasted turbulence or icing if it means getting to the destination faster. When a company puts pres¬ sure on a captain for on-time sched¬ ule performance, there is a chance that some weather conditions will be discounted. Unfortunately, it is very com¬ mon in the airline industry to hear the phrase, "WHO CARES ABOUT THE WEATHER, WE'RE GOING TO GO ANYWAY." It is a attitude that runs from the line pilots all the way up the cor¬ porate ladder to the most senior company officers. Flights canceled for weather don't make money. 3. Fixing the Problem The gap between the profession¬ al pilot and the aviation weather system is all too evident now. The amount of weather theory taught to pilots has decreased as the aircraft technology has improved. Profes¬ sional pilots have fewer and fewer opportunities to directly consult with a meteorologist or to view weather graphic products before flying. The experience levels of professional pilots, which might compensate for the decrease in theo¬ ry and briefing, are also decreas¬ ing. Two attitudes exist in the aviation world that further the gap. The first is that more technology--even without training-- is better. Second, in the world of professional aviation planes are going to fly anyway. This gap must be decreased before safety is com¬ promised. Much of the weight of fixing the current relationship between pilots and weather falls on the airlines and the FAA. There must be a healthy regard for the basic air¬ manship skill of weather from those 36 Session 2.2 responsible for pilot training. To help bridge the gap, weather fore¬ casters working with aviation must be aware of three key rules. KEEP IT SIMPLE Keeping the weather simple and non-technical helps pilots under¬ stand what is going on with only limited amounts of meteorological training. KEEP IT REALISTIC Forecasts with many conditional statements are only confusing to pilots and disruptive of flight operations. The weather forecaster must work to insure that all prod¬ ucts are as precise as possible. KEEP IT CURRENT By keeping weather products as current as possible they are not only more realistic, but they are also more credible. Whenever possi¬ ble, forecasted weather that is different from reality should be amended, even when amendment crite¬ ria have yet to be met. 4. Conclusions Despite technology advances and an ever growing aviation segment in the United States, the professional pilots and the aviation weather system that was designed to keep them safe are growing further apart. From the flight departments there must be a heathy reevaluation of the role of weather training in preparing airmen to maintain safe flying --the primary goal of avia¬ tion. From the aviation weather system there must be an emphasis in communicating effectively with an audience with limited skill levels and a skeptical attitude about the product. There must also be an ongoing dialogue between pilots and forecasters to insure that the his¬ toric relationship between these two groups of professionals is not lost. 5. References Aviation Weather Task Force: Final Report . Research Applications Program, NCAR, 1986. Massey, R.J., 1989, Aviation weather training and the professional pilot, Preprints of the Third International Conference on the Aviation Weather System , Amer. Meteor. Soc., 307-309. Miner, T.H., and C.E. McCoy: The state of professional pilot meteorology education: how much is enough?, 1989 Proceed¬ ings: University Aviation Asso¬ ciation Fall Conference , 16-34. Miner, T.H., How much weather training is enough?, I£R, Feb¬ ruary 1990. Professional Pilots Meteorology Training Standards Conference: Final Report . T.H. Miner, editor. United States Air Force Academy Technical Report TR-89-9. 37 . Session 2.3 THE IMPACT OF TERMINAL FORECASTS ON FUEL LOAD PLANNING Jeff Hubright Delta Airlines The Delta Air Lines meteorology department utilizes a staff of 27 individuals to provide worldwide meteorological support from its office in Atlanta. One of the func¬ tions of the department is to pro¬ vide valuable input to the Flight Control Superintendents as they determine how much fuel to load on each flight. This load is largely determined by the terminal forecast for the flight's destination. The flight control superintendent is required by FAR to dispatch each flight with enough fuel to operate in a safe manner. However, it costs to carry fuel, and it is the further responsibility of the Superintendent to minimize this cost. One hundred forty North Ameri¬ can terminal forecasts are issued by the Delta surface forecasters every eight hours. These fourteen hour category forecasts are issued by two individuals, who divide their areas of responsibility at the Mississippi River. The three forecast categories are defined below: NA "No Alternate" cig/vis greater than 2000‘/3 mi No TRW activity No freezing precipitation NF "No Factor" cig/vis less than 2000'/3 mi but greater than/equal to 600 1 /2 mi No TRW activity No freezing precipitation Detailed cig/vis less than 600'/2 mi specifies TRW activity specifies freezing precipitation Adverse weather, or the fore¬ cast of adverse weather, causes flight control superintendents to increase the amount of time the flight will be able to hold inbound to its destination and to designate alternate airport(s) for the desti¬ nation. The flight control superin¬ tendents will consider items such as the volume of traffic at the specif¬ ic destination and the type of ad¬ verse weather in determining how much additional hold fuel will be required. In some cases, especially those involving convective activity, two alternate airports may be desig¬ nated to make sure the flight has valid options should the destination airport be unusable. Carrying additional fuel to enable the flight to hold or divert to an alternate airport costs money. Carrying fuel is the same as carry¬ ing passengers or cargo in that more energy has to be expended to carry more weight. A general rule of thumb used at Delta to calculate this cost is to take 3 or 4 percent of the product of the additional fuel required and the time en route. Consider, for example, a flight from Los Angeles to Atlanta with an al¬ ternate airport of Knoxville and an additional 30 minutes of hold time planned. The use of Knoxville as an alternate airport requires an addi¬ tional 4500 to 6000 pounds of fuel, (depending on aircraft type), as does the extra 30 minutes hold time. The flight, then, is carrying an additional 10000 pounds of fuel for its four hour duration. To calcu¬ late the cost of carrying this extra fuel (at 3.5%): 10000 x 4 x 0.035 = 1400 lbs. 38 Session 2.3 In other words, it takes 1400 pounds of fuel to carry 10,000 pounds of fuel for four hours. At the current fuel price of $0.10 per pound, the cost for this flight to designate an alternate and hold an additional 30 minutes is $140. While this may seem to be a trivial sum, consider the effect of multiplying this num¬ ber by hundreds, (number of daily flights into Delta hub cities), and hundreds again, (number of days per year when weather significant to flight operations could reasonably be expected to occur.) To verify the 3-4% rule of thumb, look at the example of August 2, 1991. The original Delta forecast for Hartsfield Atlanta International called for isolated thunderstorms, a detailed forecast for air mass convection. Flight control superintendents responded accordingly, designating alternate airports for flights inbound to ATL. By 1700Z the Delta surface forecast¬ er had reconsidered his original forecast and amended the ATL termi¬ nal forecast to NA. Several super¬ intendents were able to rewrite flight plans, allowing for an "apples-to-apples" comparison of the cost of designating an alternate. Three examples are shown below: JAX-ATL DFU-ATL LAX-ATL ACFT B-757 L-1011 B-767 PAX 187 302 254 Time/ Burn 0:43/6610 1:35/30250 3:54/45560 Time/Burn to TYS 0:28/4290 0:25/9350 0:28/6210 NO ALTN Time/ Burn 0:43/6490 1:35/29790 3:54/44670 Fuel Saved 120 460 890 X of fuel load req'd by altn 2.4 3.3 3.6 Flight control superintendents utilize weather information from a number of sources. In making a decision to designate an alternate, both Delta and National Weather Service forecasts are considered. When these forecasts disagree, the superintendent must decide which is the more accurate. Some will dis¬ cuss the situation with the Delta meteorologist, others will utilize the more conservative of the two forecasts, and still others will rely on their personal experience and weather expertise with the air¬ port in question. Some superinten¬ dents are influenced by conditional language in the NWS forecasts. To illustrate, the Delta forecast for ATL issued at 0700Z on 13 September 91 was NA (or no alternate) through 1700Z. The NWS forecast for Atlan¬ ta, issued at 0800Z, called for 0CNL 11/2F between 1000Z and 1300Z. Of 102 Delta flights arriving at Atlan¬ ta between 1100Z and 1400Z on the 13th, 32 carried an alternate and 70 did not. The question may be asked, "What would have happened to those 70 no alternate flights had Atlanta's weather deteriorated in fog?" A number of flights would have diverted to locations through¬ out the Southeast due to their in¬ ability to hold for sequencing into Atlanta. Both direct and indirect costs are incurred by the airline when this occurs. The direct costs such as landing fees and the addi¬ tional fuel burned to fly to and land at the diversion airport and return are bad enough, but the indi¬ rect costs - frustrated passengers, missed connections, down-line delays caused by the aircraft not being where it is supposed to be, addi¬ tional congestion and workload at the diversion airport - are more significant. It is a tribute to the 39 Session 2.3 teamwork of flight control superin¬ tendents and meteorologists at Delta that the company led the industry during 1990 with the fewest diver¬ sions, according to an FAA - direct¬ ed study. Never forgetting that the pri¬ mary concern of the airline is safe¬ ty, Delta meteorologists strive to provide flight control superinten¬ dents with concise, accurate fore¬ casts for each terminal. Category forecasts ease the burden of typing and monitoring the weather. Precise timing of the onset and ending of events and the avoidance of condi¬ tional language allow superinten¬ dents to make timely and accurate decisions. Responsive amendments aid the process, regardless of whe¬ ther the situation is deteriorating or improving more rapidly or more slowly than originally forecast. Amendments of a few hours and "good weather" amendments can make signif¬ icant differences. Waiting for the next scheduled forecast time can waste significant resources. Condi¬ tional language that covers too long of a period causes inefficient air¬ line operations and in this period of airline bankruptcies may lead to loss of jobs. Most Delta meteorologists use the NGM as their primary synoptic- scale prognostic tool. The NMC upper air and surface analyses are available, as are most of the NMC- produced facsimile products. Synop¬ tic scale surface analyses are done manually three times a day and may be supplemented by regional surface analyses at the discretion of the individual forecaster. A PC-based subscriber system allows for dis¬ playing, rectifying and looping satellite and radar imagery. This system also allows forecasters to develop their own custom prognostic charts, covering regions and using parameters the individual forecaster feels are significant. Delta meteorologists provide an invaluable service to the company by helping flight control superinten¬ dents safely plan each flight. By clearly communicating the onset of good flying weather at Delta desti¬ nations, meteorologists can also take an active role in the economic well-being of the company. 40 Session 2.4 THE FAA WEATHER R & D ACTIVITIES Arthur L. Hansen Weather Research Program Federal Aviation Administration Washington, D.C. MANUSCRIPT WAS NOT AVAILABLE AT THE TIME OF PUBLICATION 41 Session 3.1 DEVELOPMENT AND DISSIPATION OF FOG AND STRATUS Lynn L. LeBlanc Northeast Louisiana University Monroe, Louisiana MANUSCRIPT WAS NOT AVAILABLE AT THE TIME OF PUBLICATION 42 Session 3.2 SEA FOG AND STRATUS: A MAJOR AVIATION AND MARINE HAZARD IN THE NORTHERN GULF OF MEXICO G. Alan Johnson and Jeffrey Graschel National Weather Service Forecast Office New Orleans, Louisiana 1. INTRODUCTION Sea fog and stratus can affect extensive areas of the northern Gulf of Mexico, especially during the Winter and early Spring months (December - March). During this time of year, polar and/or arctic outbreaks bring colder air south across the Gulf cooling the shallow waters of the continental shelf. Cooler water from the major rivers emptying into the Gulf also adds to the cooling effect of the immediate coastal waters. Dense sea fog and the associated low ceilings and visibilities can create a major aviation hazard for over 700 daily helicopter flights to and from off¬ shore oil platforms. Over 35,000 persons are working in the Gulf in support of the oil industry, most transported by helicopters. This paper describes four types of sea fog (cooling, evaporation, frontal, and radiational) including the asso¬ ciated low ceilings and visibilities which develop across the northern Gulf. Most of the research on fore¬ casting the onset, duration, and dissipation of sea fog has been for such geographical areas as the west coast of the United States, Sea of Japan, China Sea, North Sea, and the western north Atlantic. Very little research has been conducted on this significant weather problem in the northern Gulf of Mexico. The purpose of this study was to develop techniques to help the forecaster identify those synoptic patterns which are conducive to the development of sea fog and stratus in the northern Gulf of Mexico. Graphical aids using important mete¬ orological parameters were developed to assist the forecaster. The sig¬ nificance of this study is to reduce or eliminate the loss of life and property due to aviation and marine accidents in the northern Gulf, and the immediate coastal plains. There is no numerical fog model available for use in the Gulf of Mexico. This paper will highlight a forecast technique recommended to the forecast staff at the New Or¬ leans National Weather Service Fore¬ cast Office (WSFO). An application program has been written on a per¬ sonal computer (PC) for use in plot¬ ting several key parameters that are available twice daily in bulletin format from the Nested Grid Model (NGM). 2. METHODOLOGY Climatological data for ten Supplementary Aviation Weather Re¬ porting Stations (SAWRS) located across the northern Gulf of Mexico were evaluated for the Winter and early Spring months during the years of 1985-86, 1988-89, and 1989-90. These stations are located generally along and north of a line from 28 N to the coast and west of 89 W (Fig 1). Sea fog and stratus occurs most frequently in this area. Observa¬ tions were taken from oil platforms which average near 35 m above the water. The data analyzed for this study were air temperature (Ta), dew 43 Session 3.2 GPT 97_9 _EL5 94 Q i Q? g— .v^ii 3 > ** HOU” ' / ^GLS msT LA 3 " 7R5 ~ 7^^ TRiVT^ "TV 7 TEXA' £ T46 O 0 IT 0 5R0 O 9 §3 7W2 S 0 7R8 &L S58 J l >** Tg7 71 42Q20 4201 > ■ ■ • • ■■ J — • 0 • ■■ — 0 0 “ ^0 0 - — ^ \\| broV: ' \ I 42Q02 42001 0 d ¥r ! 0 30 29 28 27 26 25 Fig. 1. Geographical map of SAWRS in the northern Gulf. BRO - Brownsville, Texas CRP - Corpus Christi, Texas HOU - Houston, Texas GLS - Galveston, Texas 7R5 - Cameron, Louisiana 7R4 - Intracoastal City, Louisiana MSY - New Orleans, Louisiana 42001, 42002, 42019 and 42020 weather buoys in the Gulf. 200 PSX - Palacios, Texas BPT - Beaumont, Texas 7R1 - Venice, Louisiana GPT - Gulfport, Mississippi m contour point temperature (Td), wind direc¬ tion and speed, ceilings, and visi¬ bilities. Sea surface temperature (SST) analyses for this study were provid¬ ed by the oceanographer at the Na¬ tional Hurricane Center (NHC) in Coral Gables, Florida. This infor¬ mation was used with other meteoro¬ logical data to develop a forecast technique for sea fog and low stra¬ tus. 3. TYPES OF SEA FOG IN THE NORTH¬ ERN GULF Sea fog develops mainly during the Winter and early Spring and can occur with several synoptic pat¬ terns. The four different types of sea fog identified in the northern Gulf were: warm advection (cool¬ ing), cold advection (evaporation/ steam), frontal (mixing), and radia- tional. Also, typical ceiling heights and visibilities have been identified with each type of fog. 44 Sea fog and the associated low ceilings and visibilities are possi Session 3.2 ble from November to April. Howev¬ er, they are more likely from Decem¬ ber to March with January and Febru¬ ary having the greatest frequency. The coldest air masses of the season usually invade the Gulf during Janu¬ ary and February creating the more ideal conditions for widespread sea fog. Of the four types of sea fog, warm advection (cooling) and cold advection (evaporation) are the most prevalent and produce the most crit¬ ical weather for aviation and marine operations in the northern Gulf. There are two main synoptic patterns that dominate during sea fog development. Warm advection (cooling) fog forms with the first pattern. Kotsch (1983) and Mullan (1984) describe the dynamics of this process very well. This pattern is characterized by warmer air with higher dewpoints flowing over colder water which is found in the shallow waters of the continental shelf. The continental shelf is delineated in Fig. 1 by the 200 m contour. The boundary layer flow is anti cyclonic around a surface high pressure sys¬ tem which is usually located in the southeast United States. The air mass spreads north or northwest over the northern Gulf and is initially maritime polar and warmer than the water (mPw). It can eventually become maritime tropical (mT) if return flow continues long enough before another cold front moves into the Gulf. Figure 2 illustrates a typical synoptic pattern during the wintertime which is conducive for advection (cooling) fog to develop. This is a stable pattern as de¬ scribed by Binhau (1985) and Hsu (1988) with the prevailing surface wind direction from southeast to southwest (120-220 degrees). Figure 3a indicates what type of wind speed is desired under certain Ta - Td conditions (Ta is air temperature and Td is dew point temperature) for sea fog. Figures 3b and 3c give another estimate of visibility asso¬ ciated with sea fog under certain conditions of water temperature (Tw), Ta, and Td. The depth of the sea fog is usually < 100 m, depend¬ ing greatly on the wind speed. This type of fog is usually extensive in coverage with a longer duration. Depending on the synoptic pattern and wind profile, this type of fog could have a duration of several days. The second synoptic pattern which characterizes the Winter re¬ gime over the Gulf is the cold ad¬ vection (evaporation) fog which is commonly known as steam fog. As described by Wessels (1979), colder air accompanied by moderate to strong wind flows south over warmer waters such as the Gulf of Mexico. The wind direction is normally from northwest to northeast (310-040 degrees). The lowest visibilities with this type of fog are found with relative humidities of 90 percent or greater and Tw - Ta s 15 C. This can be noted on Fig. 4. Ceilings and visibilities can be reduced to zero even with a north wind of 30 kt (15 m/s). This type of sea fog forms in an unstable air mass as described by Binhau (1985) and the depth of the fog is usually < 35 m. Note from Fig. 4, visibilities are generally s 3 mi (4800 m) when there is fog. Duration of steam fog is normally < 18 hours with duration of dense steam fog 6 hours or less. Table 1 gives the various ceilings, visibilities, and frequency of oc¬ currence with this type of fog. Areas of dense steam fog are usually not as widespread as fog due to warm advection. The third type of sea fog is the frontal type commonly known as 45 Session 3.2 Fig. 2. Typical synoptic pattern during the wintertime with return flow of warn air with highe** dewpoints over colder water in the northern Gulf. Scalloped area is areas of potential sea fog. Dashed lines are SST in °C. Ta - Tw CC) Fig. 3b. Warm advection (coolina)foo 46 Session 3.2 fog 69 12 15 18 Tw - Ta (°C) Fig. 4. Advection (evaporation/steam)fog Table 1. Guideline and low of values for ceilings stratus in the northern and visibilities when Gulf of Mexico from forecasting sea December to March fog Type of Sea Fog Ceilings hnds ft/m Visibilities mi/m Occurrence Frequency I Warm advection (cooling) <5/152 5-10/156-304 >10/304 <2/3200 2<6/3200-9000 >6/9000 Occasional Frequent Frequent 50 Cold advection (evaporation) (steam) <5/152 5-10/152-304 >10/304 <2/3200 2-3/3200-4800 >3/4800 Occasional Frequent Occasional 25 Frontal (along and 50-70 nm north of warm or stationary front <5/152 5-10/152-304 >10/304 <2/3200 2-4/3200-6000 >416/6000-9000 Frequent Occasional Occasional 20 Radiational (light wind - clear skies) <2/061 >2-5/061-152 i<2/0800<3200 Frequent Occasional 5 Note: Sea Fog occurs most often when SST is <20° C (68° F) the mixing fog. This type forms with a nearly stationary front or warm front in the northern Gulf. Warmer air aloft overruns a shallow layer of cold air near the surface which may not be more than 100-300 m in depth. This process has been described very well by Hsu (1988). The fourth type of sea fog is formed by radiation over a calm sea under clear skies and very light wind. This type comprises a very small percent of the sea fog epi¬ sodes across the northern Gulf. Duration and dissipation of sea fog greatly depends on the wind speed and dewpoint temperature of the air as well as the temperature of the water (Tw). A critical Tw of 20 C was identified during this study for the development of signif¬ icant sea fog (visibilities < 2 mi 47 Session 3.2 or 3200 m). With a Tw of 20 to 24 C, light to moderate fog was likely and above 24 C, no fog was observed. Our findings were similar to those of Binhau (1985). An esti¬ mate of these parameters can be derived from the forecast technique which will be described in Section 5. 4. GRAPHICAL FORECAST AIDS Several graphical aids (scatter diagrams) have been developed using climatological data for ten SAWRS in the northern Gulf along and north of 28 N and west of 89 W (Fig. 1). In addition to the parameters discussed in Section 2, sea water temperatures (Tw) were used in developing the graphical forecast aids. These aids will serve as another tool for the meteorologist to use in preparing aviation forecasts for the northern Gulf and adjacent coastal plains during the Winter and early Spring seasons. The first three graphs have been developed for advection (cooling) fog (Figs. 3a, 3b and 3c). Fig. 3a which utilizes wind speed is more representative of the area north of 28 N and west of 92 N, especially in depicting dense sea fog (visibilities < 3200 m) where the areal extent of colder water is greater. This follows the outline of the continental shelf (Fig. 1) where the sea surface temperature (SST) gradient is greatest. As mentioned earlier, if the wind ve¬ locities are too high ( > 13 m/s), fog is unlikely even if the Ta - Td is small (Fig. 3a). Dense sea fog is observed with wind speeds < 10 m/s. Hsu (1988) found similar results in his study of the north Gulf. If Ta - Td is too large (> 3 C), then fog is unlikely re¬ gardless of Ta - Tw (Fig. 3b). The fourth graph (scatter diagram) was developed for forecasting cold ad¬ vection (evaporation) fog or common¬ ly known as steam fog (Fig. 4). Visibilities normally associated with this type of fog are usually s 3 mi (4800 m). Fog is unlikely with relative humidities < 83 percent. Dense sea fog is normally found with relative humidities > 90 percent with Tw - a * 15 C. Another forecast aid which is useful to the forecaster is illus¬ trated in Table 1. This table gives the most likely ceilings and visi¬ bilities associated with each of the four types of sea fog as well as frequency of occurrence. These guidelines can be very useful de¬ spite the limited data sample (3 Winter seasons) used in this study. 5. OPERATIONAL FORECAST TECHNIQUE An operational numerical fog model is not yet available for the Gulf of Mexico. An experimental fog model is under development for the Atlantic Ocean. In view of the critical nature of sea fog and low stratus for aviation and marine operations in the northern Gulf, a local technique for forecasting these parameters was developed. An application program was written for a personal computer (PC) for use in plotting several key parameters which are available twice daily in bulletin format from the Nested Grid Model (NGM) at 6 hour intervals. These parameters are: 1) boundary layer temperature (surface - 965 mb), 2) boundary layer relative humidity (surface - 965 mb), 3) sea level pressure in mb, and 4) bound¬ ary layer wind direction and speed (surface - 965 mb). This information is plotted by computer on a map background and then the latest SST 48 Session 3.2 Fig. 5. Example of computer output (forecast) from NGM with parameters analyzed. Legend for Analysis SST T R SLP Threat Area of Sea Fog Cold Front critical isotherm of 20 C is over¬ laid. The parameters (guidance) from the NGM closely approximate the expected values near the surface. These parameters are analyzed and the potential threat area for sea fog development is identified. See Fig. 5 for an example of this prod¬ uct. Forecasters should note that in a study of return flow events in the Gulf of Mexico during the Win- station Plot Legend TT -> 18 1318 <- ODFF VV ->037 o RR -> 80 20 <- SLP T - Boundary layer temperature (°C) R - Boundary layer relative humidity (%) DOFF - Boundary layer wind direction and speed (KT) SLP - Sea level pressure (mb) VV - Vertical velocities (10* 3 mb) ters of 1989-90 and 1990-91, the NGM was about 12 h slow on a 36 to 48 h forecast in returning moisture northward across the northern Gulf. This study was accomplished by some of the forecast staff at the New Orleans WSF0. Similar results were achieved by Janish (1991) in a study of return flow events. Therefore, this should be taken into consider¬ ation when making aviation forecasts 49 Session 3.2 concerning return flow. This method is still in an experimental stage. 6. RECOMMENDATIONS FOR IMPROVEMENT IN SEA FOG FORECASTING In an effort to improve the forecasting of sea fog and low stra¬ tus ceilings in the northern Gulf, additional gridpoint data from the NGM has been requested. This addi¬ tional data from the model over the northern Gulf would be in an area along and just north of 28 N and between 89 W and 96 W. Currently the only grid point data (in bulle¬ tin format) available for forecasts in this area is along the Gulf Coast and generally in the central and south Gulf. The additional data requested would be in an area where numerous SAWRS are located. Addi¬ tional graphical aids should be developed for immediate coastal areas as well as east and west of 92 W over the water. This should give greater detail to the forecast guid¬ ance. Also, enlarging the data sample may improve the graphs a 1ittle more. 7. CONCLUSIONS This paper described the rea¬ sons for developing a technique for forecasting sea fog and low stratus in the northern Gulf. This meteoro¬ logical phenomena is a major avia¬ tion hazard in the northern Gulf. Little if any research has been done on this aviation problem in this geographical area. After evaluating and analyzing the climatological data, a series of graphical forecast charts using various meteorological parameters were developed as an aid for fore¬ casting visibilities associated with sea fog. In addition, a table of recommended ceilings and visibili¬ ties were developed for the differ¬ ent types of sea fog and stratus. Using the application program, graphical aids (scatter diagrams), and table; the aviation forecasts should be improved for the northern Gulf and adjacent coastal plains. 8. ACKNOWLEDGMENTS Dan Smith, Scientific Services Division of National Weather Ser¬ vice, Southern Region Headquarters, made available climatological data for the SAWRS in the northern Gulf. The authors wish to thank Joel Schexnayder, WSFO, New Orleans, for his helpful comments. In addi¬ tion, Jim Moser wrote the applica¬ tion program for the PC for use in the fog forecast. Also, a special thanks to Paula Bolline for typing the manuscript. 9. REFERENCES Binhau, Wang, 1985: Sea Fog. Springer-Verlag. 330 pp. Hsu, S. A., 1988: Coastal Meteorology. Academic Press, Inc., p. 52. Janish, Paul and Steven Lyons, 1991: Evolution, structure and pre¬ diction of cold air outbreaks and return flow in the Gulf of Mexico. (Soon to be published as a summary of the proceedings of the Symposium on Air-Sea Interaction and Air Mass Modi¬ fication by the AMS.) Kotsch, William J., 1983: Weather for the Mariners. Naval Insti¬ tute Press, 315 pp. Mullan, D. S., 1984: Low stratus and sea fog in the North Irish Sea. Weather (GB) 39, 235-239. 50 Session 3.2 Wessels, H. R. A., 1979: Growth and disappearance of evaporation fog during the transformation of a cold air mass. Quart. J.R. Met. 5oc, 105 , 963-977. 51 Session 3.3 THE EFFECTS OF SUMMER TIME STRATUS AT SAN FRANCISCO INTERNATIONAL AIRPORT ON THE NATIONWIDE FLOW OF COMMERCIAL AIRLINE TRAFFIC Walter J. Strach, Jr. Center Weather Service Unit Oakland Air Route Traffic Control Center Fremont, California INTRODUCTION Suppose it is 6 PM CDT and you are at the airport in Kansas City. You have checked your baggage and are ready to board your plane for San Francisco. Much to your surprise, a voice comes over the loud speaker announcing a one hour delay in take off due to weather at San Francisco. You check with the National Weather Service. The weather is clear with a west wind of 18 knots. The forecast for your time of arrival is for some cloud cover at 1200 feet but no rain, no storms, and good visibili¬ ty. So, why the ground delay? GROUND DELAYS INTO SAN FRANCISCO More ground delay programs are is¬ sued for San Francisco International than any other major airport. For 1989, the last year for which we have statistics, there were over 200 delay programs. This number is more than one and one half times the number issued for the second highest delay airport, Newark (Fig. 1). It's not that the weather is partic¬ ularly harsh at San Francisco, it's not. The weather is mild. Thunder¬ storms rarely occur. Fog is rarely a problem. So why the delays? There are three reasons: 1. Close proximity of the main runways. 2. Large volume of traffic during peak arrival times 3. Summer stratus requiring instrument approach proce dures. NATIONAL GROUND DELAY PROGRAMS ALL MONTHS...1989 AIRPORT OF PROGRAMS NUMBER San Francisco 202 Newark 124 Boston 118 Kennedy 113 O'Hare 113 PhiLadelphia 110 LaGuardia 101 Denver 86 Saint Louis 66 Charlotte 63 Atlanta 61 Dallas/Ft. Worth 58 Midway 52 Seattle 45 Pittsburg 23 Detroit 22 Minneapolis 16 Houston International 8 Los Angeles 8 National 6 Orlando 6 Memphis 5 Dulles 5 Baltimore 5 Cincinnatti 4 S. Florida 2 Cleveland 1 Nashville 1 Portland, OR 1 TOTAL 1445 Figure 1. 52 Session 3.3 CLOSE PROXIMITY OF THE MAIN RUNWAYS The runways at San Francisco Inter¬ national Airport were built in the early 1950's, adjacent to San Fran¬ cisco Bay. Under normal conditions, aircraft land on the two main run¬ ways, 28R and 28L; landing from the east southeast toward the west northwest (Figs. 2a & 2b). The air¬ port has Instrument Landing System (ILS) equipment which allows most aircraft to land under just about any weather condition. The traffic patterns are such that aircraft coming in from the north, east, south, and west are merged together 5-15 miles east southeast of the airport in an area known as the approach zone (Fig. 3). If weather conditions are clear, and the arriving aircraft can see one another, the aircraft are allowed to land side by side using the two runways at the same time. This al¬ lows for an acceptance rate of up to 52 aircraft landings per hour. If arriving aircraft cannot see one another in the approach zone, con¬ trol personnel will have to separate aircraft so they land in single file, one after the other. The re¬ sult is that the arrival rate is decreased from 52 per hour to 33. This is a problem whenever there are clouds with bases below 3500 feet or visibility below 5 miles over the approach zone of San Francisco In¬ ternational Airport. The arrival rate would remain con¬ siderably higher if the main runways were built farther apart. From cen¬ ter line to center line the main runways are only 750 feet apart. Under ideal conditions they would be 4200 feet apart. Unfortunately, due to urbanization, there is not suffi¬ cient room to add another runway to the south, west or northwest. San Francisco Bay, to the east and northeast, cannot be filled to cre¬ ate additional land because of envi¬ ronmental restrictions. SUMMER STRATUS Ocean water temperatures along the northern California coast are in the 50's year around due to a cold northerly current and upwelling. Upwelling is the process by which surface water is drawn out to sea allowing cooler water from the depths of the ocean to rise to near the surface. Cold water under the semi permanent Pacific high sets up a marine inver¬ sion. Cooling of the air from below allows stratus and stratocumulus to form under the inversion. These clouds are present along the Cali¬ fornia coast most of the summer from May through September (Fig. 4). During the day, California's interi¬ or valleys frequently heat up into the 90's. As the warm air rises, thermally induced low pressure de¬ velops over the interior. By mid afternoon (22 UTC), it is not uncom¬ mon to see a pressure gradient of 4 mb form between San Francisco, near the coast, and Sacramento in the central valley, a distance of about 75 miles. This strong pressure gra¬ dient sets up a sea breeze which brings the cooler marine air into San Francisco Bay (Fig. 5). The influx of marine air can result in a ceiling at San Francisco Inter¬ national Airport as early as 6 PM (01 UTC). At other times, the marine air aided by night time cooling will result in a ceiling as late as mid¬ night to 7 AM (07-14 UTC). 53 Session 3.3 SAN FRANCISCO TERMINAL CONTROL (GROUP I) $ E AREA ^Sn'S-'K'r 1 ••• • . . -- / • . J y-'-; ' . '' , ’ >v'‘V£{v«ySv .•< -'i ‘ • y ! -- ■ £*♦' ^ * * ' »v sfc*"*«* ■—«<«Figure 2a. San Francisco Bay showing location of San Francisco %%T r T International Airport. wa II «J RUNWAY 28R Figure 2b. Orientation of the runways at San Francisco International Airport. 54 Session 3.3 55 Figure 3. Arrival and departure pattern into the San Francisco Bay.area. Session 3.3 Figure 4- Typical summer weather pattern for California. reeze: Session 3.3 By morning, San Francisco Bay is frequently filled with stratus clouds. Cloud bases and tops are usually fairly uniform. On a day to day basis, bases will generally range from 800-1500 feet and tops 1500-2500 feet. Morning solar insolation normally causes the cloud cover over San Francisco Airport to become scat¬ tered by between 9:00-11:00 AM local time (1600-1800 UTC). However, some¬ times it will be an hour later be¬ fore aircraft in the approach zone can see one another well enough to land in pairs...using the two run¬ ways simultaneously. RUSH HOUR TRAFFIC Just as our freeways experience rush hour traffic, so do our airports. Figure 6 shows the normal hourly arrival rate of air traffic at San Francisco Airport. Note there are two major peaks. One is for morning arrivals between 10:30 AM and 12:00 noon (1730-1900 UTC) and another during the evening between 8:00 and 9:00 PM (0300-0400 UTC). These times correspond to the times when the stratus is burning off in the morn¬ ing and coming in during the eve¬ ning. During the night, the airport can accommodate arriving aircraft de¬ spite the stratus because the traf¬ fic volume is low. However during the evening and late morning when volume is high, it sets up a serious problem. Significantly more aircraft want to land than can be accommodat¬ ed by the landing rate. So what can be done with the extra aircraft? NATIONAL GROUND DELAY PROGRAM Fifteen years ago, when more air¬ craft arrived at an airport than the airport could handle, controllers would put the aircraft into a hold¬ ing pattern. There, an aircraft would remain until there was room for the aircraft to land. As the number of aircraft has increased and the price of fuel has soared, this procedure has become outdated. Today, to cut down on peak arrivals, aircraft are spaced out by delaying departures. This is done by using a ground delay program. OAKLAND AIR ROUTE TRAFFIC CONTROL CENTER Requests for a ground delay program for San Francisco International Airport frequently originate with the Federal Aviation Administra¬ tion's Traffic Management Unit at the Oakland Air Route Traffic Con¬ trol Center. Each day they examine the hourly statistics for scheduled arrivals at San Francisco International Airport. For those times that the arrival rate is expected to exceed the ac¬ ceptance rate, the traffic manage¬ ment unit will request a national ground delay program. A reduced runway acceptance rate could be because of runway mainte¬ nance or due to weather problems. CENTER WEATHER SERVICE UNIT Collocated with the traffic manage¬ ment unit is a group of NWS meteo¬ rologists in the Center Weather Service Unit (CWSU). One of the responsibilities of the CWSU meteo¬ rologist is to advise traffic man¬ agement unit concerning weather that 57 Session 3.3 SFO Diurnal Variation of Ceilings 2000 ft or Less SFO Hourly Arrival Demand Figure 6. Diurnal variation of the probability of a stratus ceiling at San Francisco International are indicated in heavy solid lines. Narrow lines indicate a typical hourly demand for arrivals. Dased lines indicate the acceptance rate of arrivals for clear weather (55 per hour) and for stratus ceiling (33 per hour). 58 Session 3.3 will be a factor in flow of traffic in and out of San Francisco Airport. Weather factors which affect the flow of traffic at the airport in¬ clude: 1. Clouds in the approach zone lower than 3500 feet 2. Visibility over the ap¬ proach zone less than 5 miles. 3. Wind direction if it is other than the west north¬ west when velocity is more than 10 knots. 4. Wind velocity of more than 20 knots when wind direc¬ tion is from the west northwest. 5. Precipitation causing a wet runway. By far, the most frequent occurrence of weather related traffic flow problems is due to clouds over the approach zone below 3500 feet, espe¬ cially during the summer coastal stratus season. AIR TRAFFIC CONTROL SYSTEM COMMAND CENTER When a ground delay program is deemed necessary, Oakland Center's Traffic Management Unit will request one by telephoning the FAA's Air Traffic Control System Command Cen¬ ter, (ATCSCC). The ATCSCC is located at the FAA Headquarters in Washington D.C. and is responsible for maintaining a smooth flow of air traffic across the United States. They are support¬ ed by a collocated group of National Weather Service meteorologists of the Central Flow Weather Service Unit. If Oakland Center wants a national ground delay program for San Fran¬ cisco, the decision has to be made at least 5 hours ahead of time to catch the air traffic along the east coast before those aircraft depart. It takes 4-5 hours for an aircraft to fly across the country. If the request is made any later, the ground delay program usually is made just for the airfields under control of the following western Air Route Traffic Control Centers: Seattle Salt Lake City Oakland Denver Los Angeles Albuquerque See Fig. 7. Once a ground delay program has been agreed upon, the ATCSCC will put it into a computer system which will adjust the takeoff times of those flights departing for San Francisco. NATIONAL WEATHER SERVICE FORECAST OFFICE SAN FRANCISCO Aviation terminal forecasts for San Francisco are used as a guide by the ATCSCC. These forecasts are written by the National Weather Service Forecast Office in San Francisco (WSFO SF0). During the hours the Center Weather Service Unit at Oakland ARTCC is closed, 9:00 PM - 5:30 AM (0400-1230 UTC), the ATCSCC relies heavily on the terminal forecast written by WSFO SFO as a guide in determining whether weather will be a factor in limiting traffic into San Francisco Airport. 59 ARTCCs & LONG-RANGE RADARS Session 3.3 60 Figure 7. Outlined area indicates airspace under the control of the western air route traffic control centers. Session 3.3 COORDINATION Since the ATCSCC and Oakland Center TMU are receiving weather forecasts from three NWS sources, CFWSU Wash¬ ington, CWSU at Oakland ARTCC, and WSFO San Francisco, it is important to maintain forecast consistency among weather units. When necessary this is usually best done through telephone coordination. (Fig. 8) To minimize forecast inconsistencies between meteorologists at the CWSU Oakland and WSFO San Francisco, the CWSU prepares a preliminary terminal forecast for San Francisco Airport and sends it to the WSFO SFO avia¬ tion forecaster prior to the issu¬ ance of the 0200 UTC and 1900 UTC terminal forecast. There is no mete¬ orologist on duty at the CWSU at the time of the 1000 UTC terminal fore¬ cast. Additionally, there is an on going visitation program between meteorol¬ ogists at CWSU Oakland and WSFO San Francisco. Personnel at each office are encouraged to visit the other office and work an occasional shift. This allows each to meet the people from the other office and become familiar with their operation. There are times when consistency is not always possible. This is because She objectives of the product or unit are not the same. The aviation terminal forecast written by WSFO SFO is written for the airport ter¬ minal itself. The forecasts prepared by the CWSU are for the approach zone of San Francisco Airport. The weather over these areas is not necessarily the same. There are times when stratus will be over the approach zone of San Fran¬ cisco when the airport is clear. This happens most often in the morn¬ ing when stratus clears over the airport before it does so over the approaches to the runways. There are also times when a haze layer aloft will reduce slant range visibilities over the approach zone but not seriously affect the hori¬ zontal visibility at the airport. SUMMARY Because the main runways at San Francisco International Airport were built to close together, the airport cannot handle the large volume of aircraft during peak arrival times if there are clouds in the approach zone below 3500 feet. This is a common occurrence in the summer months. To minimize the result traffic flow problems, the FAA's ATCSCC issues ground delay programs to spread out the flow of traffic. Since many of these delays are due to weather, the FAA is highly dependent of accurate weather forecasts from NWS meteorol¬ ogists. 61 Session 3.3 62 Figure 8. Diagram showing the flow of information regarding the decision to issue a ground delay program for San Francisco. Session 3.4 AN OBJECTIVE FORECASTING AID FOR SUMMERTIME LOW CLOUDS DURING SAN FRANCISCO INTERNATIONAL AIRPORT'S EVENING RUSH Henry W.N. Lau National Weather Service Forecast Office San Francisco (Redwood City), California 1. INTRODUCTION The closeness of San Francisco In¬ ternational Airport's (SFO) runways to each other makes the operation of the airport sensitive to the weather elements, such as low clouds below 3500 (thirty-five hundred feet). Between the months of May and Sep¬ tember, low clouds made up of stra¬ tus and stratocumulus, frequent the coastal sections of California (Fig. 1). Their presence over the coastal sections between the Golden Gate Bridge and Half Moon Bay (known in this manuscript as the San Mateo coast, Fig. 2) during the afternoon and evenings represents a threat to SFO's evening peak arrival period (known as the "rush") between 03 and 04 UTC (2000 to 2100 Local Time (LT)). These low clouds can cause delays of up to two hours if not forecasted. Because of today's high jet fuel prices, the demand for on-time per¬ formance and SFO's sensitivity to the weather elements, the SFO termi¬ nal forecast (FT and TAF), issued by the National Weather Service Fore¬ cast Office at San Francisco (WSFO SFO), is extremely critical to the airlines. It is used by them to determine the amount of fuel for the flight, and, whether or not the flight will be delayed. The Central Flow Weather Service Unit (CFWSU) and their Federal Aviation Adminis¬ tration's (FAA) Air Traffic Control System Command Center (ATCSCC) in Washington D.C. also count heavily on the SFO FT. The FAA's lingo when these clouds (usually a ceiling of six-tenths or more sky cover) lower the arrival rate into SFO to a minimum is known as "IFR (Instrument Flight Rules)" or 33 rate (aircraft an hour). When the arrival rate is at maximum, the term "VFR (Visual Flight Rules)" or 52 rate is used. It is assumed, and most commonly it is the case, that a ceiling results in a 33 rate. Also, seldom does SFO experience IFR rate due to scattered (less than six- tenths sky cover), but it does hap¬ pen. For the aviation forecasters at WSFO SFO and Oakland Center Weather Ser¬ vice Unit (OAK CWSU), determining whether a low ceiling will be over SFO before 03 UTC or during the "rush" can be an agonizing task. This is due to the unpredictable nature of the low clouds, the poor data coverage over the eastern Pa¬ cific and the San Mateo coast, and the terrain of the San Francisco peninsula. Typically, the initial ceiling oc¬ curs after 07 UTC (midnight LT), but occasionally it materializes just before the start of the 03 UTC rush. This scenario shall be known in this manuscript as "early return" or "early onset." This paper discusses a procedure (or scheme) subjectively formulated to assist the forecasters in preparing and updating the 19 UTC (1200 LT) 63 Session 3.4 Fig. 1. Stratus/stratocumulus clouds shrouding a good part of the California coastal sections on July 30, 1991 at 00 UTC. 64 Session 3.4 GOLDEN )’■«} ^1 ! ^ JA1’ SAX FAANClSCOO L r > ▲ f V sO vN St . ^ ‘ -X-'xX xk ■, ‘•'3—sy.ft';i . r' ± \ %r*\. AinViV^w kvu ■ i o «««•.• ?, |U*ca\ ; i 0 $ur flat* ^ *> Y L_T!\ - \ • '• f - [, « \ *’ * . Ai P PC» T ic 0/L'VC»¥0*E o ALVAAAOO i -* 1 • SSk?s.v. *alC il*o iH*0*T *AlO AkTO O HOrrCTT HUS » V^KLVOM? saxjose state collece .H.Svcw V' »f3 -w?llVI|« V ‘ f. % ' 'i ‘. ) \ A.APO.T \ r ; \ I I '*. 1 > L •rrr.r.' it U^> \ fr,( C.^ n Wtt*. x , x „. Fig. 2. The map of the San Francisco Bay Area and its vicinity. The "■^• M indicates the location San Francisco International Airport (SFO). 65 Session 3.4 SFO terminal forecast. The scheme's goal is to alleviate some of the uncertainties (of the presence of low clouds during the SFO's evening "rush") in the 19 UTC FT. This is accomplished by recognizing certain synoptic scale patterns (both at the mid-levels and/or at the surface) that are found to have an influence on the weather at SFO. The paper also touch on the basic principles of low cloud forecasting, with particular attention given to SFO. 2. THE 19 UTC FT The SFO 19 UTC FT is critical to the users for it contains the "rush" hour forecast. The CFWSU, in partic¬ ular, uses this product to brief their FAA counterparts shortly after its receipt to see, initially, if a delay program is needed for SFO during the ensuing 10 hours. The ATCSCC preferably would like the SFO 19 UTC F? to be as accurate as possible, especially for the "rush." This sometimes is not feasible. An "in-house" FT is issued by OAK CWSU no later than 23 UTC after coordina¬ tion with WSFO SFO and CFWSU. Many flights destined for SFO from points along the East Coast would not be affected by-a delay program imple¬ mented at 23 UTC. However, flights originating elsewhere would be pre¬ vented from taking off on schedule. 3. DATASET The dataset consist of three sum¬ mers. They are from July 1, 1989 to September 20, 1989; and June 1, 1990 and '91 to September 20, 1990 and '91. The main criteria for this study is the presence of low clouds over the San Mateo coast. Areal coverage was then subjectively assigned, such as patchy; cloudy for areas of or wide¬ spread low clouds; and increasing for clouds increasing over the area. Exempted from this study were days where convective showers and strati¬ form rain occurred during the "rush." Terms were assigned according to the cloud condition during and prior to the "rush." Those evenings when SFO was "IFR rate" during or prior to the "rush" were designated "posi¬ tive." The "IFR rate" can be due either to a ceiling that material¬ ized over the airport, scattered amount of clouds situated over the final approach zone, or a ceiling that persisted throughout the day. Cases where the sky was clear during the "rush" were classified as "nega¬ tive." In addition, evenings where a scattered amount of low clouds was observed but did not affect the flow of traffic were termed "marginal." The pressure gradient values between Areata (ACV) minus (SFO), and SFO minus Sacramento Executive Airport (SAC) were documented on a daily basis, at two hour intervals between 18 and 00 UTC (1100 and 1700 LT, respectively). The SFO wind compo¬ nent was also recorded with the pressure gradient values. Most of the 00 UTC analyses of the NGM's and NMC's surfaces, the NGM's 500 mb initial vorticity and heights, and Oakland's radiosonde were retained, particularly on days when SFO was threatened by low clouds. These data were then subjec¬ tively reviewed. GOES satellite pictures were avail¬ able for a few of the cases. 66 Session 3.4 A statistical test was applied to the two pressure gradient components to determine their averages, and of which set of the components corre¬ spond best to the early onset cases. 4. DISCUSSION The review of the data revealed that at least 93 percent of the early return cases involved a synoptic scale disturbance or trigger, such as an active surface front or bound¬ ary, a deepening and/or approaching trough between the West Coast states and about 135 west longitude (135w), or an approaching shortwave or vor- ticity maxima with a westerly or northerly component (Table 1). Although low clouds are common over the coastal sections along the San Mateo coast, it does not necessarily mean these clouds will spread into San Francisco Bay (SFO Bay) over¬ night nor that they will move over SFO before 04 UTC. Of the 230 days where clouds were present along the San Mateo coast during the after¬ noons and evenings, 51% (117/230) were negative, 15% (34/230) were marginal, and 33% (75/230) were positive (Table 2). Included in the statistics are two cases where the San Mateo coast was clear during afternoon, only to experience IFR rate during the "rush" due to fron¬ tal systems reaching the SFO Bay area just prior to the start of the "rush." Many of the WSF0 SF0/0AK CWSU fore¬ casters monitor two pressure gradi¬ ent components (the difference be¬ tween ACV and SFO and SFO and SAC) to determine whether the low clouds (and amount of it) will enter SFO Bay overnight. The author applied this concept for short term fore¬ casting purpose for SFO and noticed a weak correlation between specific values (of these components) at a reference time of 22 UTC and early onset. A value of 3.4 mb (millibars) was determined to distinguish the terms "offshore" and "onshore" pattern in the ACV to SFO component. As shown in Table 2, 36% (70/195) of the early onset cases involved "onshore" (less than 3.4 mb at 22 UTC with the presence of a trigger). Only 7% (2/27) did an early onset occurred under "offshore" (due to a trigger). More importantly however is the inland SFO to SAC pressure gradient where a weak correlation (about 60%) exist between early onset and the critical value of 3.6 mb at 22 UTC. From Table 2, 63% (45/72) of the early onset cases were associated with values 3.6 mb or more at 22 UTC. The topic of "offshore" and "onshore" will be discussed further in the sections to follow. Since the 19 UTC FT is critical to many users, the author took a closer examination of the three summers of data, which then revealed that there are certain synoptic patterns asso¬ ciated with early onset. 5. LOCATION OF SFO AND ITS SUR¬ ROUNDING As in Figure 1, San Francisco Inter¬ national Airport is located on the west shore of San Francisco Bay. The San Bruno Mountain, located 5 miles to the north and northwest, rises to around 1300 feet. The Santa Cruz Mountains, with elevations of 700 to 1900 feet, extend from the south through the northwest. The ridge immediately west of SFO is 4 miles away with an elevation of 1300 feet. The broad San Bruno Gap with minimum elevation of 150 feet 67 Session 3.4 Table 1. The yearly distribution of the five types of triggers. TYPE 1989 1990 1991 TOTAL 1. Deepening/approaching trough 2 3 13 18 2. Perturbation in the westerlies 7 9 14 30 3. Back side of 500 mb trough 2 5 0 7 4. Pre-frontal stratus 0 4 4 8 5. Front with Vorticity advection 1 7 1 9 Table 2. The statistical breakdown by combining the two surface pressure gradient components with the triggers. For days to be accounted in the table, low clouds must be present over the San Mateo coast during the afternoon and evenings, and of any early onset cases that occurred at SFO. Legends: Negative (NEG) = VFR rate during the SFO's "rush"; Marginal (MAR) = scattered low clouds of less than six-tenths during the "rush", but was not a factor; Positive (POS) = IFR rate during the "rush", which includes cases where SFO had a ceiling (six-tenths or more cloud cover) for most, if not all of the day, and scattered low clouds at IFR rate # Surface pattern/trigger NEG MAR POS TOTAL %P0S 1 . Onshore/trigger/^3.6 mb 6 9 44 59 75 2. 0nshore/trigger/<3.6 mb 15 6 23 44 52 3. Onshore/no trigger/^3.6 mb 9 5 3 17 18 4. Onshore/no trigger/<3.6 mb 61 14 0 75 0 5. Offshore/trigger/^3.6 mb 0 1 1 2 50 6. 0ffshore/trigger/<3.6 mb 4 0 1 5 20 7. Offshore/no trigger/^3.6 mb 1 0 0 1 0 8. Offshore/no trigger/<3.6 mb 18 1 0 19 0 SUBTOTAL # of cases with insufficient 114 35 72 221 data 5 1 5 11 GRAND TOTAL 119 36 77 232* * 230 days of low clouds present along the San Mateo coast. 2 days where the SM coast was clear during the afternoon only to be at IFR rate during the "rush". 68 Session 3.4 sits northwest of the complex facing runways 10L and 10R. It separates the San Bruno Mountains from the Santa Cruz range. This gap results in prevailing wind directions of northwest to west for all months of the year. The gap is also well known to local Weather Service and FAA personnel, for low clouds are com¬ monly sighted there. Another break in the terrain is the Crystal Spring Gap (elevation 850 feet, Fig. 2). It is located west of San Carlos Airport (SQL), along the Santa Cruz Mountains, or southwest of SFO. Low clouds occasionally sneaks through this gap affecting the finals. 6. PRESSURE GRADIENT COMPONENTS A. The San Francisco to (minus) Sacramento Component A notable characteristic involving positive cases is the SFO to (minus) SAC pressure gradient at 22 UTC. This pressure gradient component, which is also known as the inland gradi¬ ent, is an indicator utilized by the forecasters at WSFO SFO in bringing low clouds into SFO Bay. The statistical analysis indicated that the 22 UTC SFO to SAC gradient of 3.6 mb or more corresponded best to the early onset cases. Any in¬ stance before 21 UTC (1400 LT) was unreliable. This 3.6 value (or more) is rather strong for 22 UTC (the average was 3.1 mb for negative cases). Typically, the minimum oc¬ curs during the late morning hours, which may dip as low as 1.5 mb. The significant gradient increase usual¬ ly occurs after 21 UTC, due to maxi¬ mum daytime heating in interior California. Forecasting for this value of 3.6 is rather difficult 3 to 5 hours in advance. Also, not all early onset cases involve a strong inland gradient. Thus, using it as a precursor for early onset remains subjective at the present time. B. SURFACE PATTERN/RECOGNITION AND PRESSURE GRADIENT - The Areata to San Francisco Pressure Gra¬ dient Component Recognizing the surface pattern is a major step in determining whether the clouds will enter SFO Bay over¬ night. There are generally two pat¬ terns that are of concern to SFO's weather; that is "onshore" and "off¬ shore." To determine whether it is "onshore" or "offshore", we use the pressure gradient between ACV and SFO (Fig. 1). 1. OFFSHORE (ACV to (minus) SFO gradient of 3.4 mb or more) Occasionally, a ridge of high pres¬ sure nosing into the Pacific North¬ west and northern California from the eastern Pacific Ocean will re¬ sult in an "offshore" component across the area including the SFO Bay Area (Fig. 3a). This pattern is known to the fore¬ casters at WSFO SFO as "offshore." Strocker (1945), noted that this pattern is associated with (at most) patchy coastal low clouds. It is also safe to add that it is associ¬ ated with minimal low cloud penetra¬ tion into SFO Bay overnight (given the absence of a trigger) and there¬ fore a 52 rate for SFO's evening rush. It was determined from the statisti¬ cal analysis that a value of 3.4 mb or more at 22 UTC between ACV and SFO will keep SFO at a VFR rate through the evening "rush." The most SFO is likely to report (without the influence of a trigger) during this time is scattered amount of clouds 69 Session 3.4 coastal low clouds and fog. _ V®eZFr-26JL91 Fig. 3b. Surface pattern associated with clearing of low clouds over the ocean fran the north during the day. The clearing line reached Golden Gate Bridge area at around 00 UTC and continued south for an additional 20 miles before spreading north to Pt. Reyes the ensuing 10 or so hours. 70 Session 3.4 Fig. 3c. Another pattern associated with minimal coastal low clouds and fog. Fig. 4a. Onshore pattern associated with the potential for widespread coastal low clouds and fog. widespread coastal low clouds and fog. - 71 Session 3.4 with the bulk of them located to the north of the complex. This is de¬ spite a very strong inland gradient of more than 3.6 mb at 22 UTC. The same result applies to a strengthen¬ ing field after 00 UTC. This rule is violated if there is an "active" surface front or boundary located between ACV and SFO that is moving towards SFO. Subsequently, SFO is likely to receive a ceiling before 04 UTC. The definition of "active" is low clouds ahead of or along the feature that can be accompanied by positive vorticity advection. It should be emphasized that a value of 5.0 mb (ACV to SFO) gradient at 00 UTC does not constitute clear or scattered clouds in the bay over¬ night, for the gradient might re¬ verse (to below 3.4 mb) during the ensuing 12 hours. Such a scenario can result in scattered to broken clouds at SFO occurring around day¬ break and lasting until 15 UTC (0800 LT). The low clouds over the coastal sections will more than likely be patchy at 03 UTC, but can be wide¬ spread by 12 UTC. At times it is difficult to tell how far or much clearing there will be during the afternoon over the coast¬ al sections while the 19 UTC SFO FT is being prepared. There were sever¬ al occasions where the clearing line (usually from north to south) reached the Golden Gate Bridge at 00 UTC, only to retreat north to near Pt. Reyes during the ensuing 12 hours. Figure 3b illustrate the surface analysis for such a scenar¬ io. The shaded area indicates the low clouds. Both SFO and Oakland International Airport (OAK) observed a brief ceiling around sunrise. A resemblance to Figure 3a is that the isobaric alignment is offshore north of Pt. Arena, with the heat trough present along the coast south of there. It may be accompanied by a heat low (Fig. 3c). The position of the trough axis is critical. This pattern is also related to minimal low cloud penetration into SFO bay overnight despite its pronounced presence over the coastal sections and a strong inland pressure gradi¬ ent of at least 3.6 mb during the afternoon and evenings. 2. ONSHORE (ACV to (minus) SFO Gradient of less than 3.4 mb) In the second surface pattern, called "onshore", the ACV to (minus) SFO pressure gradient is less than 3.4 mb at 22 UTC (including negative values). This basic pattern is one where the isobars parallel much, if not all of the northern and central coast (Fig. 4a). It favors the exis¬ tence and widespread penetration of low clouds into SFO Bay overnight, a common pattern associated with early return. The most familiar pattern for early onset bears a close resemblance to Fig. 3c. The only difference is that the heat trough axis falls inland across the Sacramento and San Joaquin Valleys (Fig. 4b). Also included under "onshore" is a weak¬ ening offshore pattern with the value falling below 3.4 mb by 22 UTC. At least 93% of the positive cases fell under "onshore." Thus, if the models depicts this pattern, and widespread low clouds are likely to maintain their presence over the coastal sections through at least 00 UTC, then one should investigate (from the forecast charts and latest available data) for any of the trig¬ gers about to be mentioned. 72 Session 3.4 7. INFLUENCE OF LOW CLOUDS BY THE INVERSION AND TOPOGRAPHY A common summertime feature along the California coast is a tempera¬ ture inversion. The base and strength varies from time to time, but is usually below 1200 feet and moderate to strong at 00 UTC. Below the inversion is the marine layer usually accompanied by stratus and fog. The low cloud penetration into SFO Bay is influenced by the topography of the San Francisco peninsula and the temperature inversion. It re¬ stricts the low clouds from invading the bay in force when the base of the inversion is below 1000 feet (given there is no trigger). The terrain also keeps the low clouds away from the airport complex and the final approach zone. The major area where the low clouds enter the bay is between the Golden Gate and just north of the airport. The sec¬ ond area of low cloud intrusion into the bay is the Crystal Springs Gap. There are several scenarios in the way the low clouds enters the bay overnight. The most common setting is low clouds moving into the bay at around dusk, mainly north of SFO. This is due to a low inversion of below a thousand feet. The low clouds tend to pile over the east side of the bay with OAK and Alameda Naval Air Station (NGZ) first to report a ceiling (around 02 UTC (1900 LT)). The clouds will then spread south reaching the San Jose area around 12 UTC before curling north towards SQL. In the mean time, a low ceiling materializes over SFO around 09 UTC (0100 LT). The SQL area can be the last place to re¬ ceive a ceiling. There are other occasions where SFO is the last location to receive a ceiling. Forecasting where the inversion base will be during the evening is diffi¬ cult. The Oakland RAOB is not reli¬ able due to its inland location. Neither is the S0SF temperature. The site is only at the 580 foot eleva¬ tion of the San Bruno Mountain northwest of the airport. Chalk Mountain (CKSCI, elevation 1600 feet), can give an indication of where the inversion stand (Thomas 1990). It is located on the west side of the Santa Cruz Mountains (maximum elevation 1609 feet), southwest of San Jose. Other sources of data that can be of use are cloud top measurements from pilot reports of SFO Bay, and Half Moon Bay's observation (HAF) which appears in pilot report format. These sources are available on an occasional ba¬ sis. They can be retrieved through AFOS pilot report collective PRCCA. The terrain west through northwest of SFO complicates the intrusion of the low clouds into SFO Bay during the night. At times the low clouds will move in over the airport com¬ plex from the gap only to suddenly retreat. At other times, the low clouds will sneak through the Crys¬ tal Springs Gap, thus affecting the finals. Most often though, the low clouds moves over SFO in force, especially well after 04 UTC. 8. EARLY CEILING ONSET TRIGGERS (SURFACE AND 500 MB) It was determined that any of the five synoptic features assisted the earlier than normal arrival of the low clouds at SFO. They are as fol¬ lows: 1. The deepening mid-level trough a cross the eastern Pacific (about 135w eastward) including California. Height falls up to 50 meters in a 12 hour period are possible, but not 73 Session 3.4 necessary, and could have occurred 12 to 24 hours prior to onset. Two versions are shown here, involving a vort max dropping south towards northern California (Fig. 5a), and a short wave trough off the central California coast (Fig. 5b). Low clouds are typically present along the coast prior to onset. This pat¬ tern is usually associated with "all-day" low cloudiness, a after¬ noon burn-off, or partial clearing such that "scattered" IFR may occur. 2. A mid-level shortwave trough/vorticity maximum embedded in a southwest (Fig. 6a) or westerly flow (Fig. 6b) that is forecast to be near the California coast or passing through the Bay Area during the ensuing 12 to 24 hours. The perturbation can be part of a broad 500 mb trough over the eastern Pa¬ cific. "All-day" low clouds, an afternoon burn-off, or partial clearing such that "scattered" IFR can occur with this pattern. 3. A mid-level long wave trough with its axis just east of the Bay Area. The trough is expected to deepen overnight due to a vort max dropping south along the Oregon coast or the immediate offshore waters (Fig. 7). Other characteris¬ tics include: 1) a moist air mass with the absence of an temperature inversion; 2) a strong inland (SFO to (minus) SAC) pressure gradient of more than 3.6 mb at 22 UTC; and 3) the low clouds can be a patch over the Santa Cruz Mountains during the afternoon, only to increase substan¬ tially around sunset. It is most common in the month of June. 4a. Pre-frontal low clouds associ¬ ated with a shallow front (or bound¬ ary) that is expected to reach the Oregon and northern California coast during the ensuing 12 to 24 hours. Low clouds can be fragments of the main cloud band such as in the visu¬ al satellite photo in Figure 8a, or widespread along and ahead of the front (Fig. 8b), including the San Mateo coast during the afternoon. Negative vorticity can be present over the Bay Area. The pressure field off the northern California coast is rather weak, thus allowing the low clouds to exist. In the June 2-3 (UTC), 1990 example, the patch of low cloud ahead of the eastward moving front experienced an increase in size as it impacted the coastal sections west of SFO during the late afternoon/early evening hours. The increase in size was attributed to several factors that included orographic effect, cooler sea surface temperature off the San Mateo coast and nightfall (cooling). The advection of the low clouds into SFO Bay and SFO was aided by a strong inland pressure gradient of more than 3.6 mb during the after¬ noon and evening. SFO picked up a ceiling and a 33 rate shortly after 03 UTC, June 3. The September 12-13 (UTC), 1991 case, involved areas or large patch¬ es of low clouds ahead of the sys¬ tem. Although the inland pressure gradient was rather weak (maximum 3.4 mb at 00 UTC), the surface pat¬ tern was onshore (isobars paralleled the northern California coast). The existence of areas of low clouds was due to a weak pressure gradient ahead of the system. The ceiling and 33 rate occurred toward the end of the "rush." 4b. A similar type trigger is the remnants of a shallow front (or boundary) located just off or on the northern California coast at 00 UTC (Fig. 8c). Negative vorticity can be present over the Bay Area. 74 Session 3.4 Fig. 5a. Cue of two versions of a type one trigger (deepening trough). Lefthand panel depict the initial surface analysis and 1000 to 500 mb thickness. The righthand panel shows the 500 mb heights and vorticity field. Time of ceiling onset occurred during the morning hours of June 18, 1991 but lasted into the evening hours (June 19, 1991 (OTC)). Fig. 5b. Another example of a type one trigger involving a weak stationary trough along the central California coast. Tie flow at the mid-levels became cyclonic during the day of July 7, 1990 (UTC), and the trough deepened even further during the night of July 8, 1990 (UTC). Time of ceiling onset was 03 17TC, July 8, 1990. 75 Session 3.4 particular case (June 24, 1990 UTC) was 02 OTC. Fig. 6b. The second of two versions of a type two trigger...a perturbation in the westerly flow...seen in this case on 00 I7TC July 24, 1991 as the shortwave approached northern California and the Pacific Northwest. Time of the ceiling onset was 02 UTC July, 24. 76 Session 3.4 Fig. 7. An example of a type three trigger (vorticity maxima located on the backside of a 500 mb trough} that is situated over California and adjacent states. Cd June 15 1990 UTC produced scattered to broken clouds during SFO's evening "rush." 77 Session 3.4 Fig. 8a. The N3f's 500 mb heights/vorticity analysis (left hand panel) and the corresponding satellite picture of pre¬ frontal low clouds (righthand) taken at 22 UTC (June 2 , 1990)., . - - -The-patcdi of stratus (indicated by the arrow) was responsible for a ceiling at SPO about 5 hours later at 03 UTC (June 3 , 1990). Fig. 8c. The second of two versions involving a type four trigger (diffuse boundary). The arrows on the satellite picture shows the position of the boundary at 22 UTC, June 27, 1990. 78 Session 3.4 UTC as an example of low clouds ahead of a frontal system approaching the Pacific Northwest (indicated on the satel¬ lite picture taken at 0131 UTC Sept. 13, 1991) by the . arrows. SFO reported a ceiling shortly after 03 UTC. 79 Session 3.4 Fig. 9. A case involving a front with vorticity advection. Time of the ceiling onset was 01 UTC, Aug. 30/ 1990. Fig. 10. A case example of a shortwave with most of its energy ^ ' moving into the Pacific Northwest and extreme northern California. This example caused local ceiling in SFO bay during the night of Aug. I, 1991 (UTC). SFO experienced scattered "VFR" during the rush/ with a ceiling commencing at 07 UTC. 80 Session 3.4 In this example of June 27-28 (UTC), 1990, the much benign appearance of the old boundary experienced a rath¬ er rapid increase in low clouds once it came in contact with the cooler coastal waters west of SFO. Oro¬ graphic process also was a player as well as the onset of nightfall. The inland pressure gradient was more than 3.6 mb during the late after¬ noon and evening, which aided in the continual advection of low clouds into SFO Bay and SFO. SFO observed a ceiling and a 33 rate at 02 UTC June 28. 5. A surface front extending across northern California and the eastern Pacific, and is approaching the SFO Bay Area. It is accompanied by vorticity advection and low clouds over the waters, thus repre¬ senting an active front (Fig. 9). The onset time for an early ceiling depends on when the front reaches the Bay Area. Other note of interests are: - If a trigger is identified and scattered clouds exist into the late afternoon hours (e.g. 00 UTC), there is the likelihood of an early onset. - A perturbation passing either south or remaining east of the Bay Area is likely to produce at most scattered clouds (VFR) over SFO. - "Dry" fronts, identified by water vapor imagery, that moves into northern California and the adjacent coastal waters from the north usual¬ ly do not cause an early ceiling at SFO although low clouds are present along and/or ahead of the system. - SFO will more than likely experi¬ ence, at most, scattered (VFR) clouds during the "rush" when thun¬ derstorms are observed around the Bay Area during the day. The usual onset time for a ceiling (rounded off to the nearest hour) is 02 UTC with a standard deviation of one and one-half hours. 9. IFR RATE DUE TO NON-CEILING CONDITION Evenings where a scattered amount of low clouds prevailed were classified marginal. The term "scattered", however, has a ambiguous meaning (besides being subjective), particu¬ larly to the forecasters at OAK CWSU and CFW-SU. The condition over the finals is most important, more so than the airport complex itself. The amount of "scattered" clouds does not have to be five-tenths to cause an IFR rate. All it takes is a minimal one-tenth located in the final approach zone. The suspect cause of "scattered" IFR is the Crystal Springs Gap. The weather observer at SFO may have a difficult time detecting the low clouds moving over the finals from this gap espe¬ cially after nightfall due to ob¬ structions. This is where the SQL observation can come in handy, for the observer has an excellent view of the approach zone into SFO. There were not many occasions where SFO experienced an IFR rate due to "scattered" (or occasional broken) low clouds during the "rush." Only 4 cases have been documented during the last 3 summers...all caused by a trigger (involving types 1, 2, 3 and 4a). Two common scenarios leading up to the "rush" are 1) absence of low clouds prior to the "rush" over the San Mateo coast; 2) remnants of low clouds from a late daytime burn-off (around 23-01 UTC), where the rem¬ nants are enough to affect the fi- 81 Session 3.4 nals (trigger 1 and 3). A ceiling materialized over SFO between 04 and 07 UTC in each case. 10. MARGINAL CASES (SCATTERED "VER RATE") Comprising 15% of the 230 days where low clouds were present along the coast, a majority of this 15% had clouds north of the airport's com¬ plex. In other words, VFR occurred during the "rush." About 26 of the 36 involved a weak vorticity maxima/shortwave over the Bay Area during the evening. The most notable pattern is a shortwave with the bulk of its energy forecast to move into Oregon and extreme northern California (Fig. 10). The tail end of this feature, however, was forecast to brush the Bay Area overnight. The result was locally cloudy skies in the bay during and after the rush. SFO was scattered VFR during the "rush" with broken to scattered clouds between 07 and 13 UTC. It is suggested that a forecaster call for scattered clouds for SFO during the rush should such a situa¬ tion arise. 11. PREVAILING WIND DIRECTION Although more research is required to determine the relationship be¬ tween the wind pattern and the low clouds around SFO, there are clues in the wind field that can foretell the weather condition for SFO's evening "rush." The following are a few scenarios the author has no¬ ticed. If the prevailing wind direction around the SFO Bay Area indicated by the buoys (12, 26, 42) and Pigeon Point are south to southwest, SFO will more than likely stay VFR dur¬ ing the "rush." The East Bay air¬ ports such as OAK and NGZ may have ceiling at 03 UTC, but SFO can (at most) be scattered during the rush (possibly due to downslope). This is despite a SFO to SAC gradient of more than 3.6 mb at 22 UTC. A common characteristic of this pattern is the sea breeze component at SFO (northwest wind) materializing later than the usual time of 19 to 21 UTC). SFO instead, experiences an northeasterly component. This condi¬ tion is usually due to an eddy off the San Mateo coast. SFO will also stay clear through the night if the lower Sacramento Valley airports, including Travis Air Force Base (SUU), reports wind from the north during the afternoon. (This condition is associated with a strong offshore pattern which mini¬ mizes the infiltration of the marine air mass into the bay.) A majority of the early return cases were accompanied by a west-southwest to northwest wind components at SFO, with the directions ranging from 250 to 300 (true) degrees during the afternoons and evenings. Buoys (12, 26, and 42) and Pigeon Point would also show a west through northwest component. The only exception to the rule is an "active" front nearby. The wind speed was at least 12 knots. 12. PROCEDURE TO PREPARING AND UPDATING THE 19 UTC SFO FT The following is a suggested proce¬ dure for preparing the 19 UTC SFO FT. 1. Continuous monitoring of the satellite pictures over the eastern Pacific from about 135w eastward and between 42 and 30 north latitudes, 82 Session 3.4 including the California coast. Pay particular attention to the low clouds over the waters that are moving eastward towards the San Mateo coast (especially on days where the coast is clear). Determine if these clouds will impact the coast before 03 UTC. Advection of low clouds are usually associated with a vorticity maxima or fragments of the main frontal band. If there is clearing along the coast from the north, determine how far south the clearing will be (see "Surface Pat¬ tern and Recognition" for more de¬ tails). The water vapor imagery is useful in detecting vorticity maxi- mas and shortwaves. Determine from the 12 UTC progs (mainly surface and 500 mb) if a trigger is present between 135w and the California coast. If YES, and if it is moving eastward, determine if it is on track by comparing the first 12 hour period with the latest satellite imagery and other perti¬ nent information. The Kansas City satellite discus¬ sions (under AFOS - the National Weather Service's Automated Field Operational System - product headers SIMMKC and SIMPSM, and their graph¬ ics CSM and IPG) are useful in de¬ scribing important features and their trends. The suggested forecast is as fol¬ lows: - If the prognostic charts valid 00 UTC resembles any of the five trig¬ gers discussed earlier (Early Ceil¬ ing Onset and Triggers), forecast early onset (a ceiling). - If the prognostic charts valid 00 UTC does not resemble any of the triggers and you feel it will not affect the 03 UTC "rush", then fore¬ cast clear to scattered amount of clouds. - If unsure, then forecast scattered to broken amount of low clouds. After the 19 UTC FT has been sent, keep a close tab on the situation not only by monitoring the latest satellite pictures but also the following items: 2. Monitor the pressure gradients between ACV and SF0, and the SF0 to SAC gradient. Then determine from the 12 UTC progs what the surface pattern will be during the ensuing 24 hours, e.g. offshore or onshore. AFOS products such as the surface plot (pOa) or surface analysis (90i), the hourly comparison of surface observations across Califor¬ nia and adjacent states (24hchg), and the graphical display of the trend of the pressure gradients between selected locations during an 18 hour period (gdl) should be moni¬ tored closely. 3. Monitor the wind condition around the SF0 Bay Area, such as the buoys off the San Mateo Coast (#12 and 26) and Monterey Bay (#42) (found under B0YCM7); the remote DARDC (Device for Automatic Remote Data Collec¬ tion) wind sensor at Pigeon Point under SF0DARDC; the three hourly Coast Guard observations under WRKDAR; and the remote wind/temperature sensors at South San Francisco (SOSF) whose data is appended at the end of the SFO's hourly observations. 4. Try to determine where the inver¬ sion base may be during the evening. Some of the products to aid you are pilot reports (PRCCA), Chalk Moun¬ tain whose SHEFcoded (remote sensors - Standard Hydrologic Exchange For¬ mat) identifier is CKSCI - found 83 Session 3.4 under NNCRRACA1, and SOSF tempera¬ ture. Then pertaining to Table 2 on page 4, if the situation fits... ..#1: forecast an early ceiling. ..#2: forecast an early ceiling when the trigger or situation is identi¬ cal to any of the examples given. Otherwise, forecast scattered amounts (VFR) of low clouds. There is about a 50% chance of an early onset. ..#3: forecast scattered (VFR) clouds. There is about a 20% chance of an early onset. ..#4: forecast clear to scattered (VFR) clouds. ..#5: forecast an early ceiling when the feature is identical to any of the examples given. The one positive case involved a back door front. ..#6; forecast scattered (VFR) clouds, with a 20% chance of a IFR rate. ..#7: forecast clear to scattered (VFR) amount of clouds. There is about a 20% chance of an early on¬ set. ..#8: forecast clear skies. If a significant change from the original forecast is anticipated, where it will affect the normal flow of traffic into SFO, it is advisable to coordinate with OAK CWSU and WSO SFO prior to updating the SFO termi¬ nal . It should be emphasized that the scheme is not perfect and should be use with discretion. This paper is to serve as a stepping stone in understanding the "mysteries" of early onset at SFO, in hope of elim¬ inating many of the uncertainties that goes in forecasting the sky condition for SFO's evening "rush. 13. CONCLUSION The review of three summers of daily weather data for the SFO and vicini¬ ty has yield a scheme in assisting the forecasters in preparing the 19 UTC FT (and TAF) and 22 UTC update by OAK CWSU. The scheme assist the forecaster in identifying so-called triggers that is found to be the cause of an early onset of a low ceiling during the SFO's critical arrival period of between 03 and 04 UTC. The findings of these synoptic scale features, known as triggers, will increase the awareness and accuracy of SFO's approach zone forecast. In addition, there are signatures that materializes during the afternoon that dictates the outcome of the "rush." This can be accomplished by recognizing parameters such as the pressure gradient components between ACV and SFO, and SFO and SAC; the prevailing wind at SFO and vicinity, including buoys and remote wind sites; and the sky condition at NGZ and OAK, and SFO during the after¬ noon. Five triggers in the synoptic scale were identified. An example of each is included. These five triggers are also responsible for a majority of the scattered "VFR" and all scat¬ tered "IFR" conditions during SFO's "rush." The findings of this study by no means solve the problems of an early onset at SFO. The checklist should be used with caution not only be¬ cause it is subjectively prepared, but there can be additional scenari¬ os associated with early onset. Also, there are elements on the list that required further research. Only time will allow the list to be re¬ fined. 84 Session 3.4 Acknowledgements. The author wish to thank Walter J. Strach, Jr (MIC OAK CWSU) for all his valuable time and assistance, to Mike Mogil and Dick Pritchard of NESDIS for the quick response for requests of satellite pictures. In addition, thanks to S.K. Harner of NESDIS SAB, the staff of WSFO/WSO San Francisco and Oak¬ land CWSU for their inputs; to Bill Aldridge, Ernest Daghir and PRC staff for the fast, efficient ser¬ vice for additional satellite pic¬ tures. Special thanks goes to Charito Brillantes and Shirlee Lowe for editing this paper. REFERENCES NOAA/National Weather Service, 1968: Terminal Forecasting Manual for San Francisco International Airport, 11 Strocker, G.H., 1945: Forecasting Bay Region Stratus, Journal of Aeronautical Meteorology, 129. Thomas, J., 1990: Mountaintop Temperatures as a Stratus Fore¬ casting Aid, In-house tech memo (unpublished), National Weather Service Forecast Office San Francisco. 85 Session 4.1 NMC'S MONITORING AND AVIATION BRANCH: ORGANIZATION, PRODUCTS, AND FORECASTING TECHNIQUES Vincent C. McDermott Monitoring and Aviation Branch National Meteorological CFnter Camp Springs, Maryland 1. ORGANIZATION OF THE MONITORING AND AVIATION BRANCH The Monitoring and Aviation Branch (MAB) is part of the Meteoro¬ logical Operations Division of the National Meteorological Center (NMC). One of its main responsibili¬ ties is producing centralized opera¬ tional forecast guidance in support of U.S. and international aviation activities. The branch consists of the Senior Duty Meteorologists, the Aviation Section, and the Satellite and Marine Section (SMS), located at the NOAA Science Center in Camp Springs, MD, and the Central Flow Weather Service Unit (CFWSU), locat¬ ed at FAA headquarters in Washing¬ ton. A description of the activities of the CFWSU is contained in a com¬ panion paper. The Spaceflight Meteo¬ rology Group (SMG), located in Hous¬ ton, TX, once a part of MAB, was transferred to the Southern Region of National Weather Service (January 1992). Five Senior Duty Meteorologist (SDMs) are responsible for monitor¬ ing the production of NMC guidance on a round-the-clock basis seven days a week. As such, they are con¬ stantly reviewing the status of NMC computer runs and the dissemination of NMC guidance products. Senior aviation meteorologists sometimes serve as substitute SDMs. The SDMs also monitor the qual¬ ity and quantity of upper-air obser¬ vations, including radiosonde, air¬ craft, and satellite reports. The SDMs, assisted by aviation meteorol¬ ogists, may alter, purge, or keep questionable observations. This data monitoring function is an essential part of the overall process of pro¬ ducing quality aviation products. A recent analysis of the func¬ tions of the SDM position revealed a complex array of responsibilities. Among other duties, the SDMs are forecasters, monitors, supervisors, and coordinators. They act for the MAB branch chief in his absence, and act for the Director of NMC during non-administrative hours. Much of the coordination effort is spent interacting with National Weather Service's Office of System Operations (0S0). In very general terms, 0S0 collects raw meteorologi¬ cal data and transfers it to NMC's computers. NMC runs the computer models and sends model output prod¬ ucts to 0S0 for distribution to the user community. The three main 0S0 facilities which handle this func¬ tion are Tech Control, Facsimile, and the Systems Monitoring and Coor¬ dinating Center (SMCC). These facil¬ ities are the prime user contact point for dissemination problems. These and other 0S0 facilities are now being moved from Suitland, MD to Silver Spring, MD. 2. PRODUCTS AND FORECASTING TECH¬ NIQUES - AVIATION SECTION 86 Among other products, the avia¬ tion section produces low-level Session 4.1 ceiling/visibility, turbulence, and freezing level forecasts for domes¬ tic aviation; high-level significant weather progs for international aviation; and amendments of computer-produced wind forecasts. Twelve meteorologists are assigned to the section. Low-Level Products There are four low-level prod¬ ucts produced each day. Each con¬ sists of two 12 and 24 hour fore¬ casts, with valid times of 12Z/00Z; 18Z/06Z; 00Z/12Z; and 06Z/18Z. The forecast area covers the continental U.S. and portions of southern Cana¬ da, and extends from the surface to 24,000 feet. Two forecasters work on each product; one completes the ceiling/visibility forecast, while the second does the turbulence and freezing levels forecast. Ceilinq/Visibilitv . Three categories are forecast: 1) IFR - ceiling less than 1000 ft and/or visibility less than 3 miles; 2) MVFR - ceiling 1000 to 3000 ft and/or visibility 3-5 miles; 3) VFR - ceiling above 3000 ft and visibility greater than 5 miles. Areas of IFR and MVFR are depicted on the product. Primary forecast tools used are manual guidance provided by NMC's Forecast Branch; MOS guidance from the LFM run; LFM, NGM, and AVN model products, including relative humidi¬ ty fields and boundary level winds; airport climatological data; and satellite pictures. These inputs are then subjectively modified based on forecaster experience to produce the resultant forecasts. The 24-hour forecast is verified by a computer program and compared to the Tech¬ niques Development Laboratory (TDL) automated forecast. Cumulative scores are kept. Freezing levels . The surface freezing line, as well as 4000 ft, 8000 ft, 12000 ft and 16000 ft freezing levels are forecast. The forecaster focuses on upper-air height, temperature, and thickness fields, surface winds, cloud cover, and diurnal tendencies. A comparison between analyzed thickness values and observed upper air freezing levels is made, and forecasts are made using the observed correlation and forecast thickness values from the model runs. Forecasts are veri¬ fied subjectively by branch quality control focal points. Turbulence . Forecasts of moder¬ ate or greater turbulence are de¬ picted. If the Forecast Branch mete¬ orologist predicts an area of broken thunderstorms, a depiction of at least moderate-to-severe turbulence below 24,000 ft is required. In addition to model forecast of bound¬ ary layer and upper air winds, vor- ticity, thermal advection and sta¬ bility, forecasting tools include computer-produced turbulence report summaries and charts; radar summa¬ ries; satellite pictures; and com¬ puter produced probability of CAT guidance. Forecasts are verified subjectively by branch quality con¬ trol focal points. High-Level Products Packages of high-level signifi¬ cant weather progs for international aviation are produced four times daily. The valid times of the prod¬ ucts are 00Z, 06Z, 12Z, and 18Z. In order to meet facsimile deadlines and to provide enough time for the final products to reach the users, the forecasts are made 16 to 18 hours prior to valid time. 87 Session 4.1 The high-level progs cover a vertical area from 24,000 ft to 60,000 feet. The area of responsi¬ bility extends primarily from 100E eastward across the Pacific, North America, the Atlantic, and Europe to 35E, from the equator to the North Pole. Small portions of the Southern Hemisphere (from the equator to 20S, from 130W westward to the Interna¬ tional Dateline and a portion of the west coast of South America and adjacent waters from the equator to 13S, from 73W westward to 98W) are also included in the area. Two avia¬ tion forecasters work on each pack¬ age. One forecaster, usually called the "Pacific" forecaster, is respon¬ sible for the area from 100E to 100W north of 20N, and a tropical "strip" from 100E to 150W south of 20N to the equator, plus a region south of the equator from 180 eastward to 150W. The second, or "Atlantic" forecaster, is responsible for the area from 100W to 35E north of 20N, and a tropical strip from 150W to 05W south of 20N to the equator, plus the portions of the Southern Hemisphere east of 150W. The two forecasters coordinate their fore¬ casts where their areas of responsi¬ bility abut. Forecasting tools include Avia¬ tion model products, including fore¬ casts of 250-mb heights/isotachs, maximum wind levels and speeds, tropopause heights, 500-mb heights/isotachs, 500-mb vorticity, relative humidity, lifted index, surface isobars, and 1000-500 mb thickness fields; satellite pic¬ tures; cloud top estimates provided by the Synoptic Analysis Branch (SAB) of NESDIS; tropical cyclone bulletins; manual guidance from the Forecast Branch of NMC (for the contiguous U.S.); and computerized turbulence probability charts. The progs contain surface fron¬ tal positions and speed of movement, turbulence, embedded CB activity, flight level and wind speed of jets, tropopause heights, tropical cyclone information, and data on volcanic ash clouds. Forecasts are verified subjec¬ tively by branch quality control focal points. These high-level forecasts represent a large amount of work. The two forecasters who work on each package are responsible for a huge geographical area, forecast a large number of parameters, and spend a lot of effort formulating their progs. Wind Amendments Forecast wind speeds of the Aviation forecast model from 24,000 to 39,000 ft. can be amended twice daily, based on 12Z and 00Z data. Areas of wind amendments are defined by four coordinate points. The fore¬ casts can cover 12, 18, 24, and 30 hour periods from forecast time. The area of responsibility extends from 120E eastward to 30E, north of the equator. Amendments are made for wind speed changes of a minimum of 20 knots and 20% of forecast wind speed. Amendments may increase or decrease wind speeds. Wind amend¬ ments information is disseminated on facsimile and by coded message. Twenty-four hour forecasts are veri¬ fied by computer analysis. There are an average of four amendments each day. 88 Session 4.1 3. SATELLITE AND MARINE SECTION (SMS) In addition to its marine fore¬ casting responsibilities, the SMS provides international SIGMETS and Satellite Interpretive Messages (SIMs) over oceanic areas, and an area forecast (FANT) over the Atlan¬ tic for the New York Air Route Traf¬ fic Control Center (ARTCC). One supervisor and fifteen meteorolo¬ gists are assigned to the section, with five working in each of the Atlantic marine, Pacific marine, and satellite groups. International SIGMETS SIGMETs are short-term (0-4 hours) advisories of in-flight wea¬ ther which may be hazardous to air¬ craft operations. They contain in¬ formation on specified weather phe¬ nomena of an intensity and/or extent that concerns pilots and operators of an aircraft. These phenomena include active thunderstorms, tropi¬ cal cyclones, severe turbulence, and volcanic ash clouds. The forecasters use AIREPs, satellite imagery, vertical wind shear analysis and forecast charts from the Aviation model run, hemi¬ spheric turbulence report summaries, and probability of CAT forecasts from the Aviation model when making their forecasts. They coordinate with and obtain information from Meteorological Watch Offices (MWOs), Central Weather Service Units (CWSUs), and ARTCCs. The area of responsibility over the Atlantic Ocean is the New York Oce¬ anic Flight Information Region (FIR). It extends from 30N to 45N, west of 40W, to a line from 30N 70W to 42N 67W to 45N 40W. The Pacific Ocean area of responsibility is included in the Oakland Oceanic FIR. It extends from 37N 165E to 46N 166E to 57N 150W to 30N 120W. SIMS The Satellite Interpretive Message (SIM) provides a meteorolog¬ ical interpretation of satellite imagery, described in a concise narrative. It includes correlation of satellite data with conventional analyses and numerical forecasts. The areas of responsibility for the Atlantic and Pacific SIMs are the same as for the SIGMETS. Information on jetstream axes, upper air lows, upper air troughs, vorticity maxima, upper air ridges, and precipitation area is included in the SIM. Direction, speed, and acceleration are indicated for lows, vorticity maxima, ridges, and troughs. SIMS are issued every six hours for specific areas of responsibility over the Atlantic and Pacific Oceans. A graphic SIM, which illus¬ trates features mentioned in the Pacific alphanumeric SIM, is issued at the same times. The area of responsibility for the SIM, which is done for the Pa¬ cific Ocean only, is the same as for Atlantic Ocean SIGMETs. FANTS The FANT is a low-to-mid level (surface to 24,000 ft) significant weather forecast issued in alphanu¬ meric form. It is issued four times daily, at 04Z, 10Z, 16Z, and 22Z. The forecast is valid for a twelve- hour period beginning at two hours past the issue time. The area of responsibility covers the North Atlantic Ocean from 32N to 40N, west 89 Session 4.1 of a line from 32N 63W to 40N 67W, including Bermuda. The FANTs include the follow¬ ing: a synopsis, which describes the movement and any trend of the weath¬ er systems that will affect the forecast area during the valid time of the forecast; significant weath¬ er, including areas of widespread ceilings, areas of IFR conditions, heights of cloud bases and tops, heights of CBs, and causes of IFR conditions; information on icing, including areas of moderate or se¬ vere icing and freezing levels; areas of moderate or greater turbu¬ lence, including flight levels of the turbulence; and an outlook, which includes a brief statement of the movement of weather systems affecting the forecast area and the trends of the significant weather affecting the area. The outlook covers the twelve-hour period fol¬ lowing the valid time of the FANT. Forecasters utilize hemispheric surface analyses, AIREPs, radar summaries, computer model products including relative humidity fields, freezing level analyses, and upper air soundings when preparing their forecasts. SIGMETs, SIMs, and FANTs are verified subjectively by SMS person¬ nel . 4. PLANS FOR THE FUTURE A number of ongoing projects indicate that there will be a number of improvements in areas directly affecting the MAB. With respect to the quality control of upper-air observations, branch meteorologists will be monitoring data in a work station environment. Computer pro¬ grams will take care of routine corrections, leaving the meteorolo¬ gists with more time to handle dif¬ ficult correction decisions that the machine cannot make. Branch forecast products will be produced on and disseminated through work stations, improving the quality of the product and speed of dissemination. NMC is investigating a 6-hour Aviation model forecast cycle and evaluating a rapid update cycle (RUC), both of which utilize new data sources, including automated aircraft re¬ ports. New computer-generated turbu¬ lence forecasts are being worked on as wel1. 5. SUMMARY The forecasters of the Monitor¬ ing and Aviation Branch predict a wide variety of meteorological pa¬ rameters over a huge geographical area on a round-the-clock basis. They produce a large number of prod¬ ucts, which are disseminated to members of the worldwide aviation community. Plans for the future include improvements in the quality control of upper-air observations; continued improvement in the quality of forecast products through updated production and dissemination facili¬ ties; and improvements in NMC's forecast models, which will assist the meteorologists in making better forecasts. 90 Session 4.2 DEVELOPMENT AND APPLICATION OF AN ICING PREDICTOR EQUATION David W. Bernhardt National Weather Service Office Springfield, Illinois Michael R. McCarter Atmospherics, Inc. Fresno, California 1. Introduction Forecasting methods for air¬ craft icing developed and used by the United States Air Force (AWS, 1980) and the U. S. Navy (NAVEDTRA, 1974) have been shown to have an accuracy not much better than chance (Bernhardt, 1989). As a result, we undertook the development of an improved approach to icing forecast¬ ing which was based on meteorologi¬ cal parameters easily computed from operational data. These included condensate supply rate (CSR), vapor flux rate (VFR) (Rhea, 1979), adia¬ batic condensate, Appleman's (1954) -8D parameter, static sta¬ bility (Holton, 1979), and dew-point depression. Development data came primarily from measurements by the University of North Dakota Citation II aircraft (Grainger et al .. 1986) and secon¬ darily from pilot reports. The study included data from an area encompassing parts of Oklahoma, Kansas, Colorado, and Missouri. These data were taken from 1987-90 winter seasons. Known icing layers were compared to output from a lin¬ ear regression equation which was developed on data from icing layers. The resulting icing predictor equa¬ tion was used in daily forecasting support for a winter (1989-90) field season, with very good results. Also, in order to formulate an icing potential outlook, an attempt was made to predict the amount of super¬ cooled liquid water (SLW) from com¬ monly measured parameters. 2. Icing Equation Development Background The seven (7) different parame¬ ters correlated against icing were chosen to compare past research with results of our own, as in the case of -8D, and to find if other rela¬ tionships could be found, not yet documented. For each of the param¬ eters, a short summary of their definition and derivation follows. 2.1 Adiabatic Condensate Formula The adiabatic condensate (ACON) is the amount of moisture that falls out of a parcel as it is lifted from one level to another. As long as the parcel is unsaturated, the lift is by an adiabatic process, but once saturated, the process is assumed to be pseudoadiabatic. As noted by Rhea et al . (1982), ACON can be approximated by subtracting the specific humidity at the level to which the parcel was lifted from the specific humidity at the original level, i.e., AC0N(d = q (0) - q (1) (1) 91 Session 4.2 where AC0N ( , } is the adiabatic con¬ densate at the original lower level (g kg' 1 ); q. 0) is the specific humid¬ ity (g kg' 1 ) at the original level; and q (1) is the specific humidity at the level to which the parcel was lifted. No limiting thresholds were established on the value of AC0N (L) . Icing layer thicknesses varied as synoptic conditions changed. A previous study (Bernhardt, 1989) had found that through synoptic classi¬ fication of patterns accompanying icing, different thicknesses of par¬ cel lift to sustain icing can be established. In this study, a 20 hPa (approx. 220 m) lift was assumed for cases of isentropic and warm frontal lift, ranging up to 100 hPa (approx. 950 m) for vertical motions associated with moderate to strong cold fronts. This allowed lifting of thicknesses variably dependant upon the synoptic features affecting a region. 2.2 Vapor Flux Rate Formula The vapor flux rate (VFR) is the advection rate of water vapor into a layer, accounting for direc¬ tional shear moving vertically through the atmosphere. VFRs incor¬ porated specific humidities, wind speed and wind direction at the same level at which it was computed. The direction was algebraically sub¬ tracted from the wind direction at an altitude determined to be a point of condensate renewal. If there was no significant shear through the lowest 3300 m (10,000 ft) of the sounding, the condensate renewal direction was assumed to be the average wind direction in this lay¬ er. However, if an inversion exist¬ ed, this was the wind direction near the top of the inversion. Clouds were often capped at the inversion and a strong directional shear fre¬ quently occurred, therefore this was considered the point of strongest condensate renewal. The formula for vapor flux rate (Rheaet_al_., 1982) follows: VFR (i) -Ccu xV (u XC08 ( WDR) x f (2) where VFR..J is the vapor flux rate (g cm' 1 s'j; q (L) is the specific humidity at the level; V (L) is the level wind speed (cm s* 1 ); WDR is the component wind direction; TH is the thickness of the parcel lift (hPa); and g is the acceleration due to gravity (cm s' 2 ). The preceding formula gave us values which were widely variable. High values were found with stronger winds, which did not necessarily relate to higher icing incidence. Due to this problem, we decided against establishing a specific threshold cut-off point. Rather, we looked for value maxi mums in the vertical profile of the VFR, which did appear to have a relationship to icing potential. 2.3 Condensate Supply Rate Formula The condensate supply rate (CSR) is the advection rate of new water vapor into a region, which is then converted to available conden¬ sate. It is based on many of the same parameters used in computation of VFRs, but also includes the adia¬ batic condensate and width of a surface over which a vertical lift of the moisture is assumed. The lift may be either topographic or dynamic. The CSR formula -- derived from the atmospheric water balance equa¬ tion for two-dimensional, steady- 92 Session 4.2 state flow up a barrier -- was com¬ puted using a scheme similar to that outlined in the adiabatic condensate and VFR discussions (Sec 2.1 and 2.2). An up-slope plane (Fig. 1) of 150 to 200 km was used to account for gentle up-slopes often found in isentropic and warm frontal lifts. The plane was shortened to 100 km in synoptic cases involving moderate to strong cold fronts. Fig. 1. Condensate Supply rate along an equivalent potential tem¬ perature up-slope plane. Condensate is supplied from the left of the diagram with lifting occurring along isentropic surfaces. The layer and lift of the condensate is denoted by the hatching between isent ropes. The wind speed and direction of condensate renewal is indicated by the wind flag. The formula for CSR (Rhea et al., 1982) is: CSR , THxV., XCOS ( WDR) xACON , t) pxgxWDTH (3) where CSR ( , } is the condensate sup¬ ply rate at the level (mm hr' 1 ); p is the density of water (g mm-1); and WDTH is the width of the up-slope plane (cm). The other variables have been previously de¬ fined. As noted in the vapor flux rate discussion, the CSR formula also gave widely variable values, mostly relating to variations in the wind speed. Once again, no threshold limits were set on the data to allow testing of all values. One limita¬ tion placed on the data was to allow only those values that fell between 0 # C and -20*C. We found icing in temperatures as low as -40°C, but concurred with an AWS (1980) study which showed the incidence of icing drops sharply at temperatures colder than -20°C. The CSR and VFR were found to be positive indicators of icing potential. CSR went further by assuming a lift of condensate from a lower level, over a geographic dis¬ tance. This was important as con¬ tinued condensation of water vapor is necessary for the production and sustenance of SLW. 2.4 Mixing ratio The mixing ratio is a dimen¬ sionless ratio of the mass of water vapor to the mass of dry air in an air parcel (AMS, 1959). AWS (1980) established that with higher mixing ratios, there was a greater poten¬ tial for icing. Our study set a lower limit of 1 g kg’ 1 as we felt that at concentrations of less than 1 g kg' 1 , the potential hazard of icing to aircraft would be low due to lack of sufficient moisture. 2.5 Minus 8D technique Appleman's (1954) -8D method indicates layers which are super¬ saturated with respect to ice. These values are found by multiply¬ ing the dew-point depression by eight (8). The resultant value is then compared with the ambient tem¬ perature. If the value is warmer than the ambient temperature, the layer is considered supersaturated 93 Session 4.2 and thus, has a greater potential for icing. 2.6 Static Stability As defined by AMS (1959) and shown by Holton (1979), this is the stability of an atmosphere in hydro¬ static equilibrium with vertical displacements, usually considered by the parcel method. The occurrence and rate of vertical motions are highly dependant on thermal and moisture properties of the lower and middle troposphere. 2.7 Dew-point Depression The dew-point depression is the difference between the ambient and dew-point temperatures. Past studies (AWS, 1980) have shown that the potential for icing conditions greatly decreases if the dew-point depression is greater than 6°C. This study used a threshold of 6°C in concurrence with the AWS study and due to the lack of significant moisture at larger depressions. 3. Initial Steps of the Icing Equation Development Initially, several icing cases were studied to find the correlation between meteorological parameters and icing. After deriving correla¬ tions based on icing-only cases, we randomly picked other dates of no icing throughout the data period (1987-90), so we would not be biased by icing-only cases (Table 1). The following parameters were used in the equation development: adiabatic condensate, VFR, CSR, mixing ratio, minus 8D (Appleman, 1954), static stability and dew-point depression. The correlation coefficients calcu¬ lated using each individual param¬ eter are summarized in Table 1. The second column in Table 1 shows the correlation coefficient of each parameter to known icing condi¬ tions from seventy-eight (78) events. The third column shows the correlation of each parameter to a combined data set of both the 78 icing events, and non-icing condi¬ tions, including 100 random non-ic¬ ing days. This secondary data set is the more desirable situation for operational procedures, as it in¬ cludes both types of conditions. Parameter Icing-only Cabined Data Set (Null Set) Adiabatic Condensate -0.25 0.71 Dew-point Depression 0.49 -0.71 Vapor Flux Rate 0.27 0.61 Static Stability -0.58 -0.05 Mixing Ratio -0.07 0.85 Condensate Supply Rate 0.32 0.40 Minus 80 0.08 0.11 Table 1. Caparison of the different parameters and their individual correlation coefficients derived fraa an icing-only data set (Col 2). The last colian shows the correlations using a com¬ bined data set, itfiich included both icing and non¬ icing cases. The icing-only list in Table 1 indicated that even though the high¬ est positive correlation was only 0.49 (dew-point depression), this was still a significant relation¬ ship, given the number of events. Furthermore, the correlation coef¬ ficient for static stability, though negative, had an even more signifi¬ cant relationship with icing inci¬ dence. With the icing-only cases, it showed a high correlation, but when combined with non-icing situa¬ tions, its correlation fell to near zero. We found the minus 8D method performed poorly as shown by the correlation coefficients in Table 1. Given the poor correlation coef¬ ficient and poor results attained from an earlier study in the central United States (Bernhardt, 1989), it 94 Session 4.2 was dropped from further consider¬ ation and is shown only for compari¬ son. As shown in Table 1, when the non-icing data set (called the null set) was included, the results were markedly different, with mixing ratio having the highest individual correlation. This was a predictable result because of the moisture requirement for icing conditions to develop. The decision to use a combined data set was based on the need for an operational predictor. In this study, an operational pre¬ dictor was defined as a technique that successfully predicted cases of either icing or non-icing condi¬ tions. All parameters, with the excep¬ tion of static stability and minus 8D, showed a significant increase in correlation coefficient when com¬ bined with the null set (Table 1). Parameters that had an increase in their individual correlation coef¬ ficients all measured moisture in some manner. Static stability was initially included as a potential icing predictor, as it had a high correlation, but the need for a simple operational predictor and the low correlation coefficient in the combined data set forced us to elim¬ inate it. In addition to the parameters listed in Table 1, others were ana¬ lyzed to determine their correla¬ tions to icing incidence. Tempera¬ tures were correlated against icing, with significant relationships found between the 0°C to -20°C range. Because no one specific temperature range had a higher correlation than another, it was decided to limit temperatures between a 0°C to -20°C range. Furthermore, as noted earli¬ er, prior studies (AWS, 1980) indi¬ cated the potential for significant icing greatly diminishes at tempera¬ tures colder than -20°C. 4. Methodology of the Icing Equa¬ tion Development Since individual parameter values fluctuated widely in the icing and non-icing events, it was necessary to develop a methodology which eliminated subjectivity. All observed icing layers from the ear¬ lier listed samples were categor¬ ized; those less than 150 m thick, 150 to 305 m thick, and those great¬ er than 305 m thick. The null set (non-icing data) used layers of less than 610 m, 610 to 1525 m, and greater than 1525 m. These layers were broader to allow for the thick¬ nesses within the temperature spread of 0°C to -20°C. A "best scenario" was then developed for each set. The "best scenario" used the highest values of the parameters to develop the icing data set and the lowest value was used for the null data set. Depend¬ ing upon the thickness of the layer, as earlier noted, two to four values of each of the parameters were cho¬ sen, evenly spaced in the layer. In cases of icing, the values came from icing layers, whereas in the null data set, the values came from the interval between 0 # C to -20 # C (Table 2). These values were then averaged to determine mean conditions in the layer. The icing-only data set was used for comparison, because the presence of super-cooled liquid water is often a mesoscale phenomena usually occurring in layers a few hundred meters thick. The maximum icing layer thickness during this study was 1220 meters (4000 ft). The most appropriate scheme for 95 Session 4.2 operational use would be that of the null set, since this sample would be more representative of routine atmospheric conditions, which is also why the layers are somewhat thicker. They were then combined into an equation to find how this combina¬ tion could be related into an icing potential equation. The multiple linear regression equation which resulted is: Icing Data Set Null Data Set Lyr Thkness (n>) < 150 150 - 305 > 305 # of values 2 3 4 Lyr Thkns (a) # of values < 610 2 610 - 1525 3 > 1515 4 Table 2. Grouping of layer thicknesses from icing cases and the null data set. The number of values averaged in each layer is listed opposite the thickness of the layer. Approx. English equivalents for the figure values are: 150 m - 500 ft: 305 m • 1000 ft; 610 m - 2000 ft; and 1515 o - 5000 ft. The temperature range of 0°C to -20 8 C introduced a bias to the tem¬ perature correlation by limiting temperatures in the null set to between 0°C and -20°C, while non-icing conditions may occur at any temperature. Due to this problem, temperature was not used in the combined data correlation, but as stated earlier, a limit of 0 # C and -20°C was placed on the output. ICING = * 0.023 * Dew-point Depression (4) + 0.210 * Condensate Supply Rate + 0.006 * Vapor Flux Rate + 0.340 * Mixing Ratio + 0.757 * Adiabatic Condensate. Where ICING is a number giving a predicted icing intensity. The other parameters have been described earlier. As noted earlier, static stability and minus 8D were not incorporated into the final equation. The correlation coef¬ ficient yielded by comparisons of output from Eq. 4 and that of actual icing conditions was 0.90. This indicated good prediction potential between this equation and actual icing incidence. Additionally, we averaged pilot reports of icing intensity and cen¬ tered them around the synoptic hours of 0000 UTC and 1200 UTC to coincide with the rawinsonde information. Averaging of the pilot reports was also performed because of their nature, which have an element of subjectivity and could introduce a bias. These values were later used in the statistical comparison of reported icing to icing equation output. 4.1 Icing Equation All parameters thus far dis¬ cussed were weighted through linear regression to find the best fit of the range in their values to a pre¬ dicted icing value. They were all found to have a relationship to icing potential and intensity through their inherent properties relating to moisture and stability. 5. Procedures and Skill Involved in Applications of the Icing Equation 5.1 Procedures Followed in Forecast Verification Improvements in icing forecast¬ ing during this project evolved with time, experience and technique development. All forecasts issued from 6 November 1989 through 8 March 1990 at Kansas City were verified and scored. Known icing soundings were analyzed to find the success of the icing equation. This amounted to a sample size of 53 soundings over the winter. We then included an analysis of a random sample of non-icing soundings from the same period to determine if the equation over-forecasted icing potential. Finding this not a problem, these 32 random soundings were not included so the data sample would not be 96 Session 4.2 contaminated by non-icing condi¬ tions. Finally, we verified all 144 forecasts issued over the winter to determine the probability of a correct (POC) icing levels and intensity forecast and a false alarm rate (FAR). All icing forecasts were com¬ pared with actual reports of icing based on UND Citation investigations or pilot reports. The intensities and levels of reported icing were compared against the icing equation predictions. Assigning intensity values (Table 3), they were alge¬ braically compared with actual icing conditions. The minimum and maximum altitudes of icing were likewise compared with the icing equation output. Icing Intensity None Trace Light Moderate Severe Category Assigned Value 0 1 2 3 4 over-forecasting problem was par¬ tially rectified by setting indi¬ vidual parameters to zero when they did not meet earlier listed thres¬ hold values. Though the sample size for moderate to severe icing was small, five of the seven cases which occurred were under-forecast by only half of a category. This under¬ forecasting in high rates of accumu¬ lation was found to be a problem in the initial equation development and re-verified in applications of the equation to Kansas City data from 1989-90. Though slight under¬ forecasting occurred, the equation still predicted virtually all cases of icing. In forecasting altitudes of icing, the lower icing altitude was underforecast by 150 m (500 ft), while the top of the icing layer was slightly over-forecast by 35 m (115 ft) (Table 4). We considered both of these errors to be minimal. Icing Minimun Maximum Intensity Altitude Altitude Mean Std Dev Mean Std Dev Mean Std Dev -0.4 0.5 -150 m 700 m + 35 m 810 m Table 3. Listing of icing inten¬ sities from the icing equation out¬ put and the output numerical value assigned to each. The assigned values of icing intensity were arbi¬ trarily chosen to be linear. 5.2 Results of Operational Forecast Verification The icing equation had success in forecasting non-icing conditions but did have some weakness in fore¬ casting high rates of accumulation. We found some under-forecasting in cases of moderate to severe icing. The situations where over¬ forecasting occurred most frequently were with the null data set. The Table 4. Icing equation forecasts compared to verified icing reports. The altitudes are given in meters and give bases and tops of both pre¬ dicted and reported icing levels. The altitudes of predicted icing were determined by interpolat¬ ing the sounding data from raw ob¬ served data and assigning altitudes based on hypsometric equation eval¬ uations. The verified icing layer altitudes were determined primarily by UND Citation II penetrations, and secondarily from pilot reports in the study areas. In evaluating the critical suc¬ cess of our forecasts, we computed a POC and FAR based on procedures fol¬ lowed from 1987-89, then later (1989-90) when the icing equation 97 Session 4.2 was used as part of daily operation¬ al forecasting. The first data set (198788) in Table 5 illustrates the value of subjectivity and experience in the forecast product. In this case, the sounding (objective) meth¬ od incorporated some of the techni¬ ques, such as -8D and AWS proce¬ dures, which were later rejected in this study. The other method (ob¬ jective and subjective) incorporated forecaster experience. Forecast results from Denver during 1988-89 were similar to this and are not presented here. The final data set (1989-90) shows the influence of the icing equation, together with fore¬ caster experience, and a strong re¬ liance on the equation output. Forecast methods POC FAR Sounding (objective only) 1987-88 85.0% 38.0% Objective and subjective 1987-88 97.7% 8.5% Objective and subjective 1989-90 99.3% 0.7% Table 5. The probability of a correct (POC) fore- cast and false alara rates (FAR) for two forecast¬ ing data sets, Denver (1987-88) and Kansas City (1989-90). Though the icing equation was developed using data sets from two different areas, it was possible that the increased success rate (Ta¬ ble 5) was due to the geographic location of Kansas City. Often up-slope situations and other icing producing events moved into and de¬ veloped quickly in the Denver area, whereas Kansas City's upwind condi¬ tions were much more evident with the lack of major upstream topo¬ graphic features to modify the in¬ ward moisture flow. Nonetheless, informal tests using the icing equa¬ tion across the nation during winter 1989-90 showed success in all re¬ gions. These nationwide tests involved the analysis and comparison of soundings from across the United States and Canada to reported icing conditions, using only pilot reports of icing as verification. The same procedures were followed for these tests as were used in the forecast verification, with local sounding information used, and an icing po¬ tential forecast made. This test was also applied on a data set from the northern plains from 1985-87. These tests showed comparable re¬ sults to those of other geographic regions summarized earlier, and thus showed potential as a nationwide icing predictor. Comparing winter 1989-90 icing forecast results to those from Den¬ ver 1987-88 and 1988-89 (Bernhardt, 1989), it became evident that there was a continued improvement in icing forecasts. Less sophisticated CSR and VFR forecast procedures utilized from 1987-89 were improved by the inclusion of significant-level winds and a broader and more gentle up-slope. These improvements brought more confidence and success to the forecasts. This was noted by the significant drop in the FAR and closing of the gap between subjec¬ tive and objective forecasting methods (Table 5). With the success rates of icing forecasting estab¬ lished, the next step was an attempt to forecast liquid water content, since this would more closely repre¬ sent icing intensities. 6. Development of Super-cooled liquid water prediction Encouraged by the results at¬ tained from the forecasting experi¬ ment, we decided to try to forecast supercooled liquid water content. We felt that this would be the ap¬ propriate approach to eventually forecast potential icing loads based on aircraft type. Utilizing a se¬ quence of icing and non-icing data 98 Session 4.2 samples from Denver and Kansas City, we correlated aircraft data to mea¬ sured SLW contents. Initially, seven parameters were included in an icing correlation: 1) temperature, 2) dew-point depression, 3) adiabat¬ ic condensate, 4) CSR, 5) VFR, 6) aircraft-determined vertical velocities, and 7) mixing ratio. Using a multilinear regression approach again, we weighted each parameter and developed a SLW pre¬ diction equation. Results from this equation showed a correlation coef¬ ficient of 0.5 between these and measured SLW (Table 6). It was interesting to note that a higher than 0.5 coefficient was attained using a variety of combinations of the parameters. It was also note¬ worthy to see the role of vertical velocity in producing higher corre¬ lations. TEMP DEWDP ACON CSR VFR W HXRT R X X X X X X X 0.53 0.59 X X X X X X X 0.53 0.64 X X X - X X X 0.53 0.63 X X X X X 0.53 0.64 X X X X X X 0.53 0.63 X X X X - X X 0.53 0.62 X X - X X 0.53 0.64 X - X X X X 0.52 0.60 X X X X X - X 0.40 0.20 X X X X - 0.50 0.66 Table 6. The parameter combinations used to predict liquid water contents with resultant correlation coefiecients. An 'X' indicates which the parameter combinations used to derive the resultant R. Identification of the variables follows: TEMP » Temperature; DEWDP - Dew-point depression; ACON = Adiabatic condensate; CSR = Condensate supply rate; VFR = Vapor flux rate; VV = vertical motion; and MXRT = Mixing ratio. Table 6 summarizes the derived correlation coefficients and the combinations of parameters which were used to calculate them for observed liquid water contents to a predicted value. It does not in¬ clude all combinations attempted, but shows a sample of the results. The top number for each set, reading across, is the correlation coeffi¬ cient resulting from an initial data set involving icing-only cases from the Denver, Colorado area. It showed correlations from 0.50 to 0.53, until vertical motions were removed, then it dropped to near 0.40. Only one case is shown in which vertical motion is removed, but all other iterations of the data samples were similar when vertical motion was removed. This illustrat¬ ed the role vertical motions have on the sustenance of SLW in a cloud. The second set of coefficients, reading across, involved a different data set using both icing and non¬ icing data, including data from dif¬ ferent years and locations. Much high coefficients were attained. With this set, the role of vertical motion was accented, with coef¬ ficients averaging close to 0.60 with most parameter combinations. It dropped to near 0.20 when the vertical motions were removed. In this correlation of SLW prediction, 100 data samples were used in the first set and 105 were used in the second set. Computations of the signifi¬ cance level of the correlation coef¬ ficient for the first data set indi¬ cated that the results were signifi¬ cant. In the case where the corre¬ lation coefficient was 0.40, there was a 0.5% chance (Student's T, Taylor, 1982) that this high of a correlation would be found in data which was uncorrelated with a data 99 Session 4.2 set size of 100. This meant that there was a "significant" relation¬ ship between the measured and pre¬ dicted values of the SLW. This further implied that the highest correlation coefficient (0.53) could be considered 'very significant', with only a 0.007% chance of a cor¬ relation with uncorrelated data. Encouraged by these results, we decided to investigate further using another data set. This data set once again included only aircraft data, but used aircraft data from different years and locations (Denver and Kansas City). Furthermore, this set included re¬ gions with no SLW or icing. Results from this group of 105 samples con¬ cluded with a composite correlation coefficient of predicted liquid water content to measured liquid water of near 0.60. Furthermore, it showed that as temperature and mois¬ ture (dew-point depression and mix¬ ing ratio) parameters were implicit in computations of adiabatic conden¬ sate and CSR, removing these im¬ proved the correlation to slightly greater than 0.60. Additionally, it indicated a significant relationship between the SLW content and adia¬ batic condensate, CSR, VFR and ver¬ tical velocities (Table 6 - last group), with a near zero percent chance of this high a correlation with uncorrelated data. This was not really surprising because of the dynamic relationships between these parameters and SLW production. These relationships include moisture availability, advection and parcel lift. Finding predictive success using only aircraft data, we wanted to find if similar results could be attained with 12-hourly rawinsonde information. The same set of 105 icing-only days was analyzed and verified with Citation investiga¬ tions and pilot reports. This sam¬ ple showed a correlation of 0.73 using the combined data group. When the variables of temperature and dew-point depression were removed, the coefficient dropped to 0.69. The high correlation of this com¬ bined data set was encouraging as it indicated that there was yet poten¬ tially a high probability of pre¬ dicting SLW content using the tech¬ niques described earlier. Some under-forecasting still occurred but was not too severe, as the maximum liquid water contents ranged up to 0.4 g m' 3 while the maximum pre¬ dicted value was slightly higher than 0.3 g m. Most under¬ prediction occurred at the higher liquid water contents. In the final iteration, a data sample of sounding information was analyzed from the Kansas City 1989-90 season to find if a correla¬ tion or predictive value may be determined using both icing and non-icing data-sets with model-derived vertical motions. We found a problem in obtaining reason¬ ably accurate vertical velocity estimates. The National Weather Service's Nested Grid Model (NGM) 700 hPa vertical motion field was used as an initial starting point. Since these velocities are synoptic-scale values, they were about an order of magnitude lower than those measured by the Citation. Nonetheless, we proceeded with this data sample to determine if it was feasible to predict liquid water using rawinsonde data. Using 100 data samples, a cor¬ relation coefficient of 0.50 was attained. Though significant with the number of data samples, the influence of vertical motions brought the coefficient to a point 100 Session 4.2 lower than was expected. Comparing this with the near 0.70 correlation described earlier, it became apparent that more representative vertical motion estimates were inte¬ gral to the predictive capability of the equation. Adjusting the weight¬ ing of each parameter for use with NGM 700 hPa vertical motions, the following equation was formulated: SLU = 0.009 * MXRT + 0.065 * ACON + 0.106 * VFR + 0.003 * CSR + 1.119 * W (5) where SLW is the predicted super¬ cooled liquid water (g m' 3 ), and W is the NGM vertical velocity (micro¬ bars s' 1 , where 1 microbar - 1 cen¬ timeter). The other parameters were defined earlier and in Table 6. This equation yielded a pre¬ dicted SLW content which we then compared to observed (measured) conditions. The correlation coeffi¬ cient derived from this test was 0.68. As noted before, temperature and dew-point depression had little contribution to the outcome, so were dropped from consideration in this formulation. Rather, temperature limits of 0°C to -20°C were set and moisture measured by dew-point de¬ pressions were implicit in the for¬ mula's moisture parameters of CSR, mixing ratio and adiabatic conden¬ sate. Little over-forecasting oc¬ curred with this equation which indicated a greater potential appli¬ cation. 7. Conclusions The statistical procedure fol¬ lowed in developing the icing equa¬ tion demonstrated that there was a statistically significant correla¬ tion between the parameters selected and the risk of icing. Despite the limited size of the data set, we hoped that the technique would be useful in providing forecasters with guidance in predicting layers of potential icing conditions. Further testing and implementation of the icing equation during the Kansas City, Missouri, 1989-90 winter icing program showed continued improvement in icing forecasts using its output. No model output is used in the icing equation, but melding it with models, such as the NGM or the Local Analysis and Prediction System (LAPS) icing algorithm (Rasmussen and Politovich, 1990), could extend a potentially successful icing fore¬ cast past 24 h, depending on the success of the forecast model. A major quality of the equation is that parameters which are important to the production and sustenance of super-cooled liquid water are incor¬ porated into the equation. Advan¬ tages of the equation were the elim¬ ination of subjectivity and use as a predictor by the ability to look at conditions over a broad synoptic scale. By analyzing upstream sound¬ ings and finding their icing poten¬ tial, this gave the forecaster a better idea of icing potential in a forecast area. Further, the equa¬ tion output was helpful as it was objective, efficient and opera¬ tionally easy. In searching for a potential application of the equation to pre¬ dict SLW content, correlations were found between seven different param¬ eters and atmospheric liquid water content. Varying data sets have all shown a correlation, so the poten¬ tial prediction of SLW was pursued. Applying this technique to NWS 12-hourly rawinsonde data, a corre¬ lation of over 0.7 was found. Investigations continued in an attempt to predict liquid water, using readily available thermo¬ dynamic data and vertical velocity 101 Session 4.2 values from NGM output. Initial results were less than desirable, but with adjustments made for lack of good vertical motion values, much better predictive values were attained. We feel that with improved vertical wind computations, which could be derived from profilers, Doppler radar or LAPS (Rasmussen and Politovich, 1990), that the formula's predicted SLW value will more accurately represent the environmental liquid water content. This would therefore serve as a better index by which to guide forecasters through the aircraft icing hazards problem. Acknowledgements. The work on this paper was supported by Federal Avia¬ tion Administration contract DTFA01-87-C-00019. The authors would like to thank R. Rinehart for his generous statistical assistance and M. Evenson and K. Peterson for their programming support. REFERENCES Air Weather Service, 1980: Forecaster's Guide to Aircraft Icing. Air Weather Service Technical Report 80/001 . Scott AFB, IL, 58 pp. American Meteorological Society, 1959: Glossary of Meteorology , Boston, MA, edited by R. E. Huschke. Appleman, H., 1954: Design of a cloud-phase chart. Bull, of the Amer. Meteor. Soc. 35 . Boston, MA, 223-225. Bernhardt, D. W., 1989: Aircraft icing hazards forecasting and synoptic classification. Pre¬ prints, 3rd Inti. Conf. on the Aviation Weather System, Anaheim, CA, Amer. Meteor. Soc., 249-251. Grainger, C. A., D. A. Burrows and M. R. Poellot, 1986: Weather Modification Research and Wea¬ ther Modification Pilot Training . University of North Dakota, Grand Forks, ND. Final Report prepared for Bureau of Reclamation, Division of Atmo¬ spheric Resources Research, Grant No. 4-FC-81-03780, Den¬ ver, CO, Appendix A. Holton, J. R., 1979: An Introduction to Dynamic Meteorology . Academic Press, New York, 49-50. Naval Education and Training Program Development Center, 1974: Aerographer's Mate 1 & C . NAVEDTRA 10362-D. Pensacola, FL, 402-403. Rasmussen R. and M. K. Politovich, 1990: Winter Icing and Storms Project (WISP) Scientific Over¬ view, National Center for Atmospheric Research, Boulder, CO, 45 pp. Rhea, J. 0., 1979: Orographic precipitation model for hydrodynamical use. (Doctoral Thesis), Colorado State Univer¬ sity. Available from Univer¬ sity of Michigan Microfilm Service. Rhea, J. 0., J. L. Lecompte, A. W. Huggins, G. L. Hemmer, A. P. Kuciauskas and C. J. Wilcox, 1982: Volume II Interim Progress Report, Sierra Cooper¬ ative Pilot Project Forecasting Support . Electronic Techniques, Inc., Fort Collins, CO, Interim Progress Report pre¬ pared for Bureau of Reclama- 102 Session 4.2 tion, Division of Atmospheric Resources Research, Contract No. 9-09-85-V0021, Denver, CO, 5-9. Taylor, J. R., 1982: An Introduction to Error Analysis . University Science Books, Mill Valley, CA, 270 pp. 103 Session 4.3 FORECASTING AIRBORNE VOLCANIC ASH IN ALASKA Lee Kelley National Weather Service Forecast Office Anchorage, Alaska MANUSCRIPT WAS NOT AVAILABLE AT THE TIME OF PUBLICATION 104 Session 4.4 A Comparative Study of Pilot's Understanding of Low-Level Wind-Shear Terminology Robert L. Jackson Meteorologist in Charge Center Weather Service Unit, Auburn, Washington 1. Introduction The National Weather Service Forecast Office in Seattle has sponsored numerous aviation weather seminars over the last decade that have been attended by large numbers of pilots. At these seminars, a high level of confusion has been noticed regarding the difference between non-convective Low-Level Wind Shear (LLWS) and microbursts. LLWS related to microbursts has received a great deal of attention from the research and airline communities. This is evidenced by the large number of technical papers, airline training programs, and aviation training films that are available. Nonetheless, this attention has apparently failed in differentiating between non-convective LLWS and microbursts. A survey taken at a Certified Flight Instructor (CFI) clinic sponsored by the Washington State Division of Aeronautics, showed that there was confusion among those questioned about LLWS terminology (Jackson 1991). A continuation of the initial study in an area where convection was more frequent was thought to be more appropriate. The Aviation Division of the Texas Department of Transportation agreed to take the same survey at CFI clinics held in Texas. A Certified Flight Instructor teaches "hands-on" pilot training, and must hold either a Commercial or Air Transport Pilot (ATP) license. A person must attend a CFI Clinic every two years and pass a written test in order to maintain certification as a flight instructor by the Federal Aviation Administration (FAA). The CFI is generally considered among the elite of the general aviation pilots. The term "pilot" in this paper refers to general aviation pilots, i.e. pilots of small aircraft, and does not refer to pilotB of commercial airlines. 2. The Survey Both surveys were taken at CFI clinics sponsored by respective states. The Washington survey was taken in January of 1991 and the Texas survey was taken at four CFI clinics during the summer of 1991. The CFIs were asked to be truthful in their answers and not guess when answering questions. There were 134 respondents from Washington, and 126 from Texas. The list of questions and the wording of each question on the survey was reviewed by personnel from tl training department of Alaska Air. les, SEA-TAC air traffic control tower, and by the Regional Aviation Meteorologist of the National Weather Service Western Region. The following is a list of the questions that were in the survey: 1. What license do you now hold? (A) Private (B) Commercial (C) Air Transport Pilot (ATP) 2. Is there a difference between low- level wind shear (LLWS) and Microburst? 3. You are on approach and are told by the tower that there is a Low-Level Wind-Shear Alert (LLWA) in effect. Would you expect microburst activity in the area? 4 . If you see LLWS mentioned in the Aviation Terminal Forecast (FT), would you expect microburst activity in the area? 5. Does LLWS in the FT and LLWA given out by the tower mean the same thing? 6. Is the recovery procedure the same for LLWS as it is for microbursts? 7. If microburst activity is expected to occur in the terminal area, it will be indicated in the FT as: (A) LLWS (B) TRW+G50 (C) Don't know/Unsure 105 Session 4.4 8. Is a microburst always accompanied by severe turbulence? Except for questions 1 and 7, the multiple choice answers were: (A) Yes (B) No (C) Don't know/Unsure 3. Results of the Survey Fig.1. Survey response to: "What license do you now hold?" Figure 1 shows the percentage of commercial and ATP rated pilots. A previous paper concerning the survey given to CFIs in Washington state compared commercial and ATP rated pilots, and shows no significant difference in responses between the two groups (Jackson 1991). The same result is apparent in the responses from the Texas survey. Therefore, pilots in this paper are grouped by respective states, not by type of license held. Fig. 2. Survey response to: "Is there a difference between LLWS and microburst?" The majority of the pilots knew there was a difference between LLWS and Microbursts, although nearly 20% did not (Fig. 2). Fig. 3. Survey response to: "You are on approach and are told by the tower that there is a LLWA in effect, would you expect microburst activity in the area?" A Low-Level Wind-Shear Alert (LLWA) is issued by Air Traffic Control Tower (ATCT) personnel when pilot reports of LLWS are received. There are no speed or directional criteria listed for identification purposes. An LLWA may also be issued by the ATCT personnel based on information from an anemometer network called "Low Level Wind Shear Alert System (LLWAS), which has been installed at some airports. This system compares the wind speed at outlying anemometers with that at a center point. An LLWA is issued by ATCT personnel when outlying wind differ from the center value by locally determined speeds and directions (FAA, 1990). A "Microburst alert" may be issued if a microburst is identified, but it is not necessary for ATCT to make a distinction between convective and non- convective LLWS when issuing an LLWA. So, an LLWA does not automatically indicate microburst activity in the area. Figure 3 shows that nearly 50% of the pilots believed that an LLWA implied microburst acitvity. Fig. 4 . Survey response to: "If you see LLWS mentioned in the FT, would you expect microburst activity in the area?" Non-convective LLWS can be mentioned in an Aviation Terminal Forecast (FT) while convective LLWS can not. LLWS is included in the FT if there are: pilot reports of wind shear 106 causing airspeed gain or loss of 20 knots or more within 2000 feet of the surface, or vertical shears of 10 knots or more per 100 feet within 2000 feet of the surface, or if meteorological conditions are such that LLWS intensities similar to those just listed can be expected (NWS 1984). Therefore, by definition, LLWS mentioned in an FT does not indicate microburst activity, since microbursts are associated with convection. Figure 4 shows that 77% of both groups were incorrect or did not know this. Fig. 5. Survey response to: "Does LLWS in the FT, and LLWA given out by the tower mean the same thing?" It is possible for LLWS to be included in an FT and an LLWA to be issued by the ATCT for the same phenomenon. But, because of different criteria, it iB also possible for the tower to issue an LLWA without LLWS being mentioned in the FT. Fewer than 40% of the respondents of both states knew that LLWA and LLWS are not interchangeable terms. Fig. 6. Survey response to: "Is the recovery procedure the same for LLWS as it is for microburst?" The proper inflight recovery techniques for a head-on encounter with a microburst and for non-convective LLWS are opposite. Figure 6 shows that more than 60% of the respondents either did not know or answered incorrectly. A head-on encounter with a microburst at normal flight speeds allows precious little time for the pilot to determine the proper response. The wrong choice Session 4.4 may very well make the difference between a rough ride and a mishap. Thus, it is paramount that the flight crew know ahead of time what type of phenomenon they are likely to encounter and also know the proper response. Fig. 7. Survey response to: "If microburst activity is expected to occur in the terminal area, how would it be indicated in the FT?" It has been previously stated that convective LLWS cannot be mentioned in an FT. If the forecaster determines that microburst activity will occur in the terminal area, then the following forecast is suggested by the NWS: "...Occasional TRW+G50" (NWS 1984). Figure 7 shows that more than 80% of the respondents form either state didn't know or were unsure how a microburst was identified in an FT. Fig. 8. Survey response to: "Is a microburst always accompanied by severe turbulence?" The first indication of a head-on encounter with a microburst is most often an increase in head wind, and a resultant increase in the efficiency of the aircraft; turbulence is not always an indication of a microburst (Fujita 1985). Figure 8 shows that fewer than half answered this question correctly. 107 Session 4.4 4. Discussion A majority of the pilots surveyed knew there was a difference between LLWS and microbursts (Fig. 2) but less than one third knew that the recovery procedure was different for each phenomenon (Fig. 6). Less than 20% of all respondents knew how a microburst was identified in an FT. This suggests that when microburst activity is suspected and .IS mentioned in an FT by the prescribed method, most pilots do not interpret this as such. It i6 assumed that all pilots answered question one correctly, so disregarding that question, only 2% of the pilots surveyed answered the remaining seven questions correctly, and 13% got them all wrong. Additionally, more than 90% of the pilots questioned would have failed if the "passing grade" were 70%. 5. Conclusion 6. References FAA, Air Traffic Control Manual , 7110.lOf, 1989, Change 3, 1990. Fujita, T. Theodore, 1985: The Downburst . The Satellite and Mesometeorology Research Project, Dept, of Geophysical Sciences, University of Chicago. Jackson, Robert L., Low-Level Wind-Shear Terminology , Fourth International Conference on Aviation Weather Systems, June 24-28 1991, pp. 13-15, published by American Meteorological Society and French Meteorological Society. National Weather Service, Weather Service Operations Manual Chapter D-21, 1984 . Responses from both states were similar, and indicate that pilots' misunderstanding of mechanical LLWS and microbursts is not confined to one region, nor is it dependent upon the relative amount of convective activity common to that region. Thus, this study suggests a need to improve training programs that now exist and those of the future. CFIs receive recurrent training and testing and are considered among the elite of the general aviation pilots. They could be expected to score better on this questionnaire than the average pilot. Therefore, results obtained from this study are likely to be better than results that would be obtained if the questionnaire was given to non-CFI, general aviation pilots. The user community needs consistency in the use of all terms regarding LLWS phenomena. In this author's opinion, much of the confusion would be eliminated if convective and non-convective LLWS were each given separate terms. For example, LLWS of convective origin, including that associated with microbursts, could be labeled as Convective Low-Level Wind Shear or "CLLWS.* Non-convective LLWS could be labeled as -NCLLWS." Regardless of how these separate phenomena are labeled, there needs to be a distinction made between the two types. Until such steps are taken, there will continue to be a dangerously high level of confusion in the aviation community regarding microbursts and LLWS. 108 Session 4.4 ADDENDUM TO "A Comparative Study of Pilot's Understanding of Low Level Wind-Shear Terminology" Robert L. Jackson Meteorologist in Charge Center Weather Service Unit, Auburn, Washington The survey that was given to Certified Flight Instructors in Washington and Texas was also given to the attendees of the National Weather Service Aviation Workshop, held in Kansas City, Missouri, De¬ cember 10 through 13, 1991. The attendees at the conference were from a broad spectrum of the avia¬ tion meteorology community, and included administrators, forecast¬ ers, pilots, and flight dispatchers. Thirty-seven of the 151 workshop attendees responded to the survey. Responses were collected before the topic was presented to the Workshop. The following is a summary of those responses. In addition, the re¬ sponses of the CFIs of Washington and Texas were combined and included in this addendum for comparison. The responses from the CFIs are labeled below as "CFI." In response to "What license do you now hold?" it was found that less than 10% of the respondents at the workshop were pilots and there was no significant difference in the pilot's answers from those of the remainder of the group. Response to "Is there a differ¬ ence between LLWS and microburst?" Correct Incorrect Don't know Workshop 92% 3% 5% CFIs 76% 7% 17% Response to "You are on ap¬ proach and are told by the tower that there is a LLWA in effect, would you expect microburst activity in the area?" Correct Incorrect Don't know Workshop 38% 38% 24% CFIs 30% 47% 23% Response to "If you see LLWS mentioned in the FT, would you ex¬ pect microburst activity in the ar¬ ea?" Correct Incorrect Don't know Workshop 51% 19% 30% CFIs 23% 48% 31% Response to "Does LLWS in the FT, and LLWA given out by the tower mean the same thing?" Correct Incorrect Don't know Workshop 62% 11% 27% CFIs 38% 17% 45% Response to "Is the recovery procedure the same for LLWS as it is for microburst?" Correct Incorrect Don't know Workshop 49% 16% 35% CFIs 25% 43% 32% Response to "If microburst ac¬ tivity is expected to occur in the terminal area, how would it be indi¬ cated in the FT?" Correct Incorrect Don't know Workshop 43% 11% 46% CFIs 20% 28% 53% 109 Session 4.4 Response to "Is a microburst always accompanied by severe turbu¬ lence?" Correct Incorrect Don't know Workshop 65X 30X 5X CFIs 48X 24X 28X Summary While nearly 80% of the CFI group did not know how microburst activity is identified in the Avia¬ tion Terminal Forecast, it was sur¬ prising to find that nearly 60% of the participants of the Aviation Workshop did not know. Only half of the responding aviation experts at the workshop knew that the term "LLWS" in an FT does not mean that "microbursts are expected." The term LLWS does mean that non-convective LLWS is expect¬ ed. Only 5% of the respondents of the Workshop answered all questions in the survey correctly. Conclusion Surveys taken thus far suggest a common misunderstanding of the terms "LLWS" and "Microburst" among pilots and meteorologists. This study shows that the problem exists, but does not attempt to identify the cause of the problem, nor does it offer a solution to the problem. Nonetheless, it suggests that not only pilots, but aviation meteorolo¬ gists also need training in the proper use of the terms "Microburst" and "LLWS" as applied to aviation meteorology and forecasting. no OBSERVATIONS AND CONCLUSIONS ON NON-FRONTAL, LOW LEVEL TURBULENCE IN THE CENTRAL UNITED STATES Session 4.5 Steven A. Ambnm National Weather Service Office Tulsa, Oklahoma 1. INTRODUCTION Low level turbulence over relatively flat terrain often presents a difficult problem in aviation forecasting. Some studies have related low level turbulence (below 15,000 feet meun sen level) to specific climatologies (Cundy, 1989), or trough and frontal movements (Darrah, 1989). Stanton (1965) developed a nomogram that can be used to predict low level turbulence from wind speed and stability. Through advancements in technology at the National Aviation Weather Advisory Unit (NAWAU) in Kansas City, Missouri, forecasters have been able to make real-time correlations of aircraft reported, low level turbulence with satellite imagery and other data. These correlations confirm the findings of others and further suggest that, in the absence of fronts and troughs, convective thermals are the primary mechanism for producing non-frontal, low level turbulence over the central United States. Three mechanisms which can result in turbulence are: wind shear, convective thermals, and physical obstructions. Of these three, physical obstructions to the flow represent the most simple and effective means of producing aircraft reported turbulence. Wind shear also produces turbulence, but mainly in the vicinity of strong fronts or high velocity wind fields. Convective thermals result in aircraft reported turbulence, but are generally more of a nuisance than a problem. However, evidence indicates that convective thermals act as obstructions to the flow, resulting in turbulent eddies. In light winds, turbulent eddies near thermals are generally weak. Where winds are strong, the resulting eddies are also strong, and can result in widespread outbreaks of moderate or greater turbulence for small aircraft. Forecasting non-frontal, low level turbulence over the central United States then becomes a three part problem. First, the forecaster must identify areas of strong winds aloft. Second, areas where low level insolation will be strong enough to develop convective thermals must be identified. Third, the forecaster must determine if the atmospheric stability will allow thermals to rise into layers of strong winds. Where the areas overlap will then identify the area of expected turbulence. 2. FLUID FLOW WITH OBSTRUCTIONS Fluid dynamics teaches there are two types of flow: laminar and turbulent. One mechanism to disrupt laminar flow is to introduce obstructions to tliat flow which effectively increases the surface roughness aivd produces turbulent eddies. Rigorous discussions of the relationship of wind and surface roughness can be found in many textbooks including Hess (1959). These discussions generally describe roughness parameters related to wind over smooth snow or wheat fields. Larger obstructions, i.e., mountains, produce larger eddies, but that relationship is not as easily documented and formalized. Figure 1 is an excellent example of how large obstructions affect the air flow to create aircraft turbulence. On November 12, 1989, moderate northwesterly winds (25 to 40 knots) were approximately normal to the Appalachian Mountains. Skies were mostly cloudy north of Pennsylvania and generally clear from Pennsylvania southward. The lack of turbulence reports west of the mountains indicates the flow was mostly laminar. East of the mountains, the flow was so turbulent that not all the reports could be plotted clearly. Ill Session 4.5 Since there are virtually no mountain ranges through the central United States, it is realistic to assume that air flow should normally remain laminar. Observations indicate otherwise, which implies there is some other kind of obstruction present in the flow. Recent improvements in technology at the NAWAU have allowed forecasters to make real-time correlations of weather events and weather parameters. Software was developed that decodes and plots pilot reports to map backgrounds for hourly, three-hourly, and six-hourly intervals. These real-time reports could then be assimilated, along with satellite imagery and surface and upper air observations, to help show possible causes of turbulence over flat terrain. It was obvious that most non-frontal low level turbulence was reported during daylight hours when the atmosphere was most unstable. It also became apparent that in the absence of fronts, the turbulence reports tended to come from cloud free areas which were favorable for the development of convective thermals. Further examination showed tliat cloud free areas with weak winds aloft did not result in significant numbers of turbulence reports, even when lapse rates were super-adiabatic. Except for requiring clear skies, this was in agreement with Stanton's Low-level Turbulence Nomogram (1965). It was also in agreement with Byers (1944), who stated that the degree of turbulence depends on the velocity of the wind, the roughness of the surface, the vertical lapse rate, and other lesser factors. Newton and Newton (1959) also showed that strong enough upward motion due to convection distorts the wind pattern around it (see Figure 2) resulting in turbulent eddies. Scorer and Ludlam (1953) contribute to the solution by suggesting that rising convective "bubbles” produce a wake of turbulent air beneath them (see Figure 3). Hess (1959) continued that idea by stating that as bubbles rise, the wake they produce should be carried downstream. Fig. 2. General character of flow around a cir¬ cular cylindrical obstacle. Numbers denote relative hydrodynamic pressure. (From Newton and Newton (1959). It therefore seems reasonable that convective thermals act as obstructions to the air flow in a manner similar to mountainous terrain. The wind must deviate around the thermals resulting in turbulent eddies. These eddies are then carried downwind some distance before dissipating. It can be postulated that the intensity of turbulence is related to the intensity of the thermals and the wind velocity. Also, when low level insolation is reduced by clouds or nightfall, production of convective thermals diminishes or ends, as does non-frontal low level turbulence. Fig. 3. Schematic representation of the assent of a buoyant bubble through a relatively descending environment. Note the turbulent mixed wake. (From Hess (1959)). 3. CASE STUDIES 3.1 Case 1 On 27 October 1990, a strong cold front was moving through the Great Lakes and Mississippi Valley toward Michigan and Indiana Winds were strong both ahead of and behind the cold front with surface winds gusting above 30 knots over northern portions of the area. During the day, low level turbulence reports were quite numerous from the eastern Plains to Indiana and Michigan (see Figure 4). Most of the turbulence was reported after 1400 UTC and was generally below 5000 feet mean sea level. At 1630 UTC (see Figure 5), thick low and middle clouds covered extreme eastern Minnesota and the northwestern two-thirds of Wisconsin extending northward into Canada. This area of clouds moved eastward, clearing Minnesota and moving over all of Wisconsin and Lake Michigan by 1830 UTC. Reported turbulence over southeastern Minnesota generally occurred early in the day and were likely associated with shower activity. Around 1800 UTC (see Figure 6), an area of thick low clouds developed over northwestern Lower Michigan. This area of clouds increased and moved southeastward to cover all of Michigan by 2000 UTC (see Figure 7). Reported turbulence ended over northern Michigan by 1800 UTC and ended over southern Michigan by 1930 UTC. 112 Session 4.5 Fig. 4. Low level turbulence reported from 1200 UTC through 2000 UTC, 27 October 1990. Fig. 5. Visible GOES satellite imagery valid 1631 UTC, 27 October 1990. Fig. 6. Visible GOES satellite imagery valid 1831 UTC, 27 October 1990. Another area of mostly opaque cirrus extended from southern Missouri across southern portions of Illinois and Indiana to central Ohio. These clouds persisted throughout the day; eLsewhere, skies were clear. Figure 4 shows that there were few reports of turbulence over Wisconsin. It can also be seen that there were no reports of low level turbulence from southeastern Missouri to central Ohio. Both of these areas were generally cloudy throughout the day. Most of the turbulence reports came from other areas, where skies were clear. Fig. 7. Visible GOES satellite imagery valid 1931 UTC, 27 October 1990. Lower Michigan presents a particularly interesting event. Reports of turbulence ended wl»ere clouds developed and spread across the state. Even though low level winds remained strong, tliere were virtually no reports of moderate turbulence after 2100 UTC although aircraft continued to operate in the area. Pilots reported generally smooth conditions or made no mention of turbulence. 3.2 Case 2 At 1500 UTC on Noveml^er 9, 1909, a cold front extended from the middle of Lake Superior to southern South Dakota. The front moved southeast during the day and by 0000 UTC extended from southern Lake Michigan to northern Missouri and back northwest to western South Dakota and Montana. Southwest winds ahead of the front gusted between 15 and 20 knots at the surface. Behind tlie front, northwest winds gusted to more tlian 20 knots. Figure 8 shows maiierous reports of turbulence which occurred ahead of the cold ront in clear skies. Note the lack of reported .urbulence over Illinois and the amount of cloud over the state during the day (see Figures 9 and 10). Other areas with a noticeable lack of turbulence reports include Michigan, Wisconsin, and Minnesota which were generally cloudy. Conversely, several reports of turbulence were received over North Dakota and South Dakota which can be easily correlated with the area of clear skies. 113 Session 4.5 Fig. 8. Low level turbulence reported from 1200 UTC through 2000 UTC, 9 November 1989. Fig. 9. Visible GOES satellite imagery valid 1531 UTC, 9 Noveml>er 1989. Fig. 10. Visible GOES satellite imagery valid 1831 UTC, 9 November 1989. 114 3.3 Case _3 On 13 November 1989, a slow moving cold front extended from the northern Great Lakes to the Texas Panhandle. At 1500 UTC, ahead of the front, surface winds were already gusting to more than 15 knots. By 1800 UTC surface gusts of 20 knots or more were comnon from Texas and Oklahoma to Michigan and Ohio. Behind the cold front, winds were considerably lighter. Figure 11 shows all reports of light to moderate or greater turbulence from 1200 UTC to 2000 UTC. Fig. 11. Low level turbulence reported from 1200 UTC through 2000 UTC, 13 November 1989. At 1530 UTC, skies were clear across Missouri (see Figure 12). However, low level mixing developed clouds over the area. By 1830 UTC, stratocumulu8 had developed over much of the state (see Figure 13). After that time, no reports of low level turbulence were received over the cloudy areas. However, moderate reports continued over Illinois and Indiana where skies remained clear. ,Fig. 12. Visible GOES satellite imagery valid 1531 UTC, 13 November 1989. Fig. 13. Visible GOES satellite imagery valid 1831 UTC, 13 November 1989. A. WIND, I,APSE RATE, AND SKY CONDITION Wind, lapse rate, and sky condition are critical to the develo|*uent of significant outbreaks of aircraft reported turbulence. Temperatures aloft must be cool enough to allow steep lapse rates to develop during the day. Sky conditions must be conducive to strong surface heating for the development of strong tliermals. Finally, wind velocities must be sufficient to create significant turbulent flow around the thermals. 0000 Ul'C soundings were examined for the three cases. The soundings indicated tlial lapse rates were nearly dry adiabatic where low level turbulence was reported. The steep lapse rate would allow convective thermals to rise to several thousand feet before reaching equilibrium, thereby creating a deep layer of turbulence. Cloud amounts were subjectively determined using satellite imagery and the locations and times of turbulence reports for October 27, 1990 and November 13, 1989. Using 102 turbulence reports, the average cloud amount was 0.09, or approximately one tenth cloud cover. On the same days, 85 pilots provided the altitude at which turbulence was encountered. A wind velocity was subjectively interpolated both spatially and temporally for each report of turbulence. The average wind velocity at the altitude of the reported turbulence was 32 knots. Finally, equilibrium heights for potential thermals were estimated for sounding stations in each of the three cases presented. Tlie maximum temperature for the day was determined from the 0000 Ul'C sounding. Tliat temperature was plotted on the previous 1200 UTC sounding plot, to determine the inaxijiium possible equilibrium height. Virtually every report of turbulence was below the equilibrium height interpolated for its location. It is important Session 4.5 to mention however, the equilibrium height changed throughout tlie day and couid not be precisely determined at the time of each turbulence report. 5. FORECASTING LOW LEVEL TURBULENCE OVER FIjAT TERRAIN in the absence of fronts or troughs, forecasting low level turbulence involves three e ements. First, it is necessary to forecast sufficient low level insolation (sunshine) for the development of thermals. Second, the stability of the atmosphere should be conducive to the development of strong convective thermals to a considerable altitude, i.e., lapse rates should be steep. Third, it is necessary to forecast the location and strength of the wiixl irom tlie surface to that altitude. intensity of the three elements will affect the overall degree of low level turbulence. There are nunerous National Meteorological Center (NMC) guidance products and rules-of-thumb that will be helpful. 5.1 Sunshine Sunshine must be of sufficient intensity to develop a super-adiabatic lapse rate from the ground to some height. Tlie atmosphere should therefore be clear of clouds, fog, haze, or smoke tliat would prevent such strong surface heating. Forecasting the amount and intensity of sunshine is not always a simple matter. Mean relative humidity forecast data are available from NMC, botli in graphic and alpha-numeric form. These, combined with satellite and surface data, will assist in determining the amount of sunshine. The F0US, F0UM, and M0S forecasts are also helpful. The earlier examples showed that most of the turbulence occurred in areas of clear air. Only 10 of 102 reports of turbulence were determined to be in areas of greater tlian four- tenths cloud cover. This is in agreement with the World Meteorological Organization (WM0) Technical Note Number 158 (1978) that suggests cumulus cloud amounts of four oktas (four eighths) or more cause a significant reduction of insolation and general reduction in the number and intensity of convective thermals. Similarly, tlie National Weather Service (NWS) Handbook Number 3 (1972) suggests that convective thermals are often weak and few in number even when thin, or patchy, middle or high-level clouds are present. 5.2 Stability Ultimately, the stability of the atmosphere determines the height and intensity of convective thermals. A super-adiabatic lapse rate near the ground is necessary to develop a thermal. Once developed, the thermal will rise to its equilibrium height. The NWS Handbook Number 3 states that strength of the thermal is proportional to the 115 Session 4.5 depth of the super-adiabatic layer near the ground. Deeper layers produce the strongest thermals. The maximum height of convective thermals can be found by determining their equilibrium level. This can be estimated by locating the dry adiabat which corresponds to the expected afternoon high temperature. Where that adiabat intersects the morning sounding should indicate the maximum height of the thermals for that day. Occasionally, this intersection will be the result of an inversion. Although Lilts method ignores warm or cold advection, those factors should also be considered. Advection of warm air aloft will stabilize the atmosphere, resulting in weaker thermals, a lower equilibrium level, and earlier cessation of activity as the sun angle decreases. Also, low level cold advection and the accompanying increase in stability will limit the height of thermals or end their development completely. In the three cases presented earlier, soundings were examined and an equilibrium height was determined. Virtually all the reports of turbulence shown in figures A, 8, and 11 occurred below the maximum equilibrium height for those days. Additionally, nearly all the reports of smooth flight (not shown here) occurred above the equilibrium height. 5.3 Wind Speed Forecasting wind speeds and locations is a relatively easy task, given the amount of guidance available from the NMC. Forecasts for winds aloft are available for three, six, and nine thousand feet and higher. Surface and/or boundary layer winds are available at six hourly intervals from a variety of products including the FOUS, FOUM, and MOS. A general rule in forecasting turbulence in mountainous areas suggests that winds at ridge level should be 25 knots or greater. It is conuton practice at the NAWAU to use gradient winds of around 25 knots or higher in forecasting low level turbulence in the Plains. The three examples shown here suggest a wind speed of around 30 knots at flight level, which is not inconsistent with previous findings. However, since the turbulence is proportional to both wind speed and thermal strength, a change in one should be compensated by the other to produce the same degree of turbulence. It is also important to note that winds should increase in speed with altitude. Occasionally, surface winds are stronger than those aloft. When this occurs, convective thermals will quickly transfer the horizontal momenttm vertically resulting in a decrease in the low level wind. Typically, the winds aloft will change little. In this case, significant turbulence will only last a few hours, or until the momentum transfer is complete. 6. CONCLUSIONS Observations of non-frontal, low level turbulence events over relatively flat terrain indicate that the major contributing factor is the development of convective thermals which rise into a layer of moderate or greater winds. Ttje thermals act as obstacles to the laminar flow, resulting in turbulent flow. The degree of turbulence is a function of the strength of the wind field., the atmospheric stability, and the intensity of the convective thermals. Additionally, any factors that will reduce or prohibit tlie develojiment of thermals, e.g., clouds, haze, wet ground, nighttime, will also reduce or prohibit turbulent flow, in favor of laminar flow. The three cases presented showed that where significant turbulence occurred, moderate (around 20 knots) or greater winds were always present through a relatively deep layer. Lapse rates were at or near dry adiabatic, indicating the unstable conditions. In fact, analysis of surface conditions and sounding data showed that super-adiabatic conditions existed during the period in which the turbulence occurred. These super-adiabatic conditions are necessary for the development of convective thermals. The data presented also showed that turbulence diminished or ended with the development of significant cloud layers. The cloud layers reduced surface heating, which ended the production of thermals. In the cases presented, pilot reports of smooth or occasional light turbulence indicated that aircraft did continue flying. It was found that turbulence events can occur on both the warm and cold side of fronts, providing low level insolation is sufficient to produce thermals. Additionally, the height of the turbulence can be found by determining the height to which thermals will rise, i.e., the equilibrium height. This height may also be represented as a stable layer or inversion, above which the flow is generally laminar. In summary, most non-frontal, low level turbulence events may be forecast by finding areas with moderate or greater winds in combination with areas that produce convective thermals. The height of the turbulent layer will vary throughout the day as a function of the equilibrium level with respect to the individual thermals. Conditions not conducive to the development of thermals, such as cloudy skies, wet ground, etc., will generally not result in low level turbulence. Additional research should be done to further define the relationship of wind speed and the strength of thermals to turbulence. However, given that low level turbulence is a function of wind speed and convective thermals, it appears reasonable that NMC model output could be used to generate derived fields of expected low level turbulence over relatively flat terrain. 116 Session 4.5 ACKNOWLEDGEMENTS My appreciation is extended to Doug Mathews, Supervisor of the NAWAU, for his support and encouragement in this study. In addition, my thanks go to Dick Kerr and Bill Carle, at the NAWAU, for their work in developing an automated turbulence plotting procedure that allowed me to easily make correlations of turbulence reports to satellite imagery and other data. Finally, I wish to express my appreciation to Mr. Don Devore and Mrs. Bonnie Jaus for their assistance in this manuscript. REFERENCES Cundy, R.G., 1989: National Aviation Weather Advisory Unit, Kansas City, MO, personal conmunications. Darrah, R.P., 1989: National Aviation Weather Advisory Unit, Kansas City, MO, personal communica t i ons. Hess, S.L., 1959: Introduction to theoretical meteorology , 265-291, 112. Newton, C.W. and H.R. Newton, 1959: Dynamical interactions between large convective clouds and environment with vertical shear, J. Meteor. , 16 , 483-396. Scorer, R.S. and F.H. Ludlam, 1953: Bubble theory of penetrative convection, Quart. J. Roy. Meteor. Soc. , 79 , 317-341. Stanton, T.E., 1965: Low-level turbulence nomogram. (3WW/DN TN-10, 28 Apr 1965). 117 Session 5.1 WEATHER FORECASTS FOR SOARING CONTESTS Dan Gudgel National Weather Service Office Bakersfield, California Larry Burch Meteorological Services Division NWS Western Region Salt Lake City, Utah 1. INTRODUCTION Unlike jets or airplanes, sail¬ planes depend totally on the atmo¬ sphere for lift and power during flight. Sailplane pilots, also known as glider or soaring pilots, often look for lift in meteorologi¬ cal conditions that their power counterparts try to avoid. Sail¬ plane pilots look for lift in ther¬ mals and convergence boundaries, as well as over mountains. Experienced pilots love to soar along the ridges of the nation's mountains, including the Blue Ridge Mountains and Sierra Nevada Mountains. On the extreme end of soaring, pilots fly sail¬ planes in strong mountain-waves over the Sierra Nevada reaching altitudes of 50,000 feet. Sailplane pilots are very aware of micro-scale meteorology. They are hungry for as much weather in¬ formation as they can get. You'll never here a glider pilot say, "Why check the weather, I'm flying any¬ way." Many sailplane pilots can interpret atmospheric sounding, and most are familiar with the various stability indices. However, sail¬ planes are still subject to the same dangers as other aircraft. While an unstable airmass is desireable for lift, an airmass too unstable, with abundant thunderstorms, poses a safety threat to the pilot and sail¬ plane. The cessation of low-level convection due to the over develop¬ ment of clouds often forces the sailplane pilot to land in fields away from the airport. Meteorological support for sailplanes pilots poses interesting challenges for the meteorologist. This paper will describe the meteo¬ rological information that the soar¬ ing pilot needs, along with a dis¬ cussion of the weather support given during the 1991 World Soaring Cham¬ pionships (WSC-91). WSC-91 was selected to illustrate the support required at the highest level of this facet of aviation. 2. WEATHER REQUIREMENTS Weather support for sailplanes range from single pilot weather briefing to specialized support during multi-day contests. Soaring contests usually involve a loop course, consisting of 3 or 4 legs, with the size of the loop being governed by the weather forecast. A typical loop course, also known as the "task", may be 300 miles in length, but can be as long as 500 miles under ideal meteorological conditions. The task is determined on a daily basis by the tasking committee after con¬ sultation with the meteorologist. Soaring contest winners are the pilots who fly the course in the shortest elapsed time. Winners are 118 Session 5.1 awarded 1000 points for the day. Subsequent finishers are assigned points by virtue of the time differ¬ ence between themselves and the winner-of-the-day. During multi-day events the overall winner is the pilot who accumulates the highest total points. Since the glider is dependent upon the atmosphere for flight, various meteorological parameters need to be examined by the meteorol¬ ogist before briefing a soaring pilot or contest tasking committee. These parameters include: (1) Information on thermal develop¬ ment. This information is best represented by plotting thermal altitude as a function of the time of day, and thermal lift rates as a function of the time of day. (2) Trigger-temperature and time- of-day at which thermals begin to support soaring flight. (3) Various atmospheric stability indices such as the 850-to-500 millibar temperature lapse rate, "K" index, thermal index, and a soaring index. The lat¬ ter two indices are those spe¬ cifically established to quan¬ tify expected sailplane climb rates. (4) Freezing levels are important to the pilot since water is carried in the wings for bal¬ last and to achieve faster glide speeds. (5) Winds and temperatures aloft are needed by the pilots to calculate optimum speeds. (6) Sky condition, including the convective cloud base, is nec¬ essary because the race must be flown in visual meteorological conditions, obeying Federal Aviation Regulations concerning visibility and cloud separa¬ tion. (7) General information on the weather-for-the-day must be provided so that pilots are briefed not only for soaring possibilities but also for implications of severe weather. 3. WEATHER SUPPORT FOR THE 1991 WORLD SOARING CHAMPIONSHIPS (WSC-91) During late July and early August 1991, the National Weather Service provided support for the 22nd World Soaring Championships at Uvalde, Texas. WSC-91 was the "Olympics" of soaring with 114 pi¬ lots from 26 countries competing over a two week period. The contest area for WSC-91 was 140 miles by 240 miles, centered 140 miles west- northwest of the Texas coast on the Gulf of Mexico. Uvalde Airport served as the start and finish. The meteorological support team began work at 4:30 a.m. (local time) each day preparing the forecasts for the tasking committee. This weather forecast package, which was used for the contest course selection, was completed by 8:00 a.m. Once the tasking committee was briefed, the meteorologists began preparing for the mass pilot weather briefing, which was presented to all pilots and crew at the daily pilot's meet¬ ing. Figure 1 shows the meteorolog¬ ical information sheet used during WSC-91. Each pilot was given a copy of this sheet prior to the daily pilot's meeting. The meteorological information sheet was displayed on an overhead projector in conjunction with a monitor displaying satellite pictures. Overlays indicting the weather problem-of-the-day were also presented. In the course of these meetings, the contestants and crews 119 Session 5.1 were given information to enable them to make decisions for safety considerations as well as contest strategy. The weather office at WSC-91 was well equipped with meteorologi¬ cal data systems. The meteorolo¬ gists had access to high resolution satellite imagery and overlaying graphics capability from the Nation¬ al Severe Storms Forecast Center. They also had computer access to NWS alphanumeric and graphical data bases, and radar information. A personal computer provided program¬ ming ability to analyze weather plots with applications in soaring and atmospheric instability. Site weather idiosyncrasies must always be considered by the meteorologist at a soaring contest. At WSC-91, morning stratus was com¬ mon and the length of the contest day was determined by when the stra¬ tus would give way to convection. In addition, differential surface heating and resulting pressure gra¬ dients would drive a sea breeze front deep into Southwest Texas. This boundary would then act as a convection focal point varying in intensity from light to severe de¬ pending upon accompanying atmospher¬ ic characteristics. The tasks flown during WSC-91 were usually over 300 miles with the farthest pilot-chosen task being 476 miles. Pilots usually launched by 11:30 a.m. (local time) and flew until 7:30 p.m. Pilots often chose to delay their start until after the thermal convection was well under¬ way, which sometimes was as late as 1:30 p.m. It was a large undertaking to support 114 competing sailplanes. In addition to sailplanes, a fleet of sixteen towplanes were used to launch the gliders. At launch time during WSC-91, the Uvalde Airport became as busy as any major hub airport. 342 take-off and landing operations occurred in less than one-hour as the 16 towplanes launched the 114 gliders. Landing operations were also quite spectacular as many gliders finished together, despite individu¬ ally chosen start times. One of the busier days saw 22 gliders fly through the finish gate and land within a two minute interval. Win¬ ning speeds for the daily tasks reached 90 knots over the tasks of 300 nautical miles. The overall winner of the open class (sailplanes with a wingspan of greater than 15-meters) was Janusz Centka of Poland. His points total was 11,111. The second place finisher was only a mere 10 points behind the winner after 12 contest days! Besides the role that meteorol¬ ogist's have in aiding sailplane pilots in contest task selection, the large number of landing and takeoff operations highlight the need for meteorological warning concerns. Twice during WSC-91, local airport advisories were issued by the National Weather Service support unit due to microburst winds which threatened both grounded and 1anding aircraft. 4. CONCLUSION The components of a successful meteorological support team for soaring contests must have local knowledge, soaring forecast exper¬ tise, data, and personnel manage¬ ment. The management of such a team must be able to understand the needs 120 Session 5.1 of the contest, in addition to mete¬ orological forecast and warning responsibilities. At WSC-91, the team effort within the National Weather Service garnered recognition from interna¬ tional soaring meteorologists as one of the finest seen to date. Through the effort of this team, the Nation¬ al Weather Service was able to serve this category of aviation, thereby enhancing safe flight. 121 SOARING WEATHER FORECAST WORKSHEET ’\v\;.\y.;Xv.;X;X-X;.;Xv..vX\;XyXvXyyX;:XX;X;'xXyX;XvX> WIND TEMP DEG/Kt C° FT/Km 170/14 -53 40/12 180/11 -33 30/9.1 210/10 -18 24/7.3 160/15 -9 18/5.5 150/13 4 12/3.7 140/16 8 10/3.0 150/17 13 08/2.4 140/16 20 06/1.8 160/15 26 04/1.2 160/15 33 02/0.6 Freezing Level 14,000 Ft / 4.3 Km Date AUGUST 4, 1991 Contest Day SEVEN (7) Trigger Temp °F/°C 90°F / 32°C Time (Local) 1230 SFC Winds (Deg/Kt) 160/15 Max Temp °F/°C 98°F / 37°C Time (Local) 1600 - 1730 Max Alt (Ft/Km) 8,500 Ft / 2.6 Km 850-500 Lapse 29°C "K B Index 36 Showalter Index -3 Soaring Index 3.3 M/S "Tr@850 Mb -4 Lifted Index Ft/Km 13/4.0 12/3.7 11/3.4 10/3.0 09/2.7 08/2.4 07/2.1 06/1.8 05/1.5 04/1.2 03/1.0 02/0.6 TIME 11 12 13 14 15 16 17 18 19 20 TIME 11 12 13 14 15 16 17 18 19 20 High pressure over the Northern Gulf of exico combined with low pressure over West Texas/Northem Mexico is providing a south-easterly wind flow over the contest area. Increasing moisture will be noted from this type of flow. Temperatures today will be similar to yesterday with highs in the upper 90s (37 deg C.) and skies sunny in the mornings to early afternoon. Cumulus can be expected at mid-day but also the chance of thunderstorms will be increased. Afternoon cloud bases today will be 7500 to 8000 feet (2500 meters) MSL. The sea breeze will move into the contest area after 1600 hours again but unlike yesterday make better westward progress. With the sea breeze scattered thunderstorm activity is - expected. Satellite pictures and loops provided by WSI CORPORATION 122 Session 5.2 HOT AIR BALLOON PILOT WEATHER BRIEFINGS Walter A. De Voe National Weather Service Office St. Cloud, Minnesota Good morning...we will start with a little balloon history... on Novem¬ ber 21, 1783, two Frenchmen, the Montgolfier brothers, took their balloon to a Parisian park for man's first aerial voyage. A crowd of 400,000, including the King and Queen of France, assembled to wit¬ ness the event. The men set up the balloon, checked the drift of chim¬ ney smoke and the fluttering of flags and decided to launch their aerostat. The Marquis d'Arlands and a Pilatre de Rozier drifted over the city of Paris and landed safely about 30 minutes later. Aviation, manned flight was born. Ballooning became the rage of Eu¬ rope. Clumsy hot air balloons were replaced by gas-filled balloons. Later, with the invention of a light weight and powerful gasoline engine, man achieved steerage and the diri¬ gible came into existence. After several disastrous explosions of the hydrogen filled balloons and the success of heavier-than-air- machines, the sport of ballooning nearly disappeared. Balloon flight was reduced to just a few scientific efforts (they could still fly higher than airplanes) and a handful of pleasure flying aeronauts. In the past 20 years ballooning has made a dramatic comeback. The de¬ velopment of tough and inexpensive nylon envelopes and a burner system that cheaply and efficiently produc¬ es tremendous amounts of heat brought ballooning back in grand style. On November 21, 1983, exactly 200 years after the Montgolfier flight, a pilot with a French-sounding name took his balloon, the Espirit de Saint Cloud to a city park, checked the fluttering of flags and the drift of chimney smoke and decided to launch his aerostat. Two hundred years, and yet a common thread bound the Montgolfier balloon and the spirit of St. Cloud. The final decision to fly was made with simplistic meteorological informa¬ tion based on drifting smoke and fluttering flags. Balloon pilot weather briefings need to be more detailed than what can be determined by the smoke/flag method. When I call flight service and re¬ quest a briefing for a VFR local flight in a Cessna 152, the briefer is able to give me a flood of infor¬ mation. If I call the weather ser¬ vice even more details are avail¬ able. A pilot with an IFR briefing request gets the deluxe version of a briefing. However, when I identify myself as a balloon pilot planning a local flight, I suddenly feel like the Rodney Dangerfield of aviation, getting no respect and little in the way of usable balloon flight infor¬ mation. I believe the difference in the two briefings comes, not from bad intentions, but because the briefer is not familiar with (1) the flight characteristics of a balloon, (2) the regulations governing bal¬ loon flight and (3) the weather needs of a balloonist. 123 Session 5.2 This presentation is intended to present material addressing those three items. First, the balloon flight system. The envelope, the balloon itself, is made of rip-stop nylon. The gores, or up-and-down panels, extend from the throat to the equator and from the equator to the apex of the bal¬ loon in various widths and designs. With some variation the top or the side of the balloon will have pilot controlled lines that run to defla¬ tion or venting ports. Balloon enve¬ lopes come in several sizes up to well over 200,000 cubic feet. A passenger and crew basket, or gondola, is attached to the balloon envelope with aviation cabling. It is a basket when made of wicker and a gondola when made of rigid materi¬ al; most balloonist prefer the tra¬ ditional wicker. The burner system is supported on uprights connected to the basket and by flexible hose to propane fuel tanks which are strapped into the basket. The burner has a pilot light ignition system, a blast val¬ ue, and burner coils. A "burn" creates millions of instantaneous btu to heat the air in the envelope. Instrumentation in the balloon usu¬ ally consists of fuel gages, a com¬ pass, an altimeter, a rate of climb indicator, and a pyrometer to mea¬ sure the temperature at the top of the envelope. A mid-sized balloon system has a gross weight of just under 1500 pounds and depending on the ambient air temperature can safely accommo¬ date a pay load of about 600 pounds. A balloon flight starts by deploying the envelope, attaching the basket, installing the burner system and instrument package and forcing cold air into the balloon with an infla- tor fan. It takes a ground crew of 4, including the pilot, to get set up. Once the envelope is inflated and the system safety checked, the pilot aims the burner into the throat of the balloon and adds enough heat to bring the balloon into an upright position. The ground crew assists the passengers into the basket and everything is ready to "weigh off". The pilot adds heat to reach a pre-determined lift off temperature. The lift off temperature is based on the systems total weight and the outside air temperature. In the wintertime in Minnesota it is possible to fly maximum payloads with envelope tem¬ peratures of 120 to 160°F. However, in the summertime minimum payloads require temperatures of over 200 # F. Most balloon envelopes can be heated to over 250 8 F before damaging the fabric. The envelope temperature will of course also determine the flight time duration. The hotter the required flight temperature, the shorter the flight duration time. It is customary to overheat for takeoff to offset the effects of false lift created by wind flow over the top of the balloon. Once the ascension starts the aerodynamic effect of that horizontal wind flow is lost and a rapid loss of altitude can occur. The ground crew puts their weight on the outside of the basket until the pilot is satisfied that everything is ready. The pilot calls for the ground crew to stand clear and the flight begins. As soon as the bal¬ loon lifts off the flight is at the mercy of the winds. 124 Session 5.2 Once aloft, flying a balloon would seem rather simple; add heat and go up, cool off and come down. It is just a bit more involved than that and like any flying requires con¬ stant pilot concentration to the flight details. Balloon flight is very sensitive to changes in air temperature...to wind shear...di¬ rection or speed...to thermals...es¬ pecially to thermals. I was once in one so violent that I could look up from the basket and see the equator of the envelope. That was panic time! A balloon pilot must antici¬ pate the slightest changes in atmo¬ spheric conditions and be ready to deal with them; in a balloon you cannot do a 180 and fly out of trou¬ ble. Before we go drifting away on an imaginary flight let's set the balloon back down long enough to make ourselves familiar with the FAA regulations that apply to bal¬ loons and balloon pilots. FAA Advisory Circular No. 438 says, "hot air balloons are subject to the same maintenance rules that govern other types of U.S. regis¬ tered aircraft. They operate from self-contained, generated heated air and are considered, by definition, a "1ighter-than-air aircraft". Part 31 of the FAA regulations spells out the airworthiness stan¬ dards that apply to free flying balloons. The regulations require that the balloon system be inspected each 100 hours of flight and annual¬ ly by FAA approved repair stations. Often I get the impression that pilot weather briefers put balloons in the same general category as ultra-lights and hang gliders. Here is a sample of what the regulations say about those systems: (part 103.7a) Ultralight vehicles and their component parts and equipment are not required to meet the airwor¬ thiness certification standards specified for aircraft or to have certificates of airworthiness." In addition to meeting airworthiness and certification standards, bal¬ loons are registered and assigned "n" numbers just as fixed-wing cate¬ gory aircraft. National Weather Service briefers should be recording the balloon "n" numbers in pilot briefing logs (WS Form D-10). Part 61 of the FAA regulations cov¬ ers pilot and flight instructor certification. The part lists types of certificates issued...student, private, and commercial with air¬ craft category ratings in (a) air¬ plane, (b) rotorcraft, (c) glider and (d) lighter-than air. In the 1ighter-than-air category there are class ratings for either airships or free balloons. The hot air balloon pilot is subject to flight tests, log book requirements, flight reviews and proficiency tests and medical certification. Part 61 also spells out the aeronautical experi¬ ence necessary for each type certif¬ icate. Ratings are gained through flight training programs that included both ground school and dual flight in¬ struction with a certified flight instructor. Solo flight practice is also required of the student pilot. An FAA written examination must be passed at the private and commercial rating level. To obtain either rating the pilot must successfully complete an oral exam and flight test given by an FAA flight examin¬ er. At all stages of flight train¬ ing weather is included in the pro¬ gram. 125 Session 5.2 Many other parts of the FAA regula¬ tions apply to balloon flight. Part 91 prescribes general operating and flight rules. This part covers a wide variety of topics from drinking before a flight, to lights on the aircraft, to weather minimums. They all apply equally to a balloon as to any other type aircraft. Fortunate¬ ly, the regulations say nothing about the traditional champagne after a flight. Part 91 directs that "each pilot-in- command shall, before beginning a flight, familiarize himself with all available information concerning that flight. This information must include...weather reports and fore¬ casts..." The balloon pilot is obli¬ gated by regulation, if not by good sense, to get a weather briefing. Some balloonist are setting up their own pibal systems and even computer¬ izing them, but most pilots still rely on the weather service to pro¬ vide surface and winds aloft fore¬ casts. Let me suggest other briefing items that need to be stressed and why they are important to balloon flight safety: (1) Low level inversions: besides the obvious effect on low level winds the temperatures are critical to calculating allow¬ able payload weights. (2) Wind shear, especially speed, which can cause false lift that, when lost, can result in hard or unintended landings. (3) Thermals: encountering a strong thermal can cause col¬ lapsing or distortion of the balloon to the point of crashing from an inability to control the heat needed keep flying. Why do we in the National Weather Service need to concern ourselves? (1) There is a rapidly growing number of users. The sport is affordable and appealing. (2) The National Weather Service Operations Manual requires it. (3) And without a doubt...done with skill pilot weather briefing greatly improves balloon flight safety. 126 Session 5.3 PILOT REPORTS AND AIRCRAFT IDENTIFICATION: THEIR IMPORTANCE IN WEATHER SERVICE OPERATIONS Richard E. Arkell National Weather Service Forecast Office Charleston, West Virginia 1. INTRODUCTION Pilot reports are important sources of weather information. They can help the aviation forecaster produce better forecasts. They can help both the aviation forecaster and the Weather Service specialist give better briefings. And they can also help the public forecaster by giving him information not available from any other source. This paper takes a closer look at pilot reports and how they can be used in daily National Weather Service (NWS) operations. It also provides a reference on aircraft types based on the Federal Aviation Administration’s (FAA) aircraft designators used in pilot reports. 2. DATA SAMPLE The basis for statistical analyses is a sample of 12,500 pilot reports collected randomly over a two year period from December, 1989, to November, 1991, from 15. eastern states and the District of Columbia. The 15 states consist of: Rhode Island, New York, New Jersey, Pennsylvania, Delaware, Maryland, West Virginia, Virginia, Ohio, Indiana, Kentucky, Tennessee, Georgia, South Carolina, and North Carolina. Pilot reports where the aircraft was identified with a non-specific designator, such as JET, UNKN (unknown), CSNA (Cessna), or HELO (helicopter), were excluded from the sample. Duplicate pilot reports were also excluded. 3. THE PILOT REPORT 3.1 Issuance and Dissemination Pilot reports can be initiated by either the pilot or an FAA air traffic control (ATC) facility. The FAA is required to solicit pilot reports when the following conditions are reported or forecast (FAA 1991b): ceilings at or below 5,000 ft; visibility at or below 5 miles; thunderstorms; icing -greater than a trace; turbulence - moderate or greater; or wind shear. The pilot can initiate the report if he is encountering either significant weather or weather that deviates significantly from the forecast. Pilot reports can be transmitted by the pilot on one of several radio frequencies to any one of several FAA facilities including the Flight Service Station (FSS), the Air Route Traffic Control Center (ARTCC), or approach or terminal ATC. The FSS collects and disseminates the pilot reports and rebroadcasts them to other pilots. Some of the frequencies most commonly used are: Flight Watch, the Airport Advisory Service, and the VHF Omnidirectional Range (VOR). The FSS also enters the reports into the FAA computers, which routes them to the switching center in Kansas City where they enter the NWS’s Automation of Field 127 Session 5.3 Operations and Services (AFOS) system. From there they go to the Systems Monitoring and Coordination Center (SMCC) in Silver Spring, MD, where they are reformatted and sent out as pilot report collectives for each state every 20 minutes. The AFOS product format is cccPRCxx, where ccc is the local node and xx is the state abbreviation. During fair weather, around 1,000 pilot reports are received in AFOS every 24 hours. On active weather days the total can exceed 3,000; however, not all pilot reports reach the NWS. The primary reason is that, when the work load at FAA facilities is heavy on a very active day, some pilot reports are not encoded into the FAA computers. When the number of pilot reports is very large, the FSS often combines similar reports from several aircraft and enters them into the computer under a general header such as JETS. In addition, automated meteorological reports are transmitted from many instrumented commercial aircraft using ACARS (Aircraft Communications Addressing and Reporting System). These instrument platforms report a total of 150 to 400 temperature, wind, and pressure observations every 3 hours through the Meteorological Data Collection and Reporting System (MEDCARS) to the National Meteorological Center (NMC). In the future, humidity and turbulence reports may be added to these instrument platforms. 3.2 Applications Pilot reports have several applications. In the FAA, the ARTCC, approach control, and tower use them to expedite the flow of traffic and avoid hazardous weather. The FSS uses them to brief pilots, helping them find favorable routes and altitudes. In the NWS, the aviation forecaster can use pilot reports to verify or amend conditions in aviation forecasts and advisories. Both the aviation forecaster and the Weather Service specialist can use pilot reports in briefing pilots. Of particular importance are reports of low level wind shear, winds aloft, areas of turbulence and icing, and cloud decks not reported in surface observations. Pilot reports can also provide additional information, sometimes unavailable from other sources, to both the aviation and public forecasters. Examples include: pinpointing the location of fronts; comparing winds and temperatures aloft to guidance to see if the models are on track; determining the height of the freezing level, or warm air intrusions, so that the type of precipitation (rain, sleet, freezing rain, or snow), and the height of the snow line at higher elevations, can be determined; using the thickness of a cloud layer or fog to determine the persistence of the feature; determining the height of an inversion, haze layer, or smoke plume; and obtaining severe weather reports of phenomena such as a funnel cloud or downburst. 4. REPORTED FLIGHT AND WEATHER ELEMENTS 4,1 Flight Elements The format of a pilot report is shown in Figure 1. The five flight elements (Figure 1; items 1-5) are disseminating office, location, time, flight level, and aircraft type. The disseminating office is the FSS from which the pilot report originated. The location element gives the direction and distance in degrees and nautical miles from a three-character location identifier, such as an airport or navigational aid (VOR or VORTAC [VOR Tactical Air Navigation]). The location can either be a point or a segment of the route flown. The time element is the time of the report in UTC. The flight level is the altitude in feet MSL at which the aircraft is flying at the time of the report; the weather being reported does 128 Session 5.3 not have to be at this level. The aircraft type element is the FAA designator for the aircraft, which will be discussed in detail in Section 5. 4.2 Weather Elements The seven standard weather elements are sky condition, weather, air temperature, wind velocity, turbulence, icing, and remarks (Figure 1; items 6-12). The sky condition includes cloud bases and tops in hundreds of feet MSL. The weather element includes flight visibility (FV) in miles and obscuring phenomena (e.g., rain or haze). The air temperature element is given in degrees Celsius. The wind velocity element provides the wind speed and direction in degrees and knots. The turbulence element classifies the turbulence in 7 categories: NEG or SMTH for no turbulence; LGT for light turbulence; LGT-MDT for light to moderate turbulence, MDT for moderate turbulence, MDT-SVR for moderate to severe turbulence, SVR for severe turbulence, and EXTREME for extreme turbulence. The term CHOP can be added with light or moderate turbulence. The term CAT should be added for clear air turbulence. The icing element gives aircraft icing in 7 categories: NEG for none; TR or TRACE for trace, LGT for light, LGT-MDT for light to moderate, MDT for moderate, MDT-SVR for moderate to severe, and SVR for severe. The terms CLEAR, RIME, or MIXED are often added to describe the type of icing. The remarks element is for reporting low level wind shear (LLWS) and other meteorological phenomena that are not adequately described in the other elements. The normal method for reporting LLWS is in knots gained or lost (e.g., +/-10). Criteria for turbulence and icing intensities are given in the "Airman’s Information Manual," Sections 7-20 and 7-21 (FAA 1991b) and in the Weather Service Operations Manual (WSOM) Chapter D-22 (NWS 1991). Criteria for LLWS are given in WSOM Chapter D-21 (NWS 1988). Routine pilot reports are designated UA and urgent reports UUA. The threshold for urgent criteria varies to some degree depending upon the reporting FSS, the circumstances, the time of year, the altitude, and the location. In general, it includes turbulence that is moderate to severe or greater, icing that is moderate to severe or greater, LLWS, or any other meteorological phenomenon considered significantly hazardous, such as a tornado or hail. Sometimes the threshold used for icing is lowered to moderate if the condition is unusual for that time, place, and altitude. The pilot report is not required to contain all weather elements, but rather only those considered relevant or significant. The average number of elements reported, based on the sample used in this study, was 2.04 per report. The frequencies of the individual elements are given in Table 1. Turbulence and sky condition were the most common, and weather and wind velocity the least. Urgent pilot reports accounted for 9% of all reports. On days with bad weather this percentage went as high as 25%, and on fair weather days down to zero. Turbulence was the critical element to make the report urgent 47% of the time, LLWS 39%, icing 17%, and weather (such as freezing rain, hail, or funnel cloud) 3%. Note that the total is slightly more than 100% because there were, occasionally, multiple critical elements. LLWS is an unusual parameter in that it is sometimes reported in the turbulence section instead of in the remarks section, as is the customary practice. The primary reason for this variation of format is the interrelated nature of LLWS and turbulence. What one aircraft may report as LLWS on 129 Session 5.3 take-off or landing, another aircraft may report as turbulence. In most cases, the intensity of turbulence, if reported, goes in the turbulence element, and any report described as LLWS goes in the remarks. 4.3 Abbreviations and Contractions Commonly Used Most of the language and syntax in a pilot report is the same or similar to that used by the NWS. Table 2 lists some commonly used abbreviations and contractions found in pilot reports that are not commonly used by the NWS. 4.4 Frequency of Pilot Reports bv Altitude The frequency of pilot reports by altitude was computed. Most reports, 63.4% of the total, were filed below 10,000 ft MSL. Of these, an approximately equal number pilot reports were filed at altitudes below 5,000 ft MSL and between 5,000 at 9,900 ft MSL, with 29.8% and 33.6%, respectively. Between 10,000 to 19,900 ft, there were 18.6%; between 20,000 and 29,900 ft, 7.4%; and at and above 30,000 ft, 10.6%. A detailed breakdown is provided in Table 3. The large number of reports coming from lower altitudes reflects two factors. First is the large number of single and twin-engine private and small business aircraft. Aircraft equipped only for Visual Flight Rules (VFR) without pressurization or oxygen equipment are limited to altitudes of less than 12,500 ft MSL except for short periods of less than 30 minutes at altitudes of up to 14,000 ft. With oxygen, VFR aircraft can go up to 17,500 ft. Only IFR (Instrument Flight Rules) equipped aircraft with pressurization or oxygen equipment can fly at 18,000 ft or higher. The second reason for the large number of reports at lower altitudes is the active weather regime, with sharp contrasts and terrain influences, that generally exist in the lowest 10,000 ft of the atmosphere. Cruising altitudes are usually greater than 1,000 ft AGL, except for special purposes such as crop dusting. Therefore, most pilot reports filed near the surface are departures and approaches. 5. AIRCRAFT TYPES 5.1 Common Aircraft Types and Specifications To best interpret a pilot report or to give the most effective briefing, it is important to understand the type of aircraft involved. Is it single or multi-engine? Is it equipped for IFR? What is its weight and size? What is its cruising speed and ceiling? The answers to these questions will determine how to interpret weather conditions with respect to that aircraft. For example, a single-engine Cessna reporting moderate turbulence is not usually experiencing the same conditions as a Boeing 727 reporting moderate turbulence. It is not possible or necessary for the NWS aviation forecaster or briefer to know all the details and specifications of common aircraft. However, he or she can gain some knowledge on the subject in two ways. The first is through familiarization with the kinds of aircraft most common to their region. The second is through access to easy-to-use references. The FAA provides several excellent sources of information for aircraft identification. Among these are the "Aircraft Type Designator" tables (FAA 1991c and 1991d) which list 579 two to four character designators for 547 types of aircraft. (There is some duplication due to civilian and military variants.) A larger list of designators is provided by the International Civil Aviation Organization (ICAO 1991). For each aircraft type in the FAA publications, the following information is given: type and number of engines; general weight class (small, large, and 130 Session 5.3 heavy); climb rate; descent rate; SOIR (simultaneous operations on intersecting runways) group; and SRS (same runway separation) category. These specifications are designed primarily to help the air traffic controller. The FAA also publishes the "Controller Reference Aircraft Manual" (FAA 1991a), which shows color photographs and silhouettes for over 200 civilian and military aircraft, along with an expanded list of specifications. This is an excellent reference and can be a very good familiarization tool for the aviation forecaster and Weather Service specialist. An additional reference has been developed as part of this paper for NWS use. "Quick Reference - Frequent Aircraft Types and Specifications," based on "Jane’s All the World’s Aircraft" (Taylor 1982-1991), is presented in Table 4. This reference lists the 46 most common types of aircraft found in the data sample used for this report. 5.2 Most Frequent Types The most common fixed-wing powered aircraft types were divided into five general categories to examine the frequency of different aircraft types filing pilot reports. The categories are: 1) private, sport, light business, and light cargo (single prop) 2) private, business, light commuter, cargo (twin prop; < 15,000 lbs) 3) business, commuter, and cargo (twin prop; > 15,000 lbs) 4) executive jet 5) airliners and large cargo The most common types were, as expected, the smaller aircraft; categories 1 and 2 filed a combined 66.2% of the reports, 36.9% and 29.3% respectively. Airliners were next with 14.4%. Then came executive jets with 9.9%, and larger business/commuter aircraft with 9.5%. Eight individual types filed 3% or more of the reports. They are: the Piper Cherokee, Cessna 172, Beechcraft King Air, Boeing 727, Beechcraft Bonanza, Beechcraft Super King Air, Beechcraft Baron, and Piper Navajo/Mojave/Chieftain. All together, 41 types with 1.0% or more accounted for 85.3% of the reports. A detailed summary is given in Table 5. These figures show that all sectors of the aviation community contribute to the pilot report process. They also show that although there are over one thousand types of aircraft flying in the skies today, knowing something about a relatively small number of them can provide a good foundation for the aviation forecaster and briefer. 6. SOME EXAMPLES OF PILOT REPORTS Table 6 shows some examples of pilot reports taken primarily from the 12,500 pilot reports in the data base for the period December, 1989, to November, 1991. A few examples are from outside the data sample but are still within the same two year period in the same 14 states. These pilot reports illustrate both the impact of weather on aircraft, and the sense of humor it sometimes takes to be a pilot. 7. SUMMARY AND CONCLUSIONS The primary purposes of this paper were to highlight the importance of pilot reports in both aviation and public forecasting, and to provide an aircraft reference that can be used as a familiarization and briefing tool. Towards this second goal, Table 4 provides a quick reference of common aircraft types. This table can be used in conjunction with the FAA’s pictorial glossary "Controller Reference Aircraft 131 Session 5.3 Manual" (FAA 1991a), which provides color photographs for many of the aircraft listed in Table 4. While the pilot reports used in this paper were from only 15 states, the examples and references are applicable in a more general sense to the whole country. An appreciable portion of the NWS mission is related to the service of aviation. Aviation in turn has given the NWS a valuable service in return, the pilot report. References: FAA, 1991a: Controller Reference Aircraft Manual . ATG-2. Federal Aviation Administration, Oklahoma City, OK, April, 1991. FAA, 1991b: Airman’s Information Manual . Federal Aviation Administration, Washington, D.C., July 25, 1991. FAA, 1991c: Contractions . Order 7340.1L CHG 4. Federal Aviation Administration, Washington, D.C., October 1, 1991. FAA, 1991d: Air Traffic Control . Order 7110.65 CHG 7. Federal Aviation Administration, Washington, D.C., November 14, 1991. ICAO, 1991: Aircraft Type Designators . International Civil Aviation Organization, Montreal, Canada. NWS, 1988: Weather Service Operations Manual. Chapter D-21 . National Weather Service, March 11, 1988, 23 pp. NWS, 1991: Weather Service Operations Manual. Chapter D-22 . National Weather Service, May 22, 1991, 44 pp. Taylor, J., 1982-1991: Jane’s All the World’s Aircraft . London, England, 800pp. Frequency Critical element Weather Element All UA UUA triaaerincr UUA Sky (SK) .40 .43 . 08 0% Weather (WX) . 11 . 11 . 06 3% Air temp (TA) .22 .25 . 02 0% Wind Vel (WV) . 11 .13 0 0% Turbulence (TB) .55 .54 .64 51% Icing (IC) .33 .34 .18 17% Remarks fRM) .32 .30 .50 36% fmostlv LLWS1 Total Elements 2.04 2.09 1.48 107% Per Report includes 4% LLWS reported in TB Note: 91% of total were UA and 9% were UUA. Table 1. Frequency of weather elements in pilot reports. 132 Session 5.3 A/S (or AIR SPD) - air speed * APCH - approach BL (or BTWN LYRS) - between layers CA - clear above * CAT - clear air turbulence CAVU - clear and visibility > 10 DEPG - departing DPTG - departing * DWNDFT- downdraft DURC - during climb DURD - during descent ENRT - enroute EBND - east bound FA - final approach FAP - final approach * FRZLVl- freezing level FZL - freezing level * FV - forward visibility GS (or GND SPO) - ground speed HDUND - headwind HP (or HLDG PAT) - holding pattern I AO - in and out of (clouds) IAOC - in and out of clouds * IAS - indicated air speed IBND - inbound ICGIC - icing in clouds ILS - instrument landing system IMC - instrument met. conditions INC - in clouds INVOF - in vicinity of I0VC - in overcast MTW (or MTN UV) - mountain wave MXD - mixed (icing) NBND * north bound OBND - outbound OG - on ground OMTNS - over mountains OTAS - on top and smooth OTP - on top PUP - pickup (ice) RWY - runway RY - runway SBND - south bound SMTH - smooth TFC PAT-traffic pattern TKOF - take-off TLWD - tailwind TURBC - turbulence UDDF - up and down drafts UPDFT - updraft UNRSTD- unrestricted WBND - west bound * common usage but not listed in "Contractions" (FAA 1991c) Table 2. Some common abbreviations and contractions used in pilot reports. Flight Level Percentage of reports ( feet. MSP Total Subtotal surface- 4,900 29.8 surf - 1,900* 7.6 2,000 - 2,900 6.9 3,000 - 3,900 8.1 4,000 - 4,900 7.2 5,000 - 9,900 33.6 5,000 - 5,900 7.2 6,000 - 6,900 9.5 7,000 - 7,900 6.6 8,000 - 8,900 5.6 9,000 - 9,900 4.7 10,000 - 14,900 11.8 15,000 - 19,900 6.8 20,000 - 24,900 4.0 25,000 - 29,900 3.4 30,000 - 34,900 3.7 35,000 - 39,000 5.7 40.000 - 44.900 1.2 Total 100.0 Includes take-off. landing. and on ground Table 3. Frequency of pilot reports by altitude 133 Session 5.3 i FAA Designator Manufacturer Model Type Max Takeoff (lbs) Max Cruise (mph) Service Ceiling 100s ft Typical Passeng Engine Config P=prop J=iet AC69,6T Rockwell Turbo Commander, Jet Prop business 10,500 300-340 32 8-11 2P B727 Boeing 727 large airliner 200,000 600 37 160 3J B737 Boeing 737 medium airliner 115-150,000 560 37 110-150 2J B757 Boeing 757 large airliner 240,000 570 40 180 2J BA31 British Aerospac Jetstream commuter, business 15.000 300 25 10-20 2P BE02 Beechcraft 1900 commuter 16,500 305 25 21 2P BE10,90; U21F Beechcraft King Air; Ute business 11,500 270 25 8-15 2P BE20,30; C12 Beechcraft Super King Air; Huron business, commuter 13,500 350 35 8-15 2P BE33,35,36 Beechcraft Bonanza private 3,400 185-225 17-25 4-6 IP BE55.58 Beechcraft Baron priv. It business 5.200 200-230 20 6 2P BE90 Beechcraft King Air 90 business 10,000 255-285 27 6-10 2P BE99 Beechcraft Airliner commuter 10,900 280 27 17 2P C150,152 Cessna 150, 152 private, trainer 1,600 120 14 2 IP Cl 72 Cessna 172/Skyhawk private, trainer 2,400 135 13 4 IP C182 Cessna 182/Skvlane. Skvlane RG private 3.100 160-190 15-20 4 IP C208 Cessna 208/Caravan utility 7,300 210 27 2-10 IP C210 Cessna 210/Centurion light business 3,900 190-235 17-27 6 IP C310.320 Cessna 310,320; Skyknight business 5,500 225-255 19-28 5 2P C401,402,414 Cessna 401,402,414,Chancel lor business, commuter 6,300 245 26-31 8 2P C421 Cessna 421. Golden Eaqle business 7.400 270 30 8 2P C500,501 Cessna Citation I executive jet 11,600 400 41 7-9 2J C550,551 Cessna Citation II executive jet 12,600 440 43 8-12 2J DA20 Dassault-Breguet Falcon 20,200,FJF executive jet 29,000 530 42 10-16 2J DC9 McDonnell Dougl DC-9 (Series 10-50) medium airliner 92-120,000 570 35 100-120 2J FK10 Fokker 100 small airliner 98.000 535 35 112 2J FK27, FA27 Fokker/FairchiId F27 Friendship feederliner 45,000 295 29 47 2P FK50 Fokker 50 feeder liner 45,000 330 25 52 2P G2,3,4; C20 Gulfstream Gulfstream 2,3,4 executive jet 66,000 570 45 13-22 2J HS25 British Aerospac HS-125 executive jet 27,000 510 42 10-16 2J LR23.24 Gates Leariet Leariet 23.24 executive iet 13.500 525 45 8-10 2J LR25,28,31 Gates Learjet Learjet 25,28,31 executive jet 15,500 520 45 10 2J LR35,36 Gates Learjet Learjet 35,36 executive jet 18,000 525 45 10 2J MD80 through 88 McDonnell Dougl MD-80 through 88 mediun airliner 140-160,000 575 37 130-160 2J MO20,21 Mooney Mark 20,21,Ranger,M20 private 2,600 170 17 4 IP MU2 Mitsubishi Marauise. Solitaire business 10-11.500 350 30 9-12 2P PA28 Pi per Cherokee,Archer,Cadet private, trainer 2,400 140 11-14 2-4 IP Arrow II, III; Dakota private 2,800 165-195 16-20 4 IP PA31 Pi per Navajo,Chieftain, business, commuter 7,500 225-265 16-29 7-10 2P Mojave, T-1020 PA32 Pi per Cherokee 6.Lance,Saratoqa private 3.600 165-200 14-20 6 IP PA60, TS60 Piper/Ted Smith Aerostar 600, 700 business 6,000 250-310 22-30 6 2P PARO Piper Cherokee Arrow IV private 3,000 195-225 16-20 4 IP PASE, PA34 Piper Seneca II, III light business 4,500 190-220 19-25 6-7 2P PAYE Piper Cheyenne I, II business 9,200 310 21-25 6-8 2P PAZT Pi per Aztec liqht business 5.200 210-245 21-30 6 2P SF34 Saab/Fairchi Id SF-340 feederliner 27,000 315 25 39 2P SW3,4 FairchiId/Swear Metro, Merlin 4 business, commuter 15,000 320 27 12-22 2P WW23.24; AC21 IAI/Rockwell Westwind, Jet Commander executive jet 22,500 520 45 12 2J Note: Specifications averaged, weighted by approximate nunber of each variant in service. Table 4. Quick reference - Frequent aircraft types and specifications. 134 Session 5.3 1) Private, Sport, Light Business, and Light Cargo (single prop) Piper Cherokee 6.0 % PA28 Cessna 172 5.7 Cl 72 Beechcraft Bonanza 3.7 BE33,35,36 Cessna 150,152 2.9 C150,152 Cessna 182/Skylane 2.2 C182 Mooney 20, 21 2.1 M020.21 Cessna 210/Centurion 1.9 C210 Piper Cherokee Six/Saratoga 1.6 PA32 Cessna 208/Caravan 1.5 C208 Piper Seneca II, III 1.4 PASE, PA34 Piper Cherokee Arrow IV 1.0 PARO Other 6.9 Total 36.9% 2) Private, Business, Light Comnuter, Cargo (twin prop less than 15,000 lbs) Beechcraft King Air 4.0% BE 10,90; U21F Beechcraft Super King Air 3.3 BE20,30; C12 Beechcraft Baron, Cochise 3.3 BE55.58 Piper Navaj o/Chieftain/Moj ave/T-1020 3.2 PA31 Cessna 310/320/Skyknight 2.9 C310,320 Piper Cheyenne I, II 1.4 PAYE Beechcraft Airliner 1.3 BE99 Cessna 401,402,414, Chancellor 1.2 C401,402,414 Cessna 421, Golden Eagle 1.2 C421 Piper Aerostar 1.1 PA60, TS60 Mitsubishi Marquise, Solitaire 1.0 MU 2 Rockwell/Gulfstream 1.0 AC69, AC6T Turbo Commander, Jet Prop Piper Aztec 1.0 PAZT Other 3.4 Total 29.3% 3) Business, Comnuter, and Cargo (twin prop; 15,000 lbs or more) FairchiId/Swearinger Metro/Merlin 4 2.8% SW3,4 Beechcraft 1900 1.6 BE02 BAC Jetstream (prop) 1.2 BA31, HP31 Fokker/FairchiId Friendship F27 1.1 FK27, FA27 SAAB/Fairchild SF-340 1.0 SF34 Other 1.8 Total 9.5% 4) Executive Jet Cessna Citation 1, 2 2.9% C500,501,550 Lear (24 through 36) 1.8 LR23,24,25,28,31,35,36 BAC HS-125 1.3 HS25 Grumman Gulfstream 2, 3, 4 1.0 G2,3,4; C20 Dassault-Breguet Falcon 20, 200; 1.0 DA20 HU-25A; FJF IAI Westwind/Rockwell Jet Commander 1.0 WW23,24; AC21 Other 0.9 Total 9.9% 5) Airliners and Large Cargo Boeing 727 4.0% B727 McDonnell Douglas DC-9 (Series 10-50) 2.8 DC9 Boeing 737 2.5 B737 McDonnell Douglas MD-80 through 88 1.4 MD80 through 88 Boeing 757 1.0 B757 Fokker 50 1.0 F05 Other 1.7 Total 14.4% Table 5. Most frequent types of aircraft filing pilot reports (1.0% or more). 135 Session 5.3 Turbulence 02/13/90 MKL UUA /0V MKL180007/TM 1345/FL045/TP AA5/TB MDT CHOP/RM VERY ROUGH BELOW 20 HEAD HIT TOP OF CABIN COUPLE OF TIMES 02/16/90 BUF UUA /OV BUF270040/TM 2040/FL270/TP BE20/TB SVR/RM COULD NOT MAINTAIN ALT WITH FULL POWER 04/05/90 MRB UUA /OV HGR/TM 2039/FL025/TP HELO/TB SVR/RM OVR RIDGES 04/10/90 LEX UUA /OV AZQ-FLM 180020/TM 1820/FL040/TP C210/TB SVR/RM ONE PILOT/ ONE VERY SCARED PASSENGER 09/30/90 PKB UUA /OV PKB-ZZV/TM 1640/FL040/TP PARO/WX R+/TB MDT-SVR/RM PILOT LOST HAT 2 OR 3 TIMES 02/04/91 LAF UUA /OV DNV 180010/TM 1930/FL270/TP B727/TB SVR CAT 02/20/91 ACY UUA /OV CYN 270010/TM 1855/FL120/TP B727/TB EXTRM (Only extreme report in data base) Mountain Waves 04/04/90 CHO UUA /OV MOL/TM 1547/FL100/TP PA32/WV 330052/RM MOUNTAIN WAVE IN PLACE/ HARD TO MAINTAIN ALT 02/20/91 EKN UUA /OV ESL-EKN/TM 2052/FL060/TP PA31/TB MDT OCNLY SVR/RM VERY STRONG MOUNTAIN WAVES 09/27/91 CHO UUA /OV 30N CHO/TM 1155/FL065/TP C172/RM FULL POWER TO MAINTAIN ALT IN MOUNTAIN WAVE W OF RIDGE/ NO PROBLEM E/SMTH AIR WHOLE TIME Updrafts and Downdrafts 07/01/90 CRW UUA /OV ECB225020/TM 2017/FL040/TP BH06 BELL RANGER/TB SVR/RM MICRO DOWN BURST LOST 500 FEET 08/19/90 EKN UUA /OV EKN090025/TM 0205/FL065/TP C182/RM SVR DWNDFTS LOST 800 FT WITH FULL THROTTLE COULD NOT MAINTAIN ALT 06/16/91 CHA UUA /OV RMG360010/TM 1755/FL035/TP PA28/TB MDT-SVR/RM 1500 FT PER MIN UDDF 07/26/91 TRI UUA /OV TRI/TM 2018/FLUNKN/TP HP13/RM/RM FINAL RWY 5 1400 FT PER MIN UPDFT Thunderstorms 02/15/90 MEM UUA /OV GQE/TM 2135/FL120-180/TP B727/TB MDT-SVR/RM IN WX PILOT RCMDS NOT PENETRATING THIS LINE OF TSTMS 04/10/90 RMG UA /OV CSG-RMG/TM 2105/FL000/TP BE58/RM PILOT LANDED AT RMG JUST AHEAD OF THE SQUALL LINE. GLAD TO BE ON THE GROUND 06/16/91 CSV UA /OV CSV 090020-CHA/TM 1805/FL075/TP BE35/RM LN CB'S BUILDING DIVERTED CHA-WEST AROUND 07/26/91 TYS UA /OV TYS 360050/TM 1643/FL080/TP C206/RM TCU TOPS E200 BLDG RPDLY 09/14/91 CRW UUA /OV CRW 030050/TM 0015/FL310-370/TP JETS/TB SMTH OUTSIDE TSTM/RM TWO AIRCRAFT RPTD WORST TSTM THEY EVER SAW Lou Level Wind Shear 03/30/90 HLG UUA /OV HLG/TM 04/10/90 LOZ UUA /OV K20/TM 02/14/91 LYH UUA /OV LYH/TM TURB FAP 21 1655/FLUKN/TP PA28/RM LLWS LOST 30 KTSAND 100 FT DURG SHRT FA RY21 1903/TP BE58/TB SVR 020-SFC/RM LLWS -30KTS TO +15KTS 1831/FL003/TP N265/TB LLWS +40/-10KTS/RM DH8 & BA31 ACFT RPT LLWS +20/-10 KTS AND MDT Winds Aloft 02/16/90 CAK UA /OV ACO/TM 02/27/90 MRB UA /OV MRB/TM 04/05/90 HTS UA /OV HTS/TM 04/28/91 SBY UA /OV SBY/TM 2231/FL350/TP C550/TA -42/WV 242175/TB SMTH 1605/FL060/TP BE55/IC MDT RIME/RM HEADWIND 52 KTS 1735/FL160/TP UNKN/WV 2800090/RM WNDS STRGR THAN FD 1122/FL090/TP M020/RM WINDS MORE WSTRLY THAN FCST Icing 02/15/90 CMH UUA /OV APE 180012/TM 2345/FL210/TP BE20/IC SVR/RM 10 KTS LOSS A/S 02/16/90 ERI UUA /OV ERI 300030/TM 2035/FL160/TP BE10/IC SVR RIME/RM PUP 3/4 INCH IN 1 MIN 10/11/91 MFD UUA /OV TVT/TM 1751/FL058/TP M020/TA 0/IC MDT-SVR MXD 3 060 LOST ICE 3 058 12/29/91 MGW UUA /OV MGW 338006/TM 2345/FLUNKN/TP PA28/IC SVR/RM DURGD MGW PILOT DECLARED EMERGENCY Haze. Smoke 06/15/91 EWN UA /OV EWN360060/TM 1230/FL085/TP BE35/RM 085 HAZE TOP 11/01/91 GSO UA /OV GS0075014/TM 1439/FLUNKN/TPBE36/SK K LYR 045-065/WX VSBYS 2-3K 045-065/TA 12/RM DURC GSO/ VSBY UNRSTD ABV 065 11/01/91 BKW UA /OV BKW/TM 1439/FL080/TP C310/RM ACTIVE FIRE 15 SW BKW FV 2-6K TOPS E070 Table 6. interesting and illustrative examples of pilot reports. 136 Session 5.3 Tenperatures Aloft. Freezing Level 02/27/90 AGC UA /OV AGC/TM 1402/FL125/TP BE90/SK 030 OVC 115 CA/TA -05/TB NEG/IC NEG/RM TEMP ABV FRZG 030-090 DURC EBND 02/18/91 AVP UA /OV LHY 270029/TM 1936/FL150/TP SU3/WX BTWN LYRS/TB SMTH/IC TRACE/RM INCLDS DURGC MULTI LYRS/ TEMP INVERSION 053-070 10/30/91 SBN UA /OV ARR-GIJ/TM 1232/FLDURC/TP BE20/SK OVC 260-270/WX R-/TA FRZLVL 160/TB SMTH SFC-270/IC NEG Conditions less than VFR 06/17/91 SHD UA /OV SHD-PHF/TM 1237/FL030/TP C172/RM UNABLE VFR MTNS OBSCRD 10/13/91 ABE UA /OV ETX/TM 1259/FL035/TP C172/SK RTNG ABE DUE VX DETRNG 10/14/91 ELM UA /OV ELM/TM 1225/FLDURD/TP C150/SK OVC 15/RM MISSED APCH IN FOG 11/05/91 CRW UUA /OV CRW/TM 1633/FL015/TP BH06/WX FV 1/4K/RM CRW-HTS RTNG CRW Severe Weather 07/02/91 MCN UA /OV DBN 084024/TM 0112/FLUNKN/TP UNKN/RM PILOT OG TSTMS OV SBO HAIL 1 INCH, SIZE OF 50 CENT PIECE 07/02/91 FWA UUA /OV FWA 160015/TM 1930/FL250/TP JET/WX WELL DEFINED FUNNEL 11/03/91 ISP UUA /OV DPK/TM 2048/FLUNKN/TP UNKN/RM FUNNEL CLOUD NE GLEN COVE RPT BY MAN OG Airport related 02/15/90 MEM UUA /OV MEM/TM 2230/FLOG/TP UNKN/RM LTNG HIT RWY 36R CREWS CHECKING 08/07/90 HTS UA /OV PMH/TM 0233/FLUNKN/TP C550/RM COW RUNNING ALONG RWY Other Interesting 02/04/91 YNG UA /OV YNG 090020/TM 1549/FL065/TP C182/TB NEG/RM DURGD TO NEW CASTLE...A BEAUTIFUL DAY TO FLY 10/13/91 MIV UA /OV ILG-MIV/TM 1840/FL020/TP PA28/RM SEVERAL FLOCKS OF GEESE BTWN 020-027 Table 6. Continued. 137 1. UA- Session 5.3 Encoding Pilot Weather Reports (PIREP) Routine PIREP, UUA • Urgent PIREP 2./OV- Location: Use 3-letter NAVA 10 idents only. a. Fix: /OV ABC. /OV ABC 090025. b. Fix to fix: /OV ABC-DEF, /OV ABC-DEF 120020. /OV ABC 045020-DEF 120005./OV ABC-DEF-GHI. 3./TM- Time: 4 digits in GMT: /TM 0915. 4./FL- AJtitude/FUght Level: 3 digits for hundreds of feet. If not known, use UNKN: /FL095, /FL310, /FLUNKN. 5./TP- Type aircraft 4 digits maximum. If not known use UNKN: /TP L329/TP B727. /TP UNKN. 6./SK- Cloud layers: Describe as follows: a. Height of cloud base in hundreds of feet If unknown, use UNKN. b. Cloud cover symbol. c. Height of cloud tops in hundreds of feet d. Use solidus (/) to separate layers. e. Use a space to separate each sub element f. Examples: /SK 038 BKN, /SK 038 OVC 045. /SK 055 SCT 073/085 BKN 106. /SK UNKN OVC 7./WX- Weather Flight visibility reported first Use standard weather symbols. Intensity is not reported: /WX FVG2 R H. /WX FV01 TRW. 8./TA- Air temperature In CeWur If below zero, prefix with a hyphen: /TA 15, /TA -06. 9./WV- Wind: Direction and speed in six digits. AW 270045. /WV 280110. 10. /TB - Turbulence: Use standard contractions for intensity and type (use CAT or CHOP when appropriate). Include altitude only if different from /Fl_ /TB EXTRM, /TB LGT-MDT BLO-090. 11. /IC- Idng: Describe using standard intensity and type contractions. Include altitude only if different than /FL: AC LGT-MDT RIME. /IC SVR CLR 028-045. 12. /RM- Remarfca: Use free form to clarify the report Most hazardous element first /RM LLWS -15KT SFC-003 DURGC RNWY 22 JFK. Refer to FAAH 71iai0for expanded explanation of TEI coding. Examples: CRW UA /OV HVQ 022050/TM 1230/PL045/TP PA42/SK 025 OVC 038 E100 OVC/WX S-/TA -2/WV 220045/TB LGT CHOP/IC TRACE RIME/RM BTWN LYRS ROA OUA /OV PSK 360010/TH 1645/FL060/TP C208/TB SVR/RM E OF RDGS Figure 1. The structure of a pilot report. 138 Session 5.4 PILOT WEATHER BRIEFINGS THINGS TO CONSIDER AND STEPS TO FOLLOW Bernard Esposito and Vicente Carreras National Weather Service Forecast Office Miami, Florida 1. INTRODUCTION The authors have noticed that many NWS employees, not only those recently hired, do not know the proper techniques in pilot weather briefing. It is our hope that this paper will aid all employees in this facet of their job in the Weather Service. Three types of pilot weather briefings are defined by both the FAA and the NWS. They are: stan¬ dard, outlook, and abbreviated. Since one of the requirements of all NWS employees enrolled in the Pilot Weather Course is to successfully give a standard briefing, this paper will deal with the techniques needed in giving a standard briefing. 2. STANDARD PILOT BRIEFING To provide a proper briefing, the briefer needs the following information: I. APPROPRIATE BACKGROUND INFORMATION, II. ADVERSE CONDITIONS, ITT A SYNOPSIS IV. ’ CURRENT CONDITIONS (MAY BE OMITTED IF THE PROPOSED DEPARTURE TIME IS MORE THAN 2 HOURS), V. ENROUTE FORECAST, VI. DESTINATION FORECAST, VII. WINDS ALOFT, VIII. TEMPERATURES ALOFT (UPON REQUEST), AND IX. A REQUEST FOR PILOT REPORTS. The remainder of this paper will go into detail on each of the nine items above. I. APPROPRIATE BACKGROUND INFORMATION To give an adequate pilot briefing, the briefer needs to get nine items of information from the pilot on his/her proposed flight. 1. Type of flight (VFR or IFR), 2. Aircraft I.D. or Pilot Name, 3. Aircraft Type, 4. Departure Point, 5. Route of Flight, 6. Destination, 7. Altitude, 8. Time of Departure, and 9. Time Enroute. Once the briefer has this in¬ formation, he/she should have enough information to give a thorough and hopefully an accurate pilot weather briefing. The briefer should now begin gathering weather observations, forecasts, and maps from his/her computer system. Individual weather reports or forecasts should not be read unless, in the briefer's judge¬ ment, it is necessary to emphasize an important point or unless specif¬ ically requested to do so by the pilot. 139 Session 5.4 II. ADVERSE CONDITIONS The most important and possibly most difficult part of a briefing is gathering the information dealing with adverse conditions. This is done whenever atmospheric conditions are reported or forecast that might influence the pilot to alter his/her proposed flight. Conditions that are particularly significant are: low level wind shear, embedded thun¬ derstorms, reported icing, volcanic activity, and frontal zones along the route of flight. Weather advisories--FA Flight Precautions, SIGMET's (WS), AIRMET's (FA), Con¬ vective SIGMET's (WST), Center Wea¬ ther Advisories (CWA) and Severe Weather Watch Areas (AWW)--shall be given by stating the type of adviso¬ ry. AREA FORECASTS Six Area Forecasts (FA's) are issued by the National Aviation Weather Advisory Unit (NAWAU) in Kansas City. These areas are: FA1, NORTHEAST; FA2, SOUTHEAST; FA3, NORTH-CENTRAL; FA4, SOUTH-CENTRAL; FA5, ROCKY MOUNTAIN; and FA6, WEST COAST. The FA's of primary concern to the Southern Region are: SOUTHEAST (FA2)...Georgia, Florida, and Coast¬ al Waters; SOUTH-CENTRAL (FA4)...rest of Southern Region excluding New Mexico; and ROCKY MOUNTAIN (FA5)...New Mexico. Each FA consists of two sec¬ tions: H for Hazards/Flight Precau¬ tions section, and W for the Synop¬ sis and VFR Clouds/Weather section. The AFOS pil for the Southern Region will be FA2H/FA2W for the MIA area; FA4H/FA4W for the DFW area; or FA5H/FA5W for the SLC area. SIGMETS/AIRMETS Federal Aviation Regulations distinguish between small and large aircraft. The In-Flight Weather Advisory program also recognizes that distinction by the issuance of two types of advisories: (1) Sig¬ nificant (SIGMET), and (2) Airman's Meteorological Information (AIRMET). SIGMETS (Nonconvective) contain information on specified weather conditions of such severity that it should concern all aircraft. Each SIGMET automatically amends the appropriate Area Forecast (FA); therefore, SIGMET's will be issued whether or not the specified crite¬ rion was included in the FA. SIGMET's in Conterminous U.S. are valid for up to 4 hours. Nonconvective SIGMET's relevant to areas within the conterminous U.S. will be issued by NAWAU when any of the following weather phenom¬ ena occur or are forecast over an area of at least 3,000 square miles. a. Severe or extreme turbu¬ lence, or clear air turbu¬ lence (CAT), not associ¬ ated with thunderstorms. b. Severe icing not associat¬ ed with thunderstorms. c. Widespread dust storms, sandstorms, or volcanic ash lowering surface and/or in-flight visibili¬ ty to less than 3 miles. d. Volcanic eruption. SIGMET's are found in AFOS files under WS#*, where # gives the area, i.e., WS2...SIGMET's for the Southeast U.S. and WS4...SIGMET's 140 Session 5.4 for the Southwestern U.S., and * is the alphanumeric designator-- NOVEMBER, OSCAR, PAPA, QUEBEC, RO¬ MEO, UNIFORM, VICTOR, WHISKEY, XRAY, and YANKEE are used for nonconvec- tive SIGMET's. SIERRA, TANGO, and ZULU are not used because they are used for AIRMET's. AIRMET's advise of weather phenomena less severe than that of a SIGMET, and are generally of concern to single-engine and light twin-engine aircraft but may be of significance to all aircraft. AIRMET's are routinely issued every 6 hours with a 6 hour valid time for the following weather phenomenon that is occurring or is forecast to occur within an area of at least 3,000 square miles: a. Moderate icing b. Moderate turbulence c. Sustained wind of 30 knots or more d. Ceilings less than 1000 feet and/or visibility less than 3 miles affect¬ ing over 50 percent of the area at any one time e. Extensive mountain ob- scurement. If none of these weather conditions are expected to occur, a negative statement is made in the AIRMET. There are routinely three AIRMET's: SIERRA (S) for IFR condi¬ tions and mountain obscurations; TANGO (T) for turbulence and low level wind shear; and ZULU (Z) for icing. All three should be examined to see if AIRMET conditions are in existence. AIRMET's are found in AF0S files under WA#S, WA#T, and WA#Z. The # gives the area, i.e., WA2 for the Southeast U.S., and WA4 for the Southwestern U.S. CONVECTIVE SIGMETS Convective SIGMET's are issued hourly (H+55) by NAWAU for thunder¬ storms and their related phenomena. Any Convective SIGMET (WST) issuance implies severe or greater turbu¬ lence, severe icing, and low-level wind shear (gust fronts, etc.); therefore, these conditions will not be specified in the advisory. A negative message will be sent each hour (H+55) when the forecaster determines that there is no need for a convective SIGMET in the region in question. WSTs shall be issued when ei¬ ther of the following occurs and/or is forecast to occur for more than 30 minutes of the valid period re¬ gardless of the size of the area affected (i.e., including isolated): - severe thunderstorm(s) - embedded thunderstorm(s). WSTs shall also be issued when, during the valid period, wither of the following criteria occur or are forecast to occur: - a line of thunderstorms, - an area of active thunder¬ storms affecting at least 3,000 square miles. WSTs for severe thunderstorms may include specific information or tornadoes and/or occurrence of hail of 3/4-inch or greater diameter and/or wind gusts of 50 knots or greater. Tornadoes, 3/4-inch hail or wind gusts to 50 knots or greater alone are sufficient criteria for issuing a WST for severe thunder¬ storms. 141 Session 5.4 Embedded thunderstorms, for the purpose of WSTs, are defined as thunderstorms occurring within and obscured by haze, stratiform clouds, or precipitation from stratiform clouds. WSTs for embedded thunder¬ storms are intended to alert pilots that avoidance by visual or radar detection of the thunderstorm could be difficult or impossible. A line of thunderstorms is defined, for WSTs, as being at least 60 miles long with thunderstorms affecting at least 40 percent of its length. Active thunderstorms are de¬ fined, for WSTs, as thunderstorms having a VIP level of 4 or greater and/or having significant satellite signatures and affecting at least 40 percent of the area outlined. Convective SIGMET's can be found under the AFOS header WSTE for the Eastern U.S.; WSTC for the Cen¬ tral U.S.; and WSTW for the Western U.S. ALWAYS remember, whenever giving a Convective SIGMET during a briefing to give the number, i.e., Convective SIGMET 12E or Convective Sigmet 5C. Be sure it is the latest Convective SIGMET issued, and if it is close to H+55, and there is an area of thunderstorms, it would be wise to advise the pilot to get the latest information either on the ground before departure or while in-fl ight. CENTER WEATHER ADVISORIES A Center Weather Advisory (CWA), issued by NWS meteorologists at the Center Weather Service Units (CWSU) located at the 21 FAA Air Route Traffic Control Centers (ARTCC) around the country. The CWA is an unscheduled in-flight flow control, air traffic, and air crew advisory. A CWA is not a flight planning product. It is generally a Nowcast for conditions beginning within the next 2 hours and also should reflect the weather conditions in existence at the time of issuance. In the Southern Region, there are 7 CWSU's. These are: ZMA Mi¬ ami, ZJX Jacksonville, ZTL Atlanta, ZME Memphis, ZHU Houston, ZFW Ft Worth and ZAB Albuquerque. Center Weather Advisories can be found under the AFOS header by entering CWA000. Since CWA's are unscheduled, it would be best to check your individual PIL, i.e., P:CWA to see if there is an active CWA. During a briefing, it is im¬ portant to give the center and num¬ ber of the CWA. Example: ZHU CWA4. If your office does a large number of briefings, CWA's from adjacent centers should be in the AFOS data¬ base for easy recovery. III. A SYNOPSIS Provide a brief statement de¬ scribing the type, location, and movement of weather systems and/or air masses which might affect the proposed flight. The synopsis may be combined with adverse conditions when it will help to more clearly describe conditions. IV. CURRENT CONDITIONS SUMMARIZE from all available sources, e.g., SAO's, PIREP's, cur¬ rently reported weather conditions applicable to the flight. Unless the information is requested by the pilot, this element may be omitted if the proposed time of departure is beyond 2 hours. 142 Session 5.4 V. ENROUTE FORECAST SUMMARIZE from appropriate data, e.g., FA's, prognosis charts, weather advisories, etc., forecast conditions applicable to the pro¬ posed flight. Provide the informa¬ tion in a logical order, i.e., climb out, enroute, and descent. VI. DESTINATION FORECAST Provide the destination termi¬ nal forecast (FTA), including sig¬ nificant changes expected within 1 hour before and after the estimated time of arrival. It is very impor¬ tant that the briefer know how to read and understand the FTA. VII. WINDS ALOFT VIII. TEMPERATURES ALOFT (UPON REQUEST) SUMMARIZE forecast of both winds and temperatures aloft for the proposed route. Interpolate wind directions and speeds along with temperatures between levels and stations as necessary. Winds and temperatures aloft are under AFOS header FD1/2/3 (where 1, 2 or 3 is the time period) FA1/2/3 or 4 (where 1, 2, 3 or 4 is the FA fore¬ cast area). An example is FD1FA2 which is a 12 hour wind/temperature aloft forecast for the southeast U.S. The wind and temperature aloft forecasts are issued for the follow¬ ing levels: 3,000, 6,000, 9,000, 12,000, 18,000, 24,000, 30,000, 34,000 and 39,000 feet. Wind direction is indicated in tens of degrees (two digits) with reference to true north, and wind speed is given in knots (two dig¬ its). Light and variable wind or wind speeds of less than 5 knots are expressed by "9900". Forecast wind speed of 100 through 199 knots are indicated by adding 50 to the coded direction and subtracting 100 from the speed. A forecast of 250 de¬ grees at 145 knots is encoded as 7545. Forecast wind speed of 200 knots or greater are indicated as a forecast speed of 199 knots. Temperature is indicated in degrees Celsius (two digits) and is preceded by the appropriate algebra¬ ic sign for the levels from 6,000 through 24,000 feet. Above 24,000 feet, the sign is omitted since temperatures are negative. A forecast of 175849 at 30,000 feet would be read as: Wind 170 de¬ grees...58 knots with a temperature of -49C. IX. REQUEST FOR PILOT REPORTS Solicit PIREP's for the effect¬ ed areas when any of the following weather conditions exist, or fore¬ cast to occur: a. Ceilings at or below 5,000 feet, b. Visibility 5 miles or less reported at the surface or aloft, c. Thunderstorms and related phenomena, d. Turbulence of moderate intensity or greater, e. Icing of light intensity or greater, and f. Wind shear. ABBREVIATED PILOT BRIEFINGS Provide an abbreviated briefing when a pilot requests information to SUPPLEMENT mass disseminated data, update a previous briefing, or when the pilot requests that the briefing 143 Session 5.4 be limited to specific information. Conduct abbreviated briefings as follows: 1. When a pilot desires spe¬ cific information only, provide the requested information. If adverse conditions are present or forecast, advise the pilot of this fact. 2. When a pilot requests an update to a previous briefing, obtain from the pilot the time that the briefing was received and necessary background in¬ formation. To the extent possible, limit the brief¬ ing to appreciable changes in meteorological condi¬ tions since the previous briefing. OUTLOOK PILOT BRIEFING Provide an outlook briefing when the proposed departure is 6 hours or more from the time of briefing. Limit the briefing to forecast data applicable to the proposed flight. When the proposed flight is scheduled to be conducted beyond the valid time of available forecast material, provide a general outlook and then advise the pilot when com¬ plete forecast data will be avail¬ able for the proposed flight. CONCLUSION As you can see, giving a pilot briefing can be quite cumbersome. To do a proper job, extensive data gathering is necessary in order to compile the appropriate information. This takes time and experience. It is hoped that this paper helps make pilot weather briefing an easier task. ACKNOWLEDGMENTS The authors wish to thank Ray¬ mond E. Biedinger, Deputy MIC, WSFO Miami, Armando Garza, Southern Re¬ gional Aviation Meteorologist, and Lans Rothfusz, Development and Training Meteorologist, Southern Region Scientific Services Division for their beneficial comments and input leading to the preparation of this manuscript. REFERENCES National Weather Service, 1991: Aviation area forecasts. Na¬ tional Weather Service Opera¬ tions Manual « Part D, Chapter 20, N0AA/D0C, Silver Spring, Maryland. _, 1984: Aviation terminal forecasts. National Weather Service Operations Manual, Part D, Chapter 21, N0AA/D0C, Silver Spring, Mary¬ land. _, 1991: Aviation in-flight weather advisories. National Weather Service Opera¬ tions Manual . Part D, Chapter 22, N0AA/D0C, Silver Spring, Maryland. _, 1981: Wind and temperature aloft forecasts. National Weather Service Opera¬ tions Manual . Part D, Chapter 24, N0AA/D0C, Silver Spring, Maryland. _, 1984: Support to air traffic facilities. National Weather Service Operations 144 Manual, Part D, Chapter 25, NOAA/DOC, Silver Spring, Mary¬ land. Session 5.4 _, 1982: Aviation weather warnings and pilot briefings. National Weather Service Operations Manual . Part D, Chapter 26, NOAA/DOC, Silver Spring, Maryland. _, 1990: Training program for pilot weather briefers. National Weather Service Operations Manual . Part D, Chapter 82, NOAA/DOC, Silver Spring, Maryland. 145 Session 5.5 AVIATION WEATHER BRIEFING SERVICE TRAINING FOR THE FUTURE Larry G. Sharron Environment Canada Atmospheric Environment Service Meteorology Training Centre Transport Canada Training Institute Cornwall, Ontario, Canada 1. Background The delivery of Aviation Weath¬ er in Canada has been the responsi¬ bility of Transport Canada (TC) for the past several decades. Prior to 1971, the Canadian Weather Service was a branch of the Transport de¬ partment and, as such, was mandated to administer all weather related activities in Canada. In 1971 the Weather Service changed its name to the Atmospheric Environment Service (AES) and became part of the newly formed Department of Environment and Fisheries Canada. It soon became obvious that environmental concerns and fish did not make a good blend, so the department divided, with the AES remaining a major component of Environment Canada (EC). Today in Canada most weather related activi¬ ties, including the production of all aviation forecast products, are the responsibility of the AES. The delivery of aviation weather howev¬ er, remains the responsibility of Transport Canada. For the first years after the division, AES continued to provide for the aviation weather require¬ ments at Canadian airports through a cooperative effort by both depart¬ ments. In 1974 a program to upgrade the weather knowledge and dissemina¬ tion skills of Flight Service Spe¬ cialists began. For 10 years the Meteorology Training Centre of AES delivered a 4 week training program in Aviation Weather Information Services (AWIS) at a rate of 10 courses per year, each course loaded to 16 operational specialists. By the early 80s, all Canadian Flight Service Stations, more than 100, were certified to provide the Avia¬ tion Weather Information Service. The AWIS level of briefing service is based totally on informa¬ tion received in an alphanumeric format. Specialists were trained to locally produce graphic displays to depict current and forecast weather and to provide weather information for low and mid level aviation ac¬ tivities within 500nm. of the sta¬ tion. The FSS was directed to pass requests for weather information for flights in excess of 500nm. or for high level operations to a designat¬ ed AES Weather Office staffed with Weather Service Specialists and equipped with a full array of alpha¬ numeric and graphic products. In the fall of 1988 the Trans¬ port Canada Aviation Group (TCAG) adopted a job analysis to reflect an upgrade in the aviation weather briefing knowledge and skill of the Flight Service Specialist to a level comparable to the AFS briefer. The premise of this initiative was to ensure that the FSS at selected locations could provide an interpre¬ tative level of consultation to the pilots who require a detailed analy¬ sis of weather, including specific advice concerning factors which 146 Session 5.5 could affect the safety and/or effi¬ ciency of aviation activities. The advantages of this reallocation of responsibility were seen to be: a. The FSS are readily accessible by toll-free telephone; b. The FSS can provide NOTAM and flight plan service from the same toll-free call; c. FSS can provide VHF enroute briefings; and d. TC could more easily manage the complex allocation of resources devoted to the delivery of the aviation weather service. 2. The Development In June of 1989 TCAG requested AES to proceed with the development of a training program for FSS to address the knowledge and skill levels of the Aviation Weather briefing Services job analysis. The Meteorology Training Centre (MTC), a division of the AES Training Branch, was given the mandate to develop the course and to assist with establish¬ ing an equipped simulation facility for the training. As the AWBS job analysis had been developed from a selection of the AES Weather Service Specialists (WSS) job analysis, training objectives and materials from the AES training programs were selected for the course. The recruitment standards for the AES briefer has always included mathematics and physics, at least to the high school level, and conse¬ quently training has always been designed to complement that back¬ ground. There is no such requirement for FSS. In order to address the aviation orientation of the AWBS knowledge and skills inventory and to compensate for the trainee popu¬ lation lack of math and physics, considerable changes had to be made to the training materials, particu¬ larly in Theoretical Meteorology. Much of the material was modified to use more of a verbal description of meteorological concepts rather than the traditional mathematical ap¬ proach. As with all MTC training pro¬ grams, as soon as materials are ready for print in one language, the process of translation to the second language begins. In the case of the AWBS course, materials were first developed in English and, because the first French serial was not scheduled to begin until September of 1990, there was sufficient time to run an English pilot course and fine tune the materials before the French version was finalized. An operational model of an AWBS facility had not yet been developed at the time of the course develop¬ ment. The selection of products to be used for training and the deter¬ mination, purchase and installation of cost effective systems to deliver these products to the AWBS simula¬ tion lab became a significant part of the development workload. The complexities of dealing with several divisions of a major federal depart¬ ment as well as several private sector agencies within a limited time frame and budget made for some anxious moments as deadlines ap¬ proached. The lab dedicated to AWBS training at TCTI occupies approxi¬ mately 100 square meters designed to accommodate 12 trainees and up to 4 instructors. The lab is equipped with MIDS III terminals for alphanu¬ meric data, an Alden printer for satellite imagery and a PC with 147 Session 5.5 SPIES software linked to an ink jet printer to receive and print weather charts via the METSIS system. Only specified imagery and charts are selected from the full METSIS array for distribution and printing in the AWBS lab. Another PC is used to receive real time Radar imagery from the closest AES system located ap¬ proximately 100km from Cornwall. The lab has an internal telephone system to link the 12 trainee positions to the instructor desks. 3. The Course The AWBS training program con¬ centrates on increasing the special¬ ists knowledge in Theoretical Meteo¬ rology and skills in using alphanu¬ meric and graphic products to pro¬ vide an interpretative level of aviation weather services. The course spans 30 training days which is subdivided into the following training activities. Subjects Time (hours) Theoretical Meteorology 60 Climatology 6 Radar Meteorology 11 Satellite Meteorology 10 Meteorological Data 26 Weather Services 25 Familiarization Visits 6 Simulated Operations and Evaluation 30 Adninistration 6 Total 180 Each training day is divided into three 80 minute blocks and one 120 minute block. The course com¬ bines classroom academic instruction with simulation exercises, ending with 5 days of simulated operations and evaluations. Knowledge areas are tested in the classroom setting and skills are evaluated during simulat¬ ed operations. Most of the classroom activities occur between 0800 and 1200 whereas the afternoons are largely devoted to reinforcement exercises and simulations. Towards the end of the course, trainees are taken on a familiarization visit of the AES Quebec Weather Centre in Montreal. The Regional Weather Centres are to serve as the desig¬ nated resource centre for FSS AWBS outlets. 4. The Training The pilot AWBS course began in March of 1990. Since then there has been 3 serials delivered in English and, at the end of next week, the 4th French course will be completed. The courses are scheduled at a rate of 5 courses per year with a full loading of 12 trainees. French courses are generally not fully loaded due to fact that most candi¬ dates are representing only one region. The AWBS course is, for many of the specialists, the first return to formal training for many years. Although the FSS Basic training program has a total of 63 training days devoted to Meteorology, much of that training is oriented towards weather observing skills. Meteoro¬ logical theory is only covered at a very elementary level. Getting back to the classroom and nightly study has been difficult and somewhat intimidating for many AWBS trainees. The very sight of the Theoretical Meteorology reference has caused many moments of despair during the first days of each course. The program however, is de¬ signed for training, not screening. A Training Program Plan guide book contains all the administrative and evaluation procedures of the course as well as all objectives for each subject. The guide is designed to lead the trainee through the materi¬ al with space for notes following 148 Session 5.5 each objective. Progress tests are given to alert the instructional team and trainee to problem areas. Counselling and/or extra help is offered as soon as difficulties are identified. By the end of the first week, the atmosphere tends to become somewhat more relaxed but the pro¬ gram continues to demand consider¬ able concentration and study. The final academic test, which falls on day 24 of the course, marks the end of the heavy study period. The final days of the program are totally devoted to simulation, skill evaluation and familiarization, including the trip to the Weather Centre in Montreal. The skill evalu¬ ation can be a stressful time for some trainees however, the activi¬ ties are such that after hours study is general minimal. One of the rewarding features of this program is the motivation and enthusiasm that many of the trainees have clearly demonstrated for weather briefing. It would be stretching the truth a little to say that trainees develop a love for Theoretical Meteorology, however many clearly enjoy working with the products, explaining the daily fea¬ tures and discussing the details of weather. And they are good at it. The combination of a sound under¬ standing of meteorology and close working relationship with the avia¬ tion community makes for an effec¬ tive aviation weather briefer. 5. Conclusion At present TCAG plans to up¬ grade one FSS facility to the AWBS level in each of the 6 regions in Canada. Site selection, communica¬ tion equipment upgrade and local procedural changes are well underway to implement the service in the near future. The present rate of training would train sufficient specialists to staff these centres within the next two years. After that, a re¬ duced training program to accommo¬ date attrition and transfers will most likely continue with changing user requirements and technology advances making periodic adjustments to the course necessary. The process of development and delivery of the AWBS training has been, and continues to be a coopera¬ tive effort of two major federal departments of Canada. The implemen¬ tation of the AWBS program is re¬ sulting in a change to the existing Partnership, however, it is expected that both departments will continue to work together to meet the needs of the aviation community. The pro¬ gram to train specialists who can continue the tradition of providing an effective interpretative level of weather services easily accessible to the aviator in Canada, is a sig¬ nificant part of that partnership. 149 Session 6.1 NATIONAL WEATHER SERVICE TERMINAL FORECASTS AND FEDERAL REGULATIONS Joe Pedigo National Weather Service Forecast Office St. Louis, Missouri 1. INTRODUCTION National Weather Service avia¬ tion products have taken a more important role to the national avia¬ tion industry since the Federal Aviation Administration (FAA) began a more strict enforcement of Chapter 121 of the Code of Federal Regula¬ tions. Current records indicate an average of nine hundred sixty flights per day are delayed or can¬ celed. Seventy to eighty percent of those flight alterations are due to impending weather or FORECASTS OF IMPENDING WEATHER. In other words, bad weather is not the only element that can delay or cancel a flight, but even a FORECAST describing ad¬ verse weather at a destination air¬ port can cause major problems. The objective of this paper is to show how critical terminal fore¬ casts are in day-to-day operations of the national aerospace system. This paper will expand upon the various federal regulations and critical minimums that control the national aviation system. National Weather Service forecasters should be cognizant of these rules. Fur¬ thermore, possible solutions to improve NWS products and service to the aviation community will be ex¬ plored, by: (1) Enumerating some of the more significant Federal Regulations which authorize the National Weather Service to issue fore¬ casts and regulate the airline industry. (2) Listing various aircraft approaches and critical weather minimums for a particular air¬ space. (3) Relating specific terminal forecasts to these federal regulations and airport re¬ quirements, to show how impor¬ tant it is for NWS aviation forecasters to understand how their forecasts are an integral part of the national aerospace system. Special emphasis will be placed upon the use of con¬ ditional phrases in terminal forecasts. (4) Offering some suggestions as to how the National Weather Ser¬ vice can improve aviation prod¬ ucts and develop a method of in-house quality control of those products, increasing the expertise of NWS forecasters, and (5) Stressing the importance of improvement of NWS products to the aviation community AND to the National Weather Service. Technological advances in com¬ munication and increased use of private sources for aviation information makes it imperative for the NWS to respond to the meteorological needs of the national aviation industry. This paper serves to document some of the needs and shortcomings of the NWS aviation program. Hope¬ fully, NWS personnel who oversee this program will realize how tenu- 150 Session 6.1 ous this program is with the user industry, even though it is mandated by the FAA. 2. FEDERAL AND NATIONAL WEATHER SERVICE GUIDELINES...THEIR EFFECTS Every aircraft that soars through our nation's skies is care¬ fully controlled by federal regula¬ tions. Recently, a change in the interpretation of some of these regulations has resulted in a more conservative attitude on the part of the FAA regarding the dispatching and flight of aircraft from one point to another. The FAA has also been very specific in its requirements for weather reporting and information facilities by the air carriers. The FAA Code of Federal Regulations requires the aviation industry to rely on the National Weather Service or NWS-approved products for their operations. Federal Aviation Regula¬ tion (FAR) 121.101 states: "Each domestic and flag air carrier must show that enough weather-reporting services are available along each route to ensure weather reports and forecasts necessary for the operation...No domestic or flag air carrier may use any weather report to control flight un¬ less, for operations within the 48 contiguous states and the District of Columbia, it was prepared by the U.S. National Weather Service or a source approved by the U.S. National Weather Service..." The Federal Aviation Adminis¬ tration has issued a mandate to the airline industry for dispatching and flight operations advising them to plan for the worst-case scenario, rather than the best-case situation, as noted in the FAA Manual 8400.10. Chapter 7, Section 1, Paragraph 1407, titled "Policy on Conditional Phrases in Remarks Portion of Weath¬ er Forecast" states: "Weather forecasts provided by the National Weather Service (NWS) and other sources often have conditional phrases such as "occasional," "intermittent¬ ly," "chance of," or "tempo" in the remarks portions of the forecasts. These phrases sup¬ plement the main body of the forecast by indicating the probability of changing condi¬ tions during the forecast peri¬ od. These modifying phrases, used in the remarks portion of a terminal forecast (FT), indi¬ cate the weather conditions for an area within five nautical miles of a runway complex. Certain regulations concerning the selection of destination and alternate airports require that "weather reports or fore¬ casts, or any combination thereof, indicate that the weather conditions will be at or above..." the minimum weath¬ er conditions specified in those regulations. The FAA's Office of Chief Counsel has consistently interpreted these regulations to mean that the WORST weather condition in any of the reports or forecasts used to control a flight move¬ ment is the controlling factor. These interpretations make the remarks portion of a forecast as operationally significant as the main body of the forecast. Therefore, it is FAA policy THAT THE WORST WEATHER CONDI¬ TION IN THE MAIN BODY OR THE REMARKS PORTION OF A TERMINAL 151 Session 6.1 FORECAST, as well as any weath¬ er report used, is the control¬ ling factor when selecting a destination or alternate air¬ port." The NWS policy on conditional phrases is much more general. The National Weather Service Operations Manual, Part D, Chapter 21, Section 5.2, states: "...Remarks are pertinent to an area within 5 nautical miles of the center of the runway com¬ plex and amplify or change elements described in the body of the forecast group. Within the limits specified in the following instructions, condi¬ tions described in remarks must be considered whether an amend¬ ment criterion has been met. It is recognized that situa¬ tions occur which logically dictate using one or more terms in the remarks portion of a forecast group, however, these terms should be used sparingly and be as concise as possible. It should be noted that the FAA requires that the remarks por¬ tion as well as the body of the FT be considered by pilots and dispatchers in determining "legal" destinations, alter¬ nates, and fuel loads. This makes the content of remarks operationally more significant than the body of the forecast when they describe lower condi¬ tions. .." NWS policies need to be more specific, and perhaps include the aforementioned FAA policies from the FAA Manual 8400.10 in the Operations Manual. Flight movements, take-offs, and landings have very detailed requirements, and each forecaster needs to know how his(her) forecasts and conditional phrases impact local and national air traffic flow. The purpose is not to "steer" his(her) forecasts, but to teach the fore¬ caster how to advise the users what weather conditions can be expected during departure, flight, and arriv¬ al of the aircraft. Additionally, the forecaster must forecast the ceilings, visibilities, and other meteorological phenomena as accu¬ rately as possible during those times when the weather most affects the flights. Amendments of forecasts which have gone awry must be accom¬ plished as quickly as possible when it becomes evident that the forecast is incorrect...even slightly. If the forecaster has the knowledge of the consequences of his/her forecast for the aviation community, he/she will be less apt to issue a "cover- yourself" conditional phrase just because his/her confidence factor is not one hundred percent. The major airport within the St. Louis Fore¬ cast Office area of responsibility is St. Louis Lambert International Airport. This airport is the hub airport of Trans World Airlines. Approximately eighty percent of the flights in and out of Lambert Field are with TWA. In the remaining parts of this paper, examples of critical airport minimums and terminal forecasts will refer to Lambert Field. Forecasters from other areas will have different carriers and may have different minimum criteria at the airports within their forecast area of re¬ sponsibility. 152 Session 6.1 3. FEDERAL REGULATIONS REGARDING DISPATCH AND FLIGHT RELEASE All air carriers are required to adhere to the Code of Federal Regulations. Chapter 121 of the Federal Regulations contains the major rules regarding release of flights from one location to anoth¬ er. A list of some of these regula¬ tions which are profoundly affected by terminal forecasts is included in Appendix C. The reader is advised to review these regulations prior to further reading, as these regula¬ tions will be referenced in the following sections. 4. TYPES OF AIRPORT APPROACHES, ASSOCIATED MINIMUMS AT LAMBERT FIELD In this section, the following is a listing of various airport approaches and associated airport minimums for St. Louis Lambert In¬ ternational Airport. These ap¬ proaches are categorized into four types: visual, staggered visual, circling, and instrument approaches. These approaches are defined as follows: A. VISUAL APPROACH...under good weather conditions, visual approach¬ es are used. The pilot must ba able to see the airport AND the aircraft ahead of him during the approach. Forty- five percent of the yearly operations are done in this configu¬ ration. Minimums includes no clouds below 4,000 feet and visibility at least 5 miles. Details of the air¬ port capacity under this configura¬ tion are noted in Appendix A, Sec¬ tion 1. Aircraft can negotiate side-by-side arrivals and depar¬ tures. B. STAGGERED VISUAL APPROACH... used with minimum ceiling 3,500 feet and visibility 3 miles. Details of the airport capacity under this configuration are noted in Appendix A, Section 2. Thirty-five percent of yearly operations are under this configuration. C. CIRCLING APPROACH...when ceil¬ ings are too low to permit visual approaches, a circling approach may be used. The aircraft uses some type of navigational aid to get under the cloud layer. The pilot then is able to see the airport, then circles around to the desired runway and lands. Minimum condi¬ tions include a minimum ceiling of 1,000 feet and visibility of 3 miles. This procedure is listed at Lambert Field but is never used. It would be more likely to be used at an airport that has an ILS only on one end of the runway, where surface winds were unfavorable to land on the ILS runway. D. INSTRUMENT APPROACH...used when visual approaches cannot be conduct¬ ed. Types of instrument approaches include: ILS Instrument Landing System ILS/LDA Combination using Instriment Landing System and Localizer Directional Aid VOR VHF Omnidirectional Range NDB Non-Directional Radio Beacon LDA Localizer-type Directional Aid ASR Airport Surveillance Radar LOC Localizer LOCCB/C) Localizer Back/Course The ILS/LDA approach (See Ap¬ pendix A, Section 3) is the most used instrument approach. About 10 percent of the yearly operations use this configuration. The arriving aircraft use the ILS for vertical course guidance, and the LDA for left/ right course guidance, on simultaneous approach- 153 Session 6.1 es. Once on the localizers they descend 4600 feet apart, and not exactly on course with the runways. The LDA ends at 2.6 miles from the end of the runway on "final". The pilot must have the airport AND the other aircraft in sight. He then performs a small S-turn and lines up with the runway. If the pilot cannot observe the runway at that moment, he shoots a "missed approach" and tries again. When the aircraft are on the LDA approach, they follow the LDA while maintaining 4600 feet separa¬ tion. Since the runways are 1300 feet apart, the aircraft will con¬ verge once the pilots have each other in sight. Minimums for the ILS/LDA approach are ceilings of 1,200 feet, or visibilities of 4 miles on Runway 12 or 5 miles on Runway 30. Wind direction at Lambert Field is also critical with the ILS/LDA approach. If the prevailing wind direction is between 210 degrees and 330 degrees, the tower personnel can also use Runway 24 in addition to Runway 30. This is called "running simultaneous converging approaches", and allows smaller aircraft to land on Runway 24 short of the runway intersection, and thus increases airport arrival and departure capac¬ ity. The ILS Approach is the second most often used instrument approach configuration (See Appendix A, Sec¬ tion 4). It is labor intensive, and requires a minimum of ten control¬ lers to accomplish it. Airport ac¬ ceptance rates are cut in half re¬ sulting in aircraft performing a "staggered approach". During the ILS approach, each aircraft are required to maintain a separation of three miles. One plane approaches the right runway, while the other accesses the left runway. Addition¬ ally, there must be a six mile sepa¬ ration between aircraft landing on the same runway. Minimums with the ILS-type approach are ceilings of 500 feet and visibilities of 2 miles. When conditions lower to ceil¬ ings at or below 500 feet and visi¬ bilities two miles or less, all aircraft are aligned into single file. This approach lowers the acceptance rate even further (Appen¬ dix A, Section 4), with Non-Category 2 (NON-CAT 2) minimums, ceilings 200 feet and minimum visibilities 1/2 mile or Runway Visual Range (RVR) values of at least 1800 feet. RVR is the distance the pilot can see the high intensity runway lights from the touchdown zone. In con¬ trast, "visibility" is the distance a person with 20/20 corrected vision can see ordinary lights at night or objects during the day. The RVR has a readout every minute, and is usu¬ ally greater than the visibility. RVR values are appended to surface observations and are measured in hundreds of feet. For example, RVR 36 is defined as a Runway Visual Range of 3600 feet, and RVR10- im¬ plies a Runway Visual Range of below 1000 feet. "Non-Cat 2" is defined as any aircraft which does not have the instrumentation to make a Cate¬ gory II landing (defined below). Most aircraft are Non-Cat 2 air¬ craft. The ILS is composed of three components: the Localizer, the Glide-Slope, and the Markers. The Localizer is the course guidance, usually along the runway centerline. The Glide Slope gives altitude guid¬ ance to the aircraft. The markers (the outer marker, middle marker, and inner marker) are radio aids 154 Session 6.1 that help the pilot identify exactly where he is located along the ap¬ proach to the runway. E. CATEGORY II OR III APPROACHES (ONLY RUNWAY 30R HAS THIS EQUIPMENT) When weather conditions are too low for even ILS approaches, some of the wide- bodied aircraft maintain enough technology to land or take off with even lower minimums. Ap¬ proach Charts used by the pilots contain "decision heights" (DH) or "minimum descent altitudes" (MDA). At these points during the approach, the pilot must see the required visual reference (either the runway in the daytime or runway lights at night). Each pilot AND aircraft is qualified to land at a certain mini¬ mum RVR value and MDA. If the RVR is out of service, minimum visibili¬ ties to land are usually 1/2 mile. Typical minimums for CAT II and CAT III approaches include: (1) RVR values of RVR 12 or 16 (1/4 mile and DH of 100 or 150 feet for CATEGORY II approaches, and (2) RVR values down to RVR 6 and DH 50 feet for Category III ap¬ proaches. Besides the minimums set forth above, each company develops a set of Operations Specifications for each airport which is filed with the FAA. Terminal forecasts which exceed these specifications can also cur¬ tail flight activity for that air¬ line. For example, TWA uses the following specifications, which they call "Ops Specs", for Lambert Field: (1) TAILWIND RESTRICTIONS The maximum tailwind, which is the average windspeed parallel to the active runway (the run¬ way the aircraft are using), cannot exceed 10 knots. (2) CROSSWIND LIMITATIONS The crosswind component is the vector wind speed perpendicular to the active runway. For TWA, the maximum crosswind component varies between 29 and 35 knots, depending upon the aircraft type. For aircraft making a CAT II landing, the crosswinds cannot exceed 10 knots. (3) PRECIPITATION RESTRICTIONS TWA "Ops Specs" require no take-offs or landings during the occurrence of: A. Heavy freezing drizzle (ZL+) B. Moderate or heavy freezing rain (ZR OR ZR+) C. Heavy wet snow (S OR S+) D. Moderate or heavy thunder¬ storms (TRW, TRW+) Take-offs are disallowed when: Standing water, slush, wet snow exceeds a depth of one half inch on the active runway. Landings are not permitted when: Standing water, slush, wet snow exceeds a depth of one inch on the active runway. 5. TERMINAL FORECASTS AND THEIR RELATIONSHIP TO FEDERAL REGULA¬ TIONS In light of the aforementioned regulations and critical airport minimums, this section will focus on how National Weather Service termi¬ nal forecasts affect take offs, flight movements, landings, and the dispatching requirements thereof. 155 Session 6.1 FEDERAL AVIATION REGULATIONS 121.101 and 121.107 require dispatch offices to maintain adequate weather services to ensure that weather reports and forecasts will be avail¬ able along the entire route of flight. The dispatcher must ensure legal planning of these flights, using current weather observations and terminal forecasts. Additional¬ ly, dispatchers must also keep abreast of up-to-the-minute weather conditions, particularly when chang¬ es may impact flight operations until the aircraft reaches the ar¬ rival gate. According to FEDERAL AVIATION REGULATION 121.613, the dispatcher is restricted to release a flight only when the destination point is at or above landing minimums. Under minimum Non-CAT II conditions, the ceiling forecast is not as important as the visibility. For most air¬ craft a visibility 1/2 mile is the minimum allowed (see ILS Approach). Suppose the following terminal forecast was in effect: FT1 STL CO X 1/2F Non-CAT II aircraft can still oper¬ ate using the single-file ILS ap¬ proach. However, if the forecast was written as: FT2 STL CO X 1/4F 3014 The airport would be shut down for all aircraft except the CAT II or III aircraft due to a visibility forecast less than 1/2 mile. The CAT II flights would still be able to arrive and depart on Runway 30R. (See CAT II Approach above). If the terminal forecast for St. Louis was: FT3 STL C6 X IF SLGT CHC CO X OF L-F the visibility noted in the condi¬ tional language of this forecast would completely shut down the air¬ port to all arriving and departing flights (FAR 121.613). This holds true even though the prevailing forecast conditions were well above minimums. Ceilings and visibilities are not THE ONLY conditions for which the terminal forecasts can close the airport to incoming traffic. For example: FT4 STL C2 X 1/4F 1312G22 Only CAT II aircraft would be al¬ lowed to land, due to visibilities being forecast less than 1/2 mile. However, at Lambert Field, only Runway 30R has CAT II instrumenta¬ tion. Since the forecast wind direc¬ tion is nearly parallel to the ac¬ tive runway, the "tailwind restric¬ tion" guidelines would shut down the airport (average tailwind speed would be in excess of 10 knots). Crosswinds observed OR forecast can also shut down the airport. If the following terminal forecast was in effect: FT5 STL C2 X 1/4F 2112 Then, this forecast would close the airport. In this case, ceiling and visibility forecast would allow a CAT II landing on Runway 30R. How¬ ever, the crosswind component ex¬ ceeds 10 knots (see Appendix B). 156 Session 6.1 6. TERMINAL FORECASTS...ALTERNATE AIRPORT AND FUEL REQUIREMENTS A. The Effect of Terminal Forecasts on Fuel Consump¬ tion This section focuses on how faulty terminal forecasts cause spiraling fuel costs to the aviation industry. FAR 121.619 emphasizes the requirement for an alternate airport on the Dispatch Release if the des¬ tination observed AND forecast wea¬ ther is not at least a ceiling of 2,000 feet and a visibility of 3 miles within one hour either side of expected arrival time. This is very important to major air carriers, because if an alternate IS required, FAR's 121.641, 121.643, and 121.645 specify a significant additional fuel supply to be carried aboard. In most cases, major airlines are under U.S. registry, and can operate in the U.S. and all foreign countries. They are identified as "flag carri¬ ers" in the Federal Regulations. When alternate landing sites are required, jets must carry enough fuel to: 1) fly to and land at the airport to which it is dispatched, and 2) carry enough fuel to reach the most distant alternate, still retaining 30 minutes of fuel on-board. Using these guidelines, a total fuel consumption is calculat¬ ed, based on normal cruising speeds. Then, an additional fifteen percent of the calculated fuel consumption is required to adhere to Regulation requirements. The Manager of Operations for Trans World Airlines, Don Eick, stated that on a normal day there are 334 TWA flights into St. Louis. With forecast conditions below 2,000 feet and/or a visibility of less than 3 miles, FAR 121.619 would require those flights to carry an extra 2,000,000 pounds of fuel. To carry this amount of fuel, aircraft would burn 500,000 pounds of fuel (90,000 gallons) just to carry the extra weight! Since diesel fuel costs $.65 per gallon, TWA must spend about $60,000 per day to transport the additional fuel to adhere to this regulation. General aviation aircraft are affected even more, since many cannot navigate in poorer weather conditions. In general, CAT II flights can become very expensive, since CAT II- equipped alternate airports are often 250 to 350 miles away from the destination airport. Multiplying these costs times the number of airlines at each air¬ port, for five hundred or more major airports, the financial costs can easily result in millions of dollars each day. Besides the financial aspect of the loss, there are mas¬ sive losses of precious natural resources, and untold hours of pas¬ senger inconvenience. B. Alternate Airport Selec¬ tion Based Upon Terminal Forecasts Once it is determined that an alternate is needed (destination airport is less than 2000 ft or 3 miles) the dispatcher must select an airport that meets the alternate minimum. These minimums are usually 400 feet and 1 mile or 600 feet and 2 miles. Smaller airports might require alternate minimums of 800 feet and 2 miles or 1000 feet and 3 miles (Note FAR's 121.621, 121.623). Consider a flight from New York to St. Louis. The terminal forecast for St. Louis: FT6-1 STL C16 ovc 5H 1206 157 Session 6.1 Ceilings are less than 2000 feet. Therefore, accordingly to FAR 121.619, an alternate airport is required. The dispatcher decides to use Springfield, Missouri (SGF) as the alternate, due to a snow storm further north. The Springfield fore¬ cast is: FT6-2 SGF C9 x 4f 1207 Alternate minimums for Springfield are 600 feet and 1 1/4 miles. Sup¬ pose the flight is in progress, and the SGF terminal forecast is amended to: FT6-3 SGF FT AMD 1 C5 X 2F 1207 According to FAR 121.625, the dis¬ patcher will have to change the alternate airport while the aircraft is in flight, even though the weath¬ er at St. Louis did not change. If the amendment of the SGF forecast had been: FT6-4 SGF FT AMD 1 CIO 0VC 4F SLGT CHC C5 X 2F the conditional phrase of this fore¬ cast would ALSO have required the dispatcher to change the alternate airport. Considering the current fuel on-board, if none of the nearby alternate airports meet alternate minimums, the aircraft will be re¬ quired to stop for additional fuel prior to reaching the destination airport. 7. CENTRAL FLOW CONTROL MINIMUMS The significant increase in air traffic flow in the National Aero¬ space System has created the need for a nationwide program to provide an orderly progression to the flight system. This section focuses on the central flow control system. When ceilings at St. Louis are observed or forecast to go below 800 feet or visibilities below 3 miles, the Central Flow Control Facility in Washington D.C. can invoke a program limiting the number of flight opera¬ tions for the various airports in¬ volved. This is accomplished by initiating a national ground delay program. Fuel costs and massive congestion are reduced when this program is activated. Aircraft are delayed at the departure airports and traffic is controlled both in the air and on the ground. (Other airports may have different mini¬ mums.) 8. CRITICAL AIRPORT MINIMUMS FOR ST. LOUIS Critical weather conditions which affect St. Louis Lambert In¬ ternational Airport have been summa¬ rized into a one-page list (see Attachment B). Similar lists should be compiled for every major airport and be supplied to the National Weather Forecast Office with fore¬ cast responsibility for that air¬ port. 9. RECOMMENDATIONS FOR NATIONAL WEATHER SERVICE ACTION ITEMS The National Weather Service has found itself in a difficult position regarding the issuance of terminal forecasts and other avia¬ tion forecast products to the avia¬ tion industry. The NWS has rested in the shadow of the FAA mandate, thinking that effective and proper aviation information is being sup¬ plied to the nation. However, sev¬ eral presentations at the National Aviation Workshop told a different story. Pockets of disenchantment 158 Session 6.1 from several users have surfaced resulting in a crossover from re¬ ceiving "free" NWS information to "paid" forecasts supplied by private interests. A number of major airlines have embarked on the Enhanced Weather Information System (EWINS) program which collects, evaluates, and dis¬ seminates weather data, including the authority to issue weather fore¬ casts for the control of flight movements (Baker, 1992). This is being done because the NWS is not supplying the quality forecasts that should be available with today's technology. The National Weather Service needs to make some IMMEDIATE chang¬ es. The following are several recom¬ mendations for consideration: RECOMMENDATION 1...Issue Specific Forecasts Aviation forecasters should be familiar with airport minimums at each airport within the forecast area of responsibility. When criti¬ cal weather conditions exist, they should write their forecasts as concise as possible. Conditional language is often over-used and should be introduced judiciously. Consider the following two terminal forecasts: FT 7-1 STL FT C50 OVC 6R-F 3115 SLGT CHC C7 OVC 1/2L-F FT 7-2 STL FT C7 OVC 1/2L-F From an airline's standpoint, the two terminal forecasts above gener¬ ate the same restrictions to the flight. When conditions lower into the "critical value areas" forecasters should do their best to forecast exactly what they expect will hap¬ pen, rather than state several pos¬ sible conditions. If there truly is a "chance", forecast it. Do not issue a "cover-your-bases" forecast. Jackson (1984) noted that Unit¬ ed Airlines conducted an eight-month study of aviation forecasts written for two major airports. The study showed that, "Conditions requiring a weather alternate, and thus extra fuel, were being forecast at nearly twice the rate of actual occur¬ rence." RECOMMENDATION 2...Amend Out-Of-Tolerance Forecasts Quickly Forecasts should be amended with haste when they are out of tolerance, especially when airport "critical values" are involved. Too often, forecasters review a fore¬ cast, noting that the body of the forecast is within tolerance. Even though the weather noted in condi¬ tional phrases are no longer needed, forecasters fail to amend the fore¬ casts thus leaving the aviation industry in a quandary. In changing conditions, during worsening OR improving weather situations, time is of essence and amend promptly! RECOMMENDATION 3...Improve Training The Flight Service gives each briefer a four-month training course at the FAA Academy in Oklahoma City, followed by six to twelve months of intensive training on-station, be¬ fore they are deemed qualified to brief pilots. In addition, each Area Flight Service Station has a full¬ time training staff of three indi¬ viduals to work with the operational staff on-station. Staff members are required to attend ten to twelve days per year in formal training at the station. 159 Session 6.1 The National Weather Service has initiated several training pro¬ grams including the WSR-88D, the Science and Operations Officers Residence Course, and Professional Development Work Stations, as part of Modernization and Restructuring. It appears that WSR-88D-related items will be top training priority for the next two to three years. However, several presentations by the user aviation community at the December 1991 National Aviation Workshop stated that National Weath¬ er Service meteorologists are unedu¬ cated of the users' requirements. This point was further emphasized by the FAA and AOPA. Since the FAA is funding the aviation portion of the NWS budget, and the AOPA gives very large sums of monies to the industry, the NWS should formalize a training program for its forecasting staff. The pro¬ gram should be developed with sig¬ nificant input from the airline industry, the AOPA, and the FAA. Training should be jointly funded from each of these users (FAA, AOPA). Additionally, the airline industry should assist and provide training so the forecasters acquire a view from the 'airlines' prospec¬ tive. This training should be AC¬ COMPLISHED WITHOUT DELAY! In addition, when a forecaster transfers to a new office, he(she) should be given time to visit each terminal forecast site and other FAA facilities (e.g. tower, Central Weather Service Unit, Air Route Traffic Control Center) which has responsibility for his(her) forecast area. The forecaster "needs to know" the his (her) area of forecast responsibility. RECOMMENDATION 4...Change National Weather Service Forecast Policy The National Weather Service should change the current policy in which aviation forecasts must be a reflection of the public forecasts. Aviation forecasters should be al¬ lowed to issue terminal forecasts which might at times become indepen¬ dent of public forecasts. However, coordination between the aviation and public forecaster should be common practice in any type of wea¬ ther situation. Here are some typical examples: A. When the public forecasts indi¬ cate a "20", "30", or "40" percent chance for precipita¬ tion, conditional phrases are introduced into the terminal forecasts for coordination purposes. These phrases often include forecast conditions at or below 2000 feet and 3 miles, triggering the requirement for alternate airports. In addi¬ tion, the aviation forecaster often keeps the conditional phrase in the terminal forecast for extended periods of time, when a much shorter period of time could be adequate. B. In most situations when a watch is issued by the National Se¬ vere Storm Forecast Center, the public forecasts are updated. Terminal forecasts are also amended immediately with a forecast including "chc C5 X 1/2TRW+A G50" to indicate the possibility of severe storms for that airport. In most cases, that conditional phrase is valid for the entire period the watch is in effect. Howev¬ er, such conditions rarely occur for extended periods. One final reason why terminal forecasts should become independent 160 Session 6.1 of the public forecast lends cre¬ dence to the areal forecast of re¬ sponsibility. The areal coverage for terminal forecasts is an area within five nautical miles of the airport (or about eighty square miles). In contrast, the areal coverage of a zone or local forecast is often two orders of magnitude larger. This policy should be changed quickly to give the aviation fore¬ caster the freedom to tailor the forecast to the runway complex with¬ out overstepping regional policies. RECOMMENDATION 5...Develop a Method of Aviation Quality Control Aviation products issued by NWS forecasters are rarely quality con¬ trolled, except for an occasional brief evaluation by the Lead Fore¬ caster on-station. The National Weather Service needs to establish a method of quality control of the aviation products on a regular ba¬ sis. To solve the QC problem, each Region Headquarters should institute a plan whereby the Weather Service Evaluation Officer (WSEO) would continuously monitor aviation prod¬ ucts within the region for a three- week period. The WSEO should call a particular office when a product is written incorrectly, or is not con¬ sistent with current weather or trends. Advantages of this type of quality control would: 1. Free the regional aviation meteorologist to accomplish more administrative duties and supervise the overall quality control program. 2. Provide forty weeks or more of monitoring of aviation products within each region. 3. Eliminate any possible person¬ ality conflict that can develop from continued on-station qual¬ ity control. Modernization and Restructuring will result in a host of new avia¬ tion forecasters with a very limited amount of aviation experience. A planning schedule of who will be monitoring aviation forecast prod¬ ucts should be distributed the NWS forecast offices. Aviation fore¬ casters could call that person, if a particular question arises. This will become even more necessary as the METAR code comes into its own across the U.S. Additionally, the WSEO on quality control duty might also serve as a contact point for the user industry to call to coordi¬ nate forecast problems. RECOMMENDATION 6...Establish Time for a Roundtable Discussion and Interaction at the Next Aviation Workshop Another national aviation work¬ shop should be planned for the fu¬ ture with additional percentages of AOPA, FAA, and user input. The next workshop should include roundtable discussions to give attendees a chance to interact, make sugges¬ tions, and establish action items. RECOMMENDATION 7...In-House Research Should Be Done by NWS Staff Art Hansen, of FAA Weather Research and Development, stated that FAA has contracted a local university to do a research study on ceilings and visibilities. Contract monies should be retained by the National Weather Service in which climatological studies unique to aviation can be accomplished by staff members on-station. Forecast¬ ers need statistical answers to 161 Session 6.1 meteorological questions, including the probability of ceilings at vari¬ ous heights during thunderstorm activity at airports over various regions of the United States. NWS forecasts of ceilings and visibili¬ ties are often much too conserva¬ tive, usually denoted with condi¬ tional phrases. 10. CONCLUSIONS The National Weather Service needs to take immediate and drastic measures to assure that its services will be retained as the major avia¬ tion forecasting service for our nation. These changes cannot wait for the Modernization and Restruc¬ turing phase. They must begin imme¬ diately, and a public relations effort notifying the airlines and other aviation interests of these changes should be made. At the National Aviation Work¬ shop Steve Brown of AOPA asked the question, "Do we need to attack the weather?" The answer is "Yes!" At least, the National Weather Service needs to attack aviation ignorance and create a new sense of urgency with aviation forecasting quality. Larry Sharron, meteorologist with the Atmospheric Environment Service in Ontario, Canada made a statement that everyone should re¬ member as we re-evaluate the NWS position in aviation forecasting: "If you think training is expensive, try ignorance!" The National Weather Service is spending large sums of monies to do research on future aviation fore¬ casting techniques. While this is being accomplished, the users are scattering like sheep with no shep¬ herd. NWS is the most capable orga¬ nization in the nation to issue these forecasts, and the only orga¬ nization with a federal mandate to do the job. NWS needs to show the aviation industry it is READY to make major strides to improve quali¬ ty and service, so that the users can once again rely on its products. With so many changes needed in a short time, it would take more than a handful of people at the national level can tackle these problems. Perhaps a committee of qualified people in the field could be selected to help identify prob¬ lems and implement changes in how aviation products are released to the aviation industry. Conditions are ripe for change. The National Weather Service must be an integral part of change, or it will not be a part of the new avia¬ tion industry. It is time to "at¬ tack the weather with all vigor". 11. ACKNOWLEDGEMENTS The author is grateful to Dr. Richard Livingston of SSD, Central Region and Ron Przybylinski, S00 at WSFO St. Louis, Missouri, for their many beneficial suggestions on im¬ proving the manuscript. Additional thanks goes to Steve Thomas, MIC (AM) at WSFO St. Louis, for his encouragement in this study. Final¬ ly, the author would like to acknow¬ ledge Don Eick, Manager of Meteorol¬ ogy Administration, Trans World Airlines, and Randal S. Baker, Flight Operations Trainer, United Parcel Service, for their expertise, information and unselfish interest in the science of meteorology. 12. REFERENCES Baker, R. A., 1992: EWINS (Enhanced Weather Information System) National Weather Service Avia- 162 Session 6.1 tion Workshop, December 1991, Kansas City, MO. National Archives and Records Administration, 1991: Code of Federal Regulations, Aeronau¬ tics and Space, Parts 60-139. Office of Federal Register, National Archives and Records Administration (Revised, Janu¬ ary 1, 1991). U.S. Dept, of Transportation: Flight Services Order 8400,10, Chapter 7, Para. 1401-1416. Aviation Weather Information Systems, U.S. Dept, of Trans¬ portation. Federal Aviation Administration. Jackson, R., 1984: Terminal Forecast Pitfalls. Western Region Tech¬ nical Attachment 84-07, avail¬ able from National Weather Service Western Region, Salt Lake City, UT. 163 Session 6.1 APPENDIX A APPROACH MINIMUMS AND RUNWAY CONFIGURATIONS FOR ST. LOUIS LAMBERT FIELD 1. SIMULTANEOUS VISUAL APPROACH...the most often used visual approach configuration (45 percent). On runway 30 configuration some commuter aircraft can utilize Runway 24 for arrival. This capability makes runway 30 the preferred configuration. A. Minimums.Ceiling 4,000 feet Visibility 5-6 miles Note...the actual minimums required to issue a visual approach based on FAA 7110.65 is Ceiling 1,900 feet and visibility 3 miles. However the STL Tower states that the minimums mentioned earlier are the actual minimums necessary to successfully utilize the visual approach configuration. B. Arrival runways: 12L/12R or 30L/30R Departure runways: 12L/12R or 30L/30R C. Arrival capacity (arrival-priority) Runway 30 configuration 84 per hour Runway 12 configuration 84 per hour D. Arrival capacity (departure-priority) Runway 30 configuration 60 per hour Runway 12 configuration 54 per hour E. Arrival capacity (50/50 arrival/departure mix) Runway 30 configuration 60 per hour Runway 12 configuration 54 per hour 2. STAGGERED VISUAL APPROACH...the second most often used approach (35 percent) A. Minimums.ceiling 3,500 feet Visibility 3 to 4 miles B. Arrival capacity (arrival-priority) Runway 30 configuration 60 per hour Runway 12 configuration 54 to 60 per hour C. Arrival capacity (departure-priority) Both 12 and 30 configuration 54 per hour D. Arrival capacity (50/50 arrival/departure mix) Both 12 and 30 configuration 54 per hour 3. ILS/LDA APPROACH...most often used instrument approach configuration (10 percent of total flights in this configuration). Additional staffing required. 164 Session 6.1 A. Minimums.ceiling 1,200 feet Visibility 4 miles Arrival runways ILS 12R LDA/DME 12L or ILS 30R LDA/DME Departure runways 12L/12R or 30L/30R...Aircraft separation should be: on runway 12, 4350 feet and runway 30, 4600 feet. B. Arrival capacity (arrival-priority) 84 per hour C. Arrival capacity (departure-priority) 84 per hour. D. arrival capacity (50/50 arrival/departure mix) 60 per hour. 4. STAGGERED ILS...THE LOWEST NON -CAT 2 APPROACH...the second most-often used instrument approach configuration (5 percent of total flight operations are done in this configuration). A. Minimums.Ceilings 200 feet Visibility 1800 RVR B. Arrival capacity (arrival-priority) 42 per hour C. Arrival capacity (departure-priority) 36 per hour D. Arrival capacity (50/50 arrival/departure mix) 42 per hour Note, due to noise sensitivity of Runway 24 the tower does not normally use it for jet arrivals. This can, at times, reduce arrival capacity in a Runway 30 approach configuration. 165 Session 6.1 APPENDIX B Airline Critical Minimums at St. Louis FILING WITHOUT ALTERNATES Visual Approach Staggered Visual Approach ALTERNATE MINIMUMS REQUIRED Observed or Forecast ILS/LDA APPROACH VFR ATC INITIATES DELAY PROGRAMS IF: 4000 feet, 5 miles 3500 feet, 3-4 miles 2000 feet, 3 miles +/- 1 HR OF ETA 1200 feet, 5 miles on Runway 30 feet, 4 miles on Runway 12 1000 feet, 3 miles 800 feet, 3 miles ALTERNATE MINIMUMS (AT ETA AT ALTERNATE AIRPORT) STL 400 feet, 1 mile MCI 400 feet, 1 mile COU 600 feet, 1 1/2 or SGF 600 feet, 1 1/4 or Other airports, commonly sometimes LANDING MINIMUMS Staggered ILS, NON-CAT II CAT II CAT III (Ceiling does not control TAILWIND 1 3/4 miles 1 3/4 miles 400'-1 to 600'-2. 800'-2 to 1000'-3 200 feet, 1/2 mile 100 feet, 1/4 mile or RVR 12 50 feet, 1/4 mile or RVR 6 for landing minimums) Maximum 10 Knots CROSSWIND Maximum crosswind component 20-35 Knots Under CAT II conditions, Maximum is 10 Knots PRECIPITATION RESTRICTIONS ZL+, ZR OR ZR+, S OR S+ IF WET, TRW OR TRW+ (Cannot takeoff OR land with the above precipitation conditions) 166 Session 6.1 APPENDIX C Excerpts from the Code of Federal Regulations Appropriate regulations which have been referenced in this paper are included in this section. Some portions of each regulation may not be included in interest of brevity, or because that portion of the regulation does not apply to this paper. 1. Federal Aviation Regulation 121.613 DISPATCH OR FLIGHT RELEASE UNDER IFR OR OVER THE TOP "Except as provided in 121.615, no person may dispatch or release an aircraft for operations under IFR or over-the- top, unless appropriate weather reports or forecasts, or any combination thereof, indicate that the weather conditions will be at or above the authorized minimums at the estimated time of arrival at the airport or airports to which dispatched or released." 2. Federal Aviation Regulation 121.615 DISPATCH OR FLIGHT RELEASE OVER WATER: FLAG AND SUPPLEMENTAL AIR CARRIERS AND COMMERCIAL OPERATORS Section A..."No person may dispatch or release an aircraft for a flight that involves extended overwater operation unless appropriate weather reports or forecasts or any combination thereof, indicate that the weather conditions will be at or above the authorized minimums at the estimated time of arrival at any airport to which dispatched or released or to any required alternate airport." 3. Federal Aviation Regulation 121.617 ALTERNATE AIRPORT FOR DEPARTURE "If the weather conditions at the airport of takeoff are below the landing minimums in the certificate holder's operation specifications for that airport, no person may dispatch or release an aircraft from that airport unless the dispatch or flight release specifies an alternate airport located within the following distances from the airport of takeoff: ...for an aircraft having two engines...not more than one hour from the departure airport at normal cruising speed in still air with one engine inoperative. ...for an aircraft having three or more engines...not more than two hours from the departure airport at normal cruising speed in still air with one engine inoperative." 4. Federal Aviation Regulation 121.619 ALTERNATE AIRPORT FOR DESTINATION: IFR OR OVER THE TOP: DOMESTIC AIR CARRIERS 167 Session 6.1 "No person may dispatch an airplane under IFR or over-the-top unless he lists at least one alternate airport for each destination airport in the dispatch release. When the weather conditions forecast for the destina¬ tion and first alternate airport are marginal, at least one additional airport must be designated. However, no alternate airport is required if for at least 1 hour before and 1 hour after the estimated time of arrival at the destination airport the appropriate weather reports or forecasts, or any combination of them, indicate... 1. The ceiling will be at least 2,000 feet above the airport elevation, and 2. Visibility will be at least 3 miles. For the purposes of the first paragraph of this regulation, the weather conditions at the alternate airport must meet the requirements of FAR 121.625. No person may dispatch a flight unless he lists each required alternate in the dispatch release." 5. Federal Aviation Regulation 121.621 ALTERNATE AIRPORT FOR DESTINATION: FLAG CARRIERS "No person may dispatch an airplane under IFR or over-the-top unless he lists at least one alternate airport for each destination airport in the dispatch release, unless... 1. The flight is scheduled for not more than 6 hours and, for at least 1 hour before and 1 hour after the estimated time of arrival at the destination airport, the appropriate weather reports or forecasts, or any combination of them, indicate the ceiling will be: (a) At least 1,500 feet above the lowest circling MDA, if a cir¬ cling approach is required and authorized for that airport; or, (b) At least 1,500 feet above the lowest published instrument approach, or 2,000 feet above the airport elevation, whichever is greater; and (c) The visibility at that airport will be at least 3 miles, or 2 miles more than the lowest applicable visibility minimums, whichever is greater for the instrument approach procedures to be used at the destination." 6. Federal Aviation Regulation 121.625 ALTERNATE AIRPORT WEATHER MINIMUMS "No person may list an airport as an alternate in the dispatch or flight release unless the appropriate weather reports or forecasts, or any combination thereof, indicate that the weather conditions will be at or above the alternate weather minimums specified in the certificate holder's operation specifications for that airport when the flight arrives." 168 7. Session 6.1 Federal Aviation Regulation 121.629 OPERATION IN ICING CONDITIONS "No person may dispatch or release an aircraft, continue to operate an aircraft en route, or land an aircraft when in the opinion of the pilot in command or aircraft dispatcher, icing conditions are expected or met that might adversely affect the safety of the flight. No person may take off an aircraft when frost, snow, or ice is adhering to the wings, control surfaces, or propellers of the aircraft. 169 Session 6.2 EWINS (ENHANCED WEATHER INFORMATION SYSTEM) Randal S. Baker United Parcel Service Flight Operations Training 725 Beanblossom Road Louisville, KY 40213 1. INTRODUCTION Within the past two years, the Federal Aviation Administration has introduced to commercial aviation something called EWINS, which stands for Enhanced Weather Information System. It is a system for gather¬ ing, evaluating, and disseminating weather data, including the authori¬ ty to issue weather forecasts for the control of flight movements. EWINS helps an airline to make quick, flexible, and operationally efficient responses to changing meteorological conditions. Recent interpretations of the Federal Aviation Regulations would prohibit an airline from operating a flight to an airport that had a terminal forecast with a "SLGT CHC" of conditions below landing mini- mums, regardless of the current conditions or trends. From the perspective of the users, it often seems that the National Weather Service uses conditional language in terminal forecasts in lieu of fre¬ quent updates. This can and does severely hamper airline operations, even when actual observations show that the weather is well above mini- mums. With an EWINS, any EWINS-certi- fied dispatcher or meteorologist can use current observations, trends, and their meteorological knowledge and experience to issue a forecast for a flight. Historically, a num¬ ber of airlines have been allowed to let Dispatchers make their own fore¬ casts, so this is nothing new. What is new is the specific framework whereby airlines can do this, namely EWINS. Before getting into the specifics, here is some background information regarding how airlines and other commercial aviation opera¬ tors have to use available weather information. 2. BACKGROUND INFORMATION Federal Aviation Regulation 121.613 is one of several regula¬ tions that have similar language regarding the use of weather reports and forecasts. "...NO PERSON MAY DISPATCH OR RELEASE AN AIRCRAFT FOR OPERA¬ TIONS UNDER IFR OR OVER-THE- TOP, UNLESS APPROPRIATE WEATHER REPORTS OR FORECASTS, OR ANY COMBINATION THEREOF, INDICATE THAT THE WEATHER CONDITIONS WILL BE AT OR ABOVE THE AUTHO¬ RIZED MINIMUMS AT THE ESTIMATED TIME OF ARRIVAL AT THE AIRPORT OR AIRPORTS TO WHICH DISPATCHED OR RELEASED." Historically, different air¬ lines and commercial operators have had different interpretations on just how this regulation was to be applied. Even within the FAA, dif¬ ferent FAA inspectors have had dif¬ fering interpretations. Thus, some airlines were allowed to modify forecasts based on current observa¬ tions and trends; others were not. 170 Session 6.2 The FAA has recently issued an Inspector's Handbook in an attempt to standardize government policies. Here is what it has to say about this FAR: "THE FAA'S OFFICE OF CHIEF COUNSEL HAS CONSISTENTLY INTER¬ PRETED THESE REGULATIONS TO MEAN THAT THE WORST WEATHER CONDITION IN ANY OF THE REPORTS OR FORECASTS USED TO CONTROL A FLIGHT MOVEMENT IS THE CONTROL¬ LING FACTOR." "THESE INTERPRETATIONS MAKE THE REMARKS PORTION OF A FORECAST AS OPERATIONALLY SIGNIFICANT AS THE MAIN BODY OF THE FORECAST." "THEREFORE, IT IS FAA POLICY THAT THE WORST WEATHER CONDI¬ TION IN THE MAIN BODY OR THE REMARKS PORTION OF A TERMINAL FORECAST, AS WELL AS ANY WEATH¬ ER REPORT USED, IS THE CONTROL¬ LING FACTOR WHEN SELECTING A DESTINATION OR ALTERNATE AIR¬ PORT." (Underline mine) 2.1 Impact On Terminal Weather Forecasts This means that any terminal forecast that uses conditional lan¬ guage such as CHC, SLGT CHC, or OCNL followed by zero visibility can shut down an airport. All airlines, even with the most advanced equipment, require at least some visibility above zero to land. But with a forecast of below minimums visibili¬ ty, no airplane can legally begin the flight. Some of the larger airlines have meteorology staffs that make their own forecasts, but the rest would normally have to cancel, de¬ lay, or divert a flight to that particular destination. 2.2 Weather Information System The FAA requires that all com¬ mercial airlines and operators have a Weather Information System for gathering and disseminating meteoro¬ logical data. The FAA Inspector's Handbook goes on to list the basic requirements of this system. Alpha¬ numeric data would include such things as Terminal Forecasts, Winds and Temperatures Aloft, Surface Observations, Notices to Airmen, AIRMETs, SIGMETs, Convective SIGMETs, Pilot Reports, Center Wea¬ ther Advisories, and Radar Reports. The basic required weather charts would include the Surface Analysis and Surface Progs, Radar Summaries, the Severe Weather Out¬ look, Wind and Temperature Progs at various Flight Levels, Weather De¬ piction charts, the Freezing Level chart, Constant Pressure charts, the High Level Significant Weather Prog, and the Tropopause Height/Vertical Wind Shear chart. 2.3 Adverse Weather Phenomena Re¬ porting and Forecasting Subsystem Passenger-carrying airlines have some additional requirements so as to avoid such things as thunder¬ storms, clear air turbulence, and low altitude windshear. The FAR's require commercial passenger-carry¬ ing airlines to have an FAA-approved Adverse Weather Phenomena Reporting and Forecasting Subsystem. The FAA Inspector's Handbook says: "THESE SUBSYSTEMS MUST INCLUDE FORECASTING ABILITIES WHICH ARE AT LEAST EQUAL IN CAPABILITY TO GOVERNMENT WEATHER SYSTEM FORE¬ CASTING ABILITIES." 171 Session 6.2 3. EWINS Those are the basic require¬ ments on commercial operators. EWINS has requirements over and above the basics just mentioned. EWINS is not mandatory, but any airline or commercial operator can apply for EWINS certification if they wish to gain the advantages that it can bring. To qualify for an Enhanced Weather Information System, there must be advanced technical capabili¬ ties. This could include such things as dial-up radar, infrared and visible satellite imagery, and lightning detection. The FAA must approve such a system before it can be used as an EWINS. Also, it must handle ordinary weather conditions as accurately as adverse weather phenomena. The FAA Inspector's Handbook says: "AN EWINS USES REPORTED AND FORECAST WEATHER CONDITIONS NOT ONLY TO AID IN CONTROLLING DAILY FLIGHT MOVEMENTS, BUT ALSO TO PERMIT SHORT AND LONG¬ TERM OPERATIONAL PLANNING FOR ENHANCING AN OPERATOR'S CAPA¬ BILITY TO PROTECT SCHEDULES AND TO USE EQUIPMENT AND PERSONNEL WITH MAXIMUM EFFICIENCY." The basic idea is to help an airline make quick, flexible and operationally efficient responses to changing meteorological conditions. Here are some quotes taken from the FAA's Inspector's Handbook, regard¬ ing the use of EWINS. "FLIGHT MOVEMENT FORECASTS (FMF) ARE OFFICIAL WEATHER FORECASTS WHICH CONTROL SPECIF¬ IC FLIGHT OPERATIONS FOR A PARTICULAR OPERATOR." "AN AVIATION METEOROLOGIST OR A DISPATCHER WITH FLIGHT MOVEMENT FORECAST AUTHORITY MUST CONTIN¬ UOUSLY BE ON DUTY WHEN ANY FLIGHT OPERATIONS ARE IN PROG¬ RESS." "PROPERLY TRAINED AND QUALIFIED AVIATION METEOROLOGISTS AND DISPATCHERS WITH FMF AUTHORITY WHO OPERATE AN EWINS MAY BE AUTHORIZED TO PREPARE AND ISSUE FLIGHT MOVEMENT FORECASTS." "BASED ON CONCLUSIONS DERIVED FROM EWINS DATA, AUTHORIZED PERSONNEL MAY PREPARE AND ISSUE FLIGHT MOVEMENT FORECASTS... TO CONTROL FLIGHT MOVEMENTS." Further requirements include an EWINS Policy and Procedure Manual, a training program with at least the minimum specified curricula, quality assurance procedures, work facili¬ ties and equipment, and back-up capabilities to provide uninterrupt¬ ed operation should any single com¬ ponent of the system fail. UPS received EWINS approval in December of 1989, becoming the very first airline to operate under EWINS. Since that time at least 3 other airlines have become EWINS- approved, and several more have requested approval. As of December, 1991, UPS has a total of 16 individuals who are authorized to issue Flight Movement Forecasts. 15 are Dispatchers, some of whom have over 20 years of expe¬ rience. Collectively, they average 11.5 years of Dispatching experi¬ ence. In addition, UPS has a Meteo¬ rologist who has 8 years of profes¬ sional experience as a forecaster and meteorology instructor. 172 Session 6.2 4. EXAMPLES Following are two examples of Flight Movement Forecasts and how they are used at UPS. First is a recent example of a flight operating from Louisville to Atlanta. The flight is scheduled to arrive at 1530 UTC, and the required landing minimums at Atlanta are 1/2 mile visibility. ATL FT 140808 C5 BKN 50 OVC 4RW- OCNL -X 5 SCT C12 BKN 2F CHC C2 X 1/2L-F. 12Z C3 OVC 2F OCNL 1RW- CHC Cl X 1/4L-F. 16Z C12 BKN 5F CHC 2TRW. 21Z 25 SCT C80 BKN CHC C25 BKN 3TRW. 02Z MVFR CIG F.. ATL RS 0950 Mil BKN 35 OVC 4R-F 158/71/70/1604/003 ATL RS 0850 5 SCT M70 OVC 4R-F 158/71/70/3006/003/ 70704 172/ Note that the forecast at ETA (Estimated Time of Arrival) calls for 2 miles in fog with occasional 1 mile in light rainshowers and a chance of 1/4 mile in light drizzle and fog. That chance term puts it below legal minimums. Legally, without EWINS this flight would have cancel, delay, or reroute to another airport based on this forecast. However, the current observa¬ tion for Atlanta shows the visibili¬ ty holding at 4 miles in light rain and fog. The Dispatcher looked at the trend over the past several hours at Atlanta and surrounding stations, along with other meteoro¬ logical information. He concluded that 1/4 mile would not occur at the ETA, so he decided to issue a Flight Movement Forecast, as shown. The Dispatcher writes in the Flight number and date, along with the destination city code. A valid period is listed, along with his forecast that visibilities would be no worse than 1/2 mile. This is now a legal forecast for the operation of this flight to Atlanta. FLIGHT MOVEMENT FORECAST (FMF) FLIGHT NUMBER/DATE: PT 3l/lH Al/6 CITY: ATL_ ISSUED BY: ^CHEITER VALID TIME: 1300- IkCG Z FLIGHT MOVEMENT FORECAST: C 3 OVC 2P CCAjl- I CHC C2 X 1/2 l-F TERM TIMES: 1538 / Z NOTE: ATTACH THE FOLLOWING INFO : 1) NATIONAL WEATHER SERVICE FORECAST 2) A.R.T.R. 3) SEQUENCE WEATHER ...PLEASE FORWARD TO FLIGHT CONTROL SHIFT MANAGER... For documentation purposes, the Dispatcher must attach to this Flight Movement Forecast the Nation¬ al Weather Service forecast, the ARTR (Amend Release to Read), and the actual weather observations. These are retained for 90 days. Here is the ARTR that the Cap¬ tain received from the Dispatcher that contained the Flight Movement Forecast. *t***t*St**t FLIGHT RELEASE AMENDMENT I « < > > I t t * I t t CAPTAIN: ATTACH TO THE FLIGHT RELEASE OF FLT UPS2931/14 0NT-ATL ARTR-1 ATL FMF 12Z C3 OVC 2F OCNL JRW- CHC C2 X 1/2L-F TIL 16Z.. ISSUED BY SCHEITER BALANCE RELEASE SAME. FLIGHT CONTROL DISPATCHER/DUNN /1018Z NOTICE: THIS ARTR VALID ONLY WITH CAPTAIN'S CONCURRENCE. FLIGHT DISPATCHER MUST BE ADVISED OF CAPTAIN'S CONCURRENCE PRIOR TO DEPARTURE OF FLIGHT. A Flight movement forecast is all that is legally required to operate a flight. However, UPS goes one step further by issuing an ARTR, which means "Amend Release To Read." This allows the Captain a voice in the decision so that the Captain must indicate concurrence before the 173 Session 6.2 flight is allowed to operate. If the Captain does not concur, the flight does not operate. Here is what actually happened. The flight landed with visibilities well above landing minimums, as reported visibilities were between 2 and 2 1/2 miles. ATL SA 1451 8 SCT M65 OVC 21/4VR-F 173/69/65/2007/007/ VSBY 2V2 1/2/21052 172/ Here is another example for Ontario, California. Again, landing minimums are 1/2 mile visibility. At 1207 UTC Estimated Time of Arriv¬ al, the forecast calls for 1/2 mile in fog and haze with occasional zero visibility. 0NT FT 040202 CLR. 12Z C2 X 1/2FH OCNL CO X OF. 16Z -X 3FH. 20Z MVFR H.. 0NT SA 0546 CLR 10 67/60/0000/991 ONT SA 0451 CLR 20 68/59/2308/990 ONT SA 0346 CLR 20 74/59/2910/989 However, the current observa¬ tions show rather high visibilities, and in looking over the situation, the Dispatcher did not think there was going to be a problem at ETA. Here is the Flight movement forecast she issued, valid from 1100 to 1300 UTC. FLIGHT MOVEMENT FORECAST (FMF) FLIGHT HUMBER/DATE: /JfS HU h crW CITT: /Wr - ISSUED BY: C /Vracv _ VALID TIME: Z FLIGHT MOVEMENT FORECAST: ftf _ Here's what really happened. The flight landed with the following weather report: 5. EWINS VERIFICATION How well has EWINS worked over the last 2 years? Here are the statistics for the first two years of operation. It shows month by month the number of Flight Movement Forecasts issued. Not as many Flight Movement Forecasts were is¬ sued during the summer months when weather is generally better than during the rest of the year. At the bottom in parentheses is the number each month that were inaccurate. An inaccurate Flight Movement Forecast is defined as one in which the flight did not have landing minimums when it arrived over the destination so that it could not land. 1990 EWINS FLIGHT MOVEMENT FORECASTS 100 90 80 70 60 50 40 30 20 10 0 39 4 - 27 25 * ISSUED * INACCURATE 13 ad 1 B El P. ihiinWl JFMAMJJASOND 1991 EWINS FLIGHT MOVEMENT FORECASTS ONT SA 1147 CLR 7 66/50/000/984/H ALQDS 174 Session 6.2 It should be noted that EWINS does not give blanket authority to modify NWS forecasts. The person issuing the Flight Movement Forecast must be able to differentiate be¬ tween those situations that really could go below minimums versus those that are very unlikely to go below minimums. Whenever the Dispatcher agrees with the NWS forecast that the weather could go below minimums, no Flight Movement Forecast is is¬ sued. However, if based on current observations and trends the Dis¬ patcher believes the weather will stay above minimums, a Flight Move¬ ment Forecast is issued and its accuracy verified as part of UPS's quality assurance program. 6. SUMMARY Summing up the first two years of EWINS operation at UPS, a total of 843 Flight Movement Forecasts were issued. Of those, 811 were accurate, giving an accuracy rate of 96.2%. This represents 811 flights that were not delayed, canceled, or rerouted to another airport, and were able to operate normally due to EWINS. Looking at this from a busi¬ ness standpoint, this represents over 4 million packages that other¬ wise might not have been delivered on time. EWINS allows UPS to plan flight operations with weather information that is more current and more accu¬ rate, allowing the completion of more arrivals on time with no com¬ promise of safety. EWINS provides UPS the capability to better meet the needs of its customers, enhanc¬ ing UPS's reputation for being the "tightest ship in the shipping busi- 175 Session 6.3 PRELIMINARY RESULTS OF THE ENHANCED TERMINAL FORECAST RISK REDUCTION AT DENVER, COLORADO Lynn P. Maximuk National Weather Service Central Region Transition Program Manager Kansas City, Missouri 1. INTRODUCTION The requirement for up-to-date, high resolution, short-term aviation forecasts for use in aviation termi¬ nal operation has long been recog¬ nized. Terminal forecast users have indicated that they require fore¬ casts to be issued more frequently with higher resolution in the short- range (0-6 hours) portion of the forecast. Users have also indicated that amendment criteria should re¬ flect operational procedures and critical values whenever possible. Several segments of the aviation community have also expressed a need for specific element forecasts for the full period covered by aviation terminal forecasts. It is believed that new tech¬ nology, which will be available at Weather Forecast Offices (WFOs) after the Modernization and Associ¬ ated Restructuring (MAR) of the National Weather Service, will pro¬ vide forecasters with the opportuni¬ ty to provide the services discussed above. To evaluate the risks asso¬ ciated with providing these servic¬ es, a risk reduction activity was conducted at WSFO Denver, Colorado, during the period January 16, 1991 through November 26, 1991. WSFO Denver was chosen as the location for the activity due to the high temporal and spacial observational data sets and advanced processing and display capabilities of the DAR3E-II system in use at the sta¬ tion. The office also has access to doppler radar information from the Mile High radar. This paper will briefly discuss some preliminary results of the risk reduction. A complete report on the results of the activity will be completed by spring 1992. 2. OBJECTIVE OF THE RISK REDUCTION There were several objectives to be achieved during the risk re¬ duction. They were: evaluate fore¬ caster workload, evaluate advanced workstation capabilities as they related to the EFT, determine value added by the EFT, evaluate user perception of the EFT, and evaluate the effect of change in amendment criteria. 3. TRAINING All forecasters at WSFO Denver had training and experience working on the DAR3E-II workstation before the beginning of the risk reduction. All of the forecasters were experi¬ enced in issuing terminal forecasts, however several of the journeyman forecasters were relatively new to the Denver area at the beginning of the risk reduction. All forecasters also had training on interpretation of the doppler radar data before the activity began. The forecasters were provided with guidelines on the format, content, and amendment cri¬ teria for the EFT and issued prac¬ tice forecasts before the risk re¬ duction began. Before the activity started the DMIC at Denver certified 176 Session 6.3 that the staff was sufficiently prepared to issue EFTs. 4. FORMAT OF THE RISK REDUCTION WSFO Denver staffed five avia¬ tion forecast shifts per day, pro¬ viding aviation forecast double coverage for 16 hours. Between the hours of 10:00 p.m. and 6:00 a.m., one aviation forecaster issued both the conventional and enhanced termi¬ nal forecasts. During the rest of the day one forecaster was responsi¬ ble for the conventional forecasts, while the other concentrated on only the EFTs. The intent of this work distribution was to provide the EFT forecaster with an environment in which it was possible to determine the exact workload, equipment, and data sets required to produce the EFTs. The structure of the EFT was somewhat different from conventional terminal forecasts. The changes were made to attempt to better sat¬ isfy identified terminal forecast user requirements. Some of the major changes in structure are list¬ ed below. The first three hours of the EFT were specifically forecast with¬ out the use of probability terms. To provide increased temporal reso¬ lution during hours 0 through 3 in the EFT, forecasters were encouraged to indicate forecast change groups to the nearest 15 minutes. The changes were made in an attempt to get forecasters to provide more specific short range forecasts with less variable terms and wide range forecasts in that time period. Hours 3 through 24 were specif¬ ically forecast and the use of con¬ ditional terms was permitted during that time period. It should be noted that this was a change from the conventional FT in which hours 18 through 24 are an outlook rather than a specific forecast. Change groups (new forecast periods) were added to the EFTs when significant changes in forecast conditions were expected to occur. Each forecast change group began on a new line. The amendment criteria for the EFT was adjusted to reflect observed or forecast changes which directly affect aircraft operations. Fore¬ casts were amended when the observed or forecast weather changed catego¬ ry, i.e., IFR to LIFT, etc. An additional amendment threshold was added for ceilings of 200 feet and/or visibilities of one-half mile since those values are representa¬ tive of minimums for operations at many airports. 5. ISSUANCE TIMES AND VALID PERIODS Core EFTs were valid for a period of 24 hours and issued one- half hour before the beginning of the valid period. The valid periods during MST began at 05 UTC, 11 UTC, 17 UTC, and 23 UTC. Scheduled up¬ dates of the EFT were required when¬ ever one of the following conditions occurred: the current core forecast had a first period less than three hours long (this eliminated old forecasts for time which had already passed), a probability term was used before the sixth hour of the core forecast, or expected weather condi¬ tions differed from what was fore¬ cast. The ending time of the fore¬ cast period was not extended at the scheduled or non-scheduled update times. 177 Session 6.3 6. PRELIMINARY OBSERVATIONS While a full analysis of the risk reduction results has not yet been completed, some preliminary observations are presented here. A. Forecaster Workload Interviews with forecasters involved in the risk reduction indi¬ cated that the workload associated with the three EFTs in the Denver area was manageable. On quiet wea¬ ther days the forecasters had a considerable amount of free time which was utilized for professional development and research work. On rapidly changing weather days the forecaster was pressed to keep up with the forecast updates. It was the consensus of the forecasters that the maximum EFT workload which would be handled by one forecaster with a DAR3E-II type workstation would be three to six terminals. B. Workstation Capabilities Integration of data sets was critical to providing the resolution desired in the EFT. The DAR3E-II workstation provided this ability. The forecasters were universal in their stated need for two graphic and one alphanumeric screens for support of the EFT effort. The perception of the forecasters was that the most frequently used prod¬ ucts in the EFT activity were the doppler radar reflectivity and ve¬ locity displays, the meso-net sur¬ face observations, and the satellite graphics. It was interesting that the meso-net surface observations were one of the most frequently used tools in this activity. C. Scientific Value-Added by the EFT Preliminary verification re¬ sults for March and April 1991 showed that the Denver forecasters did not add to the accuracy of the EFT forecasts. Verification numbers were no better, and possibly a bit worse than previous samples for the same terminals in previous years. There are several possible reasons for these early results. Many of the forecasters were new to the Denver area and may not have been familiar with the local topographi¬ cal effects. It is also possible that in trying to add resolution to the forecasts, and eliminate the hedging with a lack of probability terms, the forecasters may have missed more forecasts than when they were permitted more frequent use of variability and probability terms. It will be interesting to see wheth¬ er this trend continues when the final verification is completed. Going into the risk reduction there was a great deal of discussion concerning the ability of the fore¬ casters to provide specific fore¬ casts for the 18 to 24 hour time period. Interviews with the fore¬ casters indicated that most fore¬ casters felt that they did have skill in providing those specific forecasts. While the skill was not perceived at the same level as short range forecasts, the forecasters did feel that they could add value to the current outlook forecast by providing specific forecasts. The forecasters also felt that there was value added to the termi¬ nal forecasts by the more frequent issuances, and the required updates. They felt that these requirements forced them to make changes to the forecasts that didn't require updat- 178 Session 6.3 ing due to the amendment criteria. In many instances these changes, due to observed or changed forecast trends, made the forecasts more up- to-date. D. User Perception of the EFT Preliminary user perceptions were obtained through interviews with several FAA Flight Service Station employees in Denver, and from comments received from other Flight Service Stations and private pilots. Virtually all of the early user comments with respect to the EFT were positive. The number one positive comment was concerning the format of the EFT. Almost all peo¬ ple interviewed, and the written comments, stated that the users liked the new format with each change group beginning on a new line. This made the product easier to user. In general, the forecast¬ ers shared this view stating that the new format made updating easier, and that the EFTs were easier to read. The users, especially the FAA personnel, were very pleased with the 18 to 24 hour specific forecast. They felt that the accuracy of that forecast period was adequate to fill their needs. The briefers stated that this aspect of the EFT was more important to them than the specific¬ ity in the early part of the fore¬ cast since much of their workload is in providing planning outlooks. Several briefers stated that they felt much more comfortable with the NWS 18 to 24 hour forecast than they did in making their own interpreta¬ tion of the conventional categorical outlook forecast. All early user comments indi¬ cated satisfaction with the more frequent issuances of the EFTs. For briefing purposes this provided FAA personnel with longer forecast peri¬ ods. With conventional terminal forecasts there are times when the period covered by a forecast drops to 15 hours, while with the EFTs this period never drops to less than 18 hours and frequently is 21 hours or more. Contrary to early verification statistics, the users perceived the EFTs to be more accurate than the conventional FTs. Its quite likely that the more frequent issuances and scheduled updates led to this per¬ ception. The users felt that fore¬ casters were issuing better amend¬ ments and that the forecasts were more representative of the weather. They noticed the greatest improve¬ ment in perceived accuracy during the early periods of the forecasts when there were less variability and probability terms. It is interest¬ ing that while the NWS was unable to demonstrate a significant improve¬ ment in forecast skill, some user segments perceived the forecast product to be better. It remains to be seen if the major airline opera¬ tions people share this perception of improved accuracy as it relates to their short range planning activ¬ ities. All early user comments indicated that the accuracy of the 18 to 24 hour forecast was suffi¬ cient to satisfy their needs. E. Effects of Change in Amendment Criteria The early reviews of the change in amendment criteria were mixed. The users felt that the changes were good, and led to forecasts being more useful operationally. Some comments were made that amendment criteria for very low ceilings and visibilities should be even more strict, possibly being site specific 179 Session 6.3 based upon local airport minimums. The forecasters were very apprehen¬ sive about the new criteria at the beginning of the risk reduction. As the activity progressed some of that apprehension disappeared as fore¬ casters became more comfortable with the change in criteria. However, several forecasters were unhappy with the amendment criteria in the lower visibility and ceiling range. They felt that they did not have the skill necessary to meet the require¬ ments, especially in the range of ceilings below 500 feet. It will be interesting to see if verification results bear out the forecasters concerns. This aspect of the EFT needs to be explored further to fit the user needs more with the fore¬ casters measured abilities. 7. SUMMARY AND CONCLUSIONS Preliminary observations from the EFT risk reduction indicate that the NWS is moving in the right di¬ rection in trying to better serve the users through the EFT. Some adjustments in the concept will be necessary as Modernization and Asso¬ ciated Restructuring (MAR) planning continues. The risk reduction also showed that some changes leading to better user services in the terminal forecast program may be possible before the MAR is completed. Chang¬ es in forecast format, with change groups beginning on a new line, increased number of issuances, and specific 18 to 24 hour forecasts may all be possible in the current NWS environment. It may also be possi¬ ble to adjust the amendment criteria to better fit user operational needs. However, significant im¬ provements in spacial and temporal forecast resolution will have to wait until advanced observational and workstation equipment and im¬ proved guidance is available. 180 Session 7.1 AVIATION HAZARD IDENTIFICATION USING DOPPLER RADAR Michael D. Eilts National Severe Storms Laboratory Norman, Oklahoma 1. INTRODUCTION The Federal Aviation Adminis¬ tration (FAA) is in the process of deploying their Terminal Doppler Weather Radar (TDWR) system at 47 airports in the continental United States (Fig. 1). At the same time, 135 WSR-88D Doppler radars are being deployed throughout the continental United States by the Departments of Commerce (DOC), Defense (DOD), and Transportation (DOT) (Fig. 2). Although these Doppler radar systems are very similar, their functional utility is quite different due to the needs of the different agencies using the system. Both of these Doppler weather radar networks will be important tools for detecting and warning of weather phenomena that are potentially hazardous to pene¬ trating aircraft. This paper will describe these two Doppler radar systems along with the automated algorithms designed to detect weath¬ er phenomena that may be hazardous to aircraft in flight. 2. TERMINAL DOPPLER WEATHER RADAR SYSTEM The principal motivation for the TDWR system is to improve air safety by warning of wind shears and precipitation in the terminal area of major airports. A 1983 National Research Council study identified low-altitude wind shear as the cause of 27 aircraft accidents and inci¬ dents, which resulted in 488 fatali¬ ties between 1964 and 1982. At least 3 other low-altitude wind shear incidents have occurred since that study, one which resulted in 137 fatalities (Evans, 1991a). Another goal of the TDWR system is to increase capacity and effi¬ ciency of operations by short-term prediction (up to 20 minutes) of wind shifts and wind shears that may impact the terminal area. The sys¬ tem has the capability of detecting, tracking, and forecasting the loca¬ tion and movement of fronts and estimating the winds behind the fronts. This information can be used by air traffic control supervi¬ sors to anticipate runway changes, as opposed to reacting to them after a wind shift occurs. To attain these goals, TDWRs will be sited between 5 and 25 km from the air¬ port, and will be sited along the extension of the principal Instru¬ ment Flight Rules (IFR) runway as much as land availability allows (Evans, 1991). The scanning strategy used by the TDWR system consists of sector volume scans of approximately 100° azimuth over the airport which up¬ date every 2.5 minutes, two low- altitude full 360° scans for gust front detection which update every 5 minutes, and a low-altitude sector scan over the airport every minute for microburst detection. Since the area of interest will generally be within 25 km of the radar, the algo¬ rithms will only ingest data out to 70 km range, ensuring coverage over the airport yet allowing for the detection of fronts that may impact the runways in 20 minutes. 181 Session 7.1 Figure 1. Planned locations of the initial 47 terminal Doppler weather radars. above site level for the contiguous United States. Hatched regions represent areas not covered below 10,000 feet. Dots repre¬ sent locations of radar towers. Figure 3. A schematic of the details fdund on a Geographical Situation Display. A microburst is displayed as a circle with the strength indicated by the number inside it. The solid curved line represents the gust front detection and the dashed lines are the 10 and 20 minute fore¬ casts. Levels of intensity of precipitation are shown in various shadings and LLWAS wind vectors are displayed for each of 12 sites. 182 Session 7.1 There are 3 main automated algorithms in the TDWR system. They are the Microburst Detection, Gust Front Detection, and Wind Shift Prediction algorithms. The FAA has invested heavily in the development and operational readiness of these algorithms because the output of the algorithms will go directly to the air traffic control users with no human intervention. Thus, the per¬ formance of these algorithms has to be very good. The Microburst Detection Algo¬ rithm identifies small-scale low- altitude divergence events which can cause an aircraft to lose a signifi¬ cant amount of airspeed while at¬ tempting to land or depart. The Gust Front Detection Algorithm de¬ tects significant fronts (i.e., wind shifts), which can cause aircraft to gain airspeed while departing or landing. Turbulence is also often associated with fronts. Finally, the Wind Shift Prediction Algorithm forecasts the location of a gust front 10 and 20 minutes in the fu¬ ture and estimates the winds on either side of the front. This information can be used by Air Traf¬ fic Control Supervisors to antici¬ pate runway shifts, rather than react to wind shifts after they have occurred. The output of these three algorithms will be displayed on a Geographical Situation Display (GSD) for air traffic control supervisors. A schematic of how the output of these algorithms will look is shown in Figure 3. Another set of runway specific output will be given to the Air Traffic Controllers on a Ribbon Display Terminal as shown in Figure 4. 3. WEATHER SURVEILLANCE RADAR - 1988 DOPPLER (WSR-88D) The WSR-88D (formerly called NEXRAD) system was developed by three agencies to meet their common operational needs. Initially, the WSR-88D system will employ four different scanning strategies: 1) two clear-air modes which complete seven or eight 360° scans between 0.5° and 4.5° elevation angle in ten minutes; 2) the precipitation detec¬ tion mode, which completes nine full 360° azimuthal scans in six minutes at elevation angles between 0.5° and 19.5°; and 3) the severe weather mode, which completes 14 full azi¬ muthal scans between 0.5° and 19.5° elevation angle. Because the WSR-88D network spacing is rather sparse in certain regions of the country, the radars will need to process data out to ranges over 300 km in some instances to be able to cover most of the continental United States. The diagram in Figure 5 shows the effec¬ tive ranges of the WSR-88D system for both the reflectivity and veloc¬ ity fields. Velocity data are col¬ lected out to 230 km in range with range resolution of 250 m and re¬ flectivity data are collected out to 460 km with range resolution of 1000 m. There are a number of automated algorithms as part of the WSR-88D system. The data flow through the different algorithms is shown in Figure 6. In total, these algo¬ rithms identify, track, and forecast the movement of storm cells, examine their reflectivity characteristics to determine the likelihood of se¬ vere weather, and examine the veloc¬ ity field to determine if there is a mesocyclone (the parent circulation of a tornado) or possible tornado 183 Session 7.1 Type of Runway wind shear Threshold winds Wind shear Headwind Location change (kts) CF 190 16 0 25 MBA 35 LD 160 22 50- RWY MBA 35 RD 180 5 25- RWY MBA 35 LA 030 23 55- 1 MF 35 RA 180 10 60- 3 MF MBA 17 LA 180 5 25- RWY MBA 17 RA 160 22 55- RWY 17 LD 180 10 60- RWY MBA 17 RD 030 23 55- RWY Figure 4. An example of the alphanumeric information shown on a Ribbon Display Terminal. A message would be read to a pilot as "Microburst Alert, Runway 35LA, Threshold wind 030 at 23, 55 knot loss, 1 mile final, centerfield wind 190 at 16, gust 25“ (from Turnbull et al ., 1989). 184 Session 7.1 RDA RPG PUP Algorithms Products 50,000 FT ANVIL sw [ - Figure 7. PREMATURE GROUND IMPACT The structure of a typical thunderstorm in the Midwest United States, and its effects on aircraft during cruise and landing. Dotted lines represent expected flight paths and solid lines are actual paths. The notation (S) indicates the likely zones of wind shear, and the wavy segments of flight paths are the result of turbulence. 185 Session 7.1 associated with the storm. In addi¬ tion, there is a series of algo¬ rithms that determine accumulated rainfall and another that profiles the winds in the atmosphere (Alberty et al., 1991). 4. AUTOMATED ALGORITHMS TO DETECT AVIATION WEATHER HAZARDS There are a number of weather phenomena that can be detected using a Doppler weather radar. These include microbursts, gust fronts and synoptic fronts, tornadoes, meso- cyclones, hail, heavy precipitation, and strong vertical shear of the horizontal wind. Most of these phenomena are associated with con¬ vective storms as is shown in the schematic in Figure 7. All of these weather phenomena are potentially dangerous to aviation. A number of automated algorithms have been de¬ veloped to detect these phenomena for the two Doppler radar systems described above. In this section, some of the important algorithms for detecting aviation weather hazards will be briefly described. And enhanced versions of the algorithms will be discussed when appropriate. The Microburst Detection Algo¬ rithm was developed for the TDWR system by the FAA. This algorithm detects low-altitude divergence signatures in Doppler radar data and also uses microburst precursor sig¬ natures aloft (e.g., convergence, rotation, or descending core aloft) to give earlier warnings of micro¬ bursts (Merritt, 1991, Campbell, 1989). A contour plot of the Dopp¬ ler velocities observed in a strong Oklahoma microburst at low-altitudes is shown in Figure 8. The Micro¬ burst Detection Algorithm examines radials of velocity data to locate runs of increasing radial velocities (which indicate radial divergence). To declare a potential microburst detection, a minimum number of runs of radial divergence have to be located near each other azimuthally, the strongest run has to be above a minimum threshold, and a minimum area has to be surpassed (Merritt, 1991). To actually declare a micro¬ burst, a potential microburst has to be declared on two consecutive scans in close proximity to each other OR a microburst precursor feature aloft has to have been detected. The Microburst Detection Algorithm has been tested on data collected near Huntsville, Alabama; Denver, Colora¬ do; Kansas City, Missouri; and Or¬ lando, Florida. The algorithm de¬ tects greater than 98% of the events with differential velocity greater than 15 m s’ and has a less than 5% probability of false alarm (Evans, 1991a). The Gust Front Detection Algo¬ rithm was also developed for the TDWR system. It detects lines of radial convergence in Doppler radar velocity fields. A contour plot of a radial velocity field associated with a strong Oklahoma gust front is shown in Figure 9. In a manner similar to the Microburst Detection Algorithm, the algorithm initially examines radials of Doppler velocity to locate runs of decreasing radial velocity (which indicate radial convergence). It then groups runs near each other into features. To declare a gust front detection, the algorithm locates features using data collected at two low-elevation angles (nominally 0.5° and 1.0°) and then vertically associates them to ensure that the detection is valid. The Gust Front Detection Algorithm detects over 80% of gust fronts that have a differential velocity >15 m s’ 1 with a probability of false alarm <6%, except in the Kansas City environment where the probability of 186 Session 7.1 Figure 8. Contoured Doppler velocity field from 9 August 1988, 1559 CST. Data were collected with the National Severe Storms Laboratory's Norman Doppler radar. Data are from approximately 100 m above ground level, and the negative velo¬ cities are towards the radar, positive away. The signature is mainly divergent which was associated with a strong micro- burst which caused wind damage west of Norman, Oklahoma. VELOCITY (M/S) Figure 9. Same as Figure 8, except for gust front con¬ vergence signature observed on 26 April 1984, 2047 CST. 187 Session 7.1 false alarms was 13% (most were caused by strong vertical wind shear associated with the low-level jet) (Hermes et al., 1990). An improved version of the Gust Front Detection Algorithm has been developed, which uses additional information observed in Doppler radar data (e.g., azi¬ muthal shear and reflectivity thin line) to help the algorithm detect a larger portion of long gust fronts (Eilts et al., 1991). The Wind Shift Prediction Algo¬ rithm relies on the Gust Front De¬ tection Algorithm to detect fronts and then it tracks them and fore¬ casts their locations 10 and 20 minutes in advance. When the algo¬ rithm tracks them (50% of fronts are tracked) it correctly forecasts their location over 95% of the time for the 10 minute forecasts and over 75% of the time for the 20 minute forecasts. A series of algorithms have been developed for the WSR-88D sys¬ tem (called the Storm Series Algo¬ rithms), that identify individual storms and create tracking informa¬ tion. Tracks indicate the past movement of thunderstorm cells and their expected movement in the next hour. When individual storms are identified, the Hail Detection Algo¬ rithm examines the reflectivity structure of the cell to determine the likelihood of hail. Enhanced versions of the Storm Series algo¬ rithms and the Hail Detection Algo¬ rithm have been developed (Witt, 1991 and Witt, 1990a). From limited testing, the enhanced version of the Hail Detection Algorithm has a Prob¬ ability of Detection (POD) of 92% with a False Alarm Ratio (FAR) of 30%. The present WSR-88D Hail De¬ tection Algorithm has a similar POD, but its FAR is 50% greater than that of the new algorithm. A Tornado Detection Algorithm has been developed for both the TDWR and WSR-88D systems (Vasiloff, 1991). Presently the TDWR system does not have a tornado detection algorithm. The WSR-88D system has a Tornadic Vortex Signature (TVS) Algorithm that relies on the Meso- cyclone Detection Algorithm to iden¬ tify regions of possible tornadoes before it is can detect tornadoes. This may not allow the WSR-88D algo¬ rithm to detect a number of smaller tornadoes which may not be associat¬ ed with mesocyclones. The Tornado Detection Algorithm, recently devel¬ oped at NSSL, identifies tornadic vortex signatures (TVS) in Doppler velocity fields independently of the detection of a larger-scale circula¬ tion. A TVS is characterized by highly localized, strong azimuthal shear. An example of a TVS from a very large tornado is shown in Fig¬ ure 10. The shaded area on the figure is the area which the algo¬ rithm declared a feature. When three features are identified, at different elevation angles from the same volume scan, the algorithm declares a tornado detection. Al¬ most all tornadoes observed with Doppler radar, at ranges less than 40 km, have TVS's associated with them. The new Tornado Detection Algorithm has a POD of 76% and an extremely low FAR. The Mesocyclone Detection Algo¬ rithm that was developed for the WSR-88D system examines radial ve¬ locity fields for azimuthal shear over several azimuths, rather than just two as does the Tornado Detec¬ tion Algorithm. An example of a single Doppler radar signature of a mesocyclone is shown in Figure 11. Recent enhancements to the WSR-88D algorithm have been made to reduce an apparent high false alarm ratio 188 Session 7.1 Figure 10. Same as Figure 8 except for Tornadic Vortex Signature observed on 22 May 1981. The hatched area is the location that the Tornado Detection Algorithm found a potential tornado. Figure 11. Same as Figure 8 except for meso- cyclone signature that was observed on 20 May 1977 with NCAR's CP-4 radar. The radar was located to the SSW of the mesocyclone. Notice that the mesocyclone signature is of larger scale than the TVS in the Figure 10 (from Brown and Wood, 1983). 189 Session 7.1 of the present Mesocyclone Detection Algorithm (Witt, 1990b). Strong vertical shear of the horizontal winds at low-altitudes can be dangerous to landing air¬ craft. Low-level jets, which occur frequently in the Central Plains, are one cause of strong vertical shears at low-altitudes. For exam¬ ple, a moderate strength low-level jet was observed with Doppler radar on May 4, 1989 near Kansas City. If an aircraft attempted to land through this jet, it would poten¬ tially lose 17 m s’ 1 airspeed in the last 500 m of descent (Figure 12) (Mahapatra and Zrnic, 1991). The Velocity Azimuth Display (VAD) Algo¬ rithm estimates the vertical profile of horizontal winds in the atmos¬ phere. An example of the output from this algorithm, running on the WSR-88D, is shown in Figure 13. 5. FUTURE PLANS The amount of data available to detect aviation weather hazards will increase greatly with the deploy¬ ment of the TDWR and WSR-88D radar networks throughout the country. The challenge is to convert these data into information that can be delivered to individuals needing it in a timely manner. Automated algo¬ rithms are an important step in this direction. Some of the algorithms developed for the TDWR and WSR-88D radar systems have been well tested and have been enhanced over a number of years. Other algorithms are in a state of infancy. It is important that all of the algorithms are eval¬ uated and enhanced in the coming years. In addition, there are plans to integrate Doppler radar data with other data sources, such as pro¬ filers; automated surface weather observing systems; aircraft-reported winds, temperature, and turbulence; and satellites, to get a complete look at weather phenomena and to forecast their movement and evolu¬ tion (McCarthy, 1991; Evans, 1991b). In addition, mesoscale models will ingest data from some of these data sources and their output will be integrated with data from an Avia¬ tion Gridded Forecast System (AGFS) (Sherretz, 1991). The FAA is pro¬ posing to use the AGFS as input to Regional and National Aviation Wea¬ ther Products Generators which will generate products specifically tai¬ lored to aviation users (McCarthy, 1991). All of the new weather sen¬ sors and the new systems being de¬ veloped to use the increased amounts of data will increase safety and increase the operational capacity of the airspace system. 6. CONCLUSIONS The deployment of the Terminal Doppler Weather Radar and WSR-88D (NEXRAD) radar networks during the next few years, throughout the con¬ tinental United States, will allow the detection and prediction of aviation hazards that was not possi¬ ble before now. Because of the large amounts of data that are col¬ lected, the short life-time of some of the hazardous weather phenomena, and the necessary timeliness of information dissemination, it is important to use automated algo¬ rithms to detect the hazards. It is also important to automatically send the information to the user, whether it be the pilot or air traffic con¬ trol personnel. Further applied research using Doppler weather radars is needed to produce better algorithms in the future. In addition, all of the present algorithms should be refined 190 Session 7.1 WIND SPEED VARIATION THROUGH MAY 4. 1989 LOW LEVEL JET AT 3 AND 6 DEGREES ELEVATION ANGLE Figure 12. Wind speed profiles during a low-level jet observed on 4 May 1989 near Kansas City using the Lincoln Laboratory's FL-2 radar. The vertical profile (a) shows a sharp peak at about 500 m altitude. An aircraft attempting to take off (at a 6° angle) or land (along a 3° qlideslope) would experience the wind variations shown in (b). Figure 13. WSR-88D Velocity Azimuth Display (VAD) wind profile product for data collected in Oklahoma City, Oklahoma, on September 20 1990. "ND" indicates insufficient data to make an estimate at that height. 191 Session 7.1 to increase their skill and further increase the safety of air travel in the United States. 7. REFERENCES Alberty, R., T. Crum, and F. Toeper, 1991: The NEXRAD Program: Past, Present, and Future; A 1991 Perspective, Preprints, 25— International Conference on Radar Meteorology (Paris, France), Amer. Meteor. Soc., pp. 1-8. Wood, V.T. and R.A. Brown, 1983: Single Doppler Velocity Signa¬ tures: An Atlas of Patterns in Clear Air/Widespread Precipita¬ tion and Convective Storms, NOAA Technical Memorandum ERL NSSL-95, 71 pp. Campbell, S.D., 1989: Use of Features Aloft in the TDWR Microburst Recognition Algo¬ rithm, Preprints, 24— Confer¬ ence on Radar Meteorology (Tallahassee, FL), Amer. Mete¬ or. Soc., pp. 167-170. Eilts, M.D., S.H. Olson, G.J. Stumpf, L.G. Hermes, A. Abrevaya, J. Culbert, K.W. Thomas, K. Hondl, and D. Klingle-Wilson, 1991: An Im¬ proved Gust Front Detection Algorithm for the TDWR, Pre¬ prints, Fourth International Conference on Aviation Weather Systems (Paris, France), Amer. Meteor. Soc, pp. J37-J42. Evans, J., 1991a: Status of the Terminal Doppler Weather Radar One Year Before Deployment, Preprints, Fourth International Conference on Aviation Weather Systems (Paris, France), Amer. Meteor. Soc., pp. J1-J6. Evans, J., 1991b: Integrated Terminal Weather System (ITWS), Preprints, Fourth International Conference on Aviation Weather Systems (Paris, France), Amer. Meteor. Soc., pp. 118-123. Hermes, L.G., K.W. Thomas, G.J. Stumpf, and M.D. Eilts, 1990: Enhancements to the TDWR Gust Front Algorithm, FAA Interim Report DOT/FAA/MR-91-3 , 55 pp. Mahapatra, P.R. and D.S. Zrnic, 1991: Sensors and Systems to Enhance Aviation Safety Against Weather Hazards, Proceedings of the IEEE , Vol, 79, No. 9, pp. 1234-1267. McCarthy, J., 1991: The Aviation Weather Products Generator, Preprints, Fourth International Conference on Aviation Weather Systems (Paris, France), Amer. Meteor. Soc., pp. 106-111. Merritt, M.W., 1991: Microburst Divergence Detection for Termi¬ nal Doppler Weather Radar (TDWR), Lincoln Laboratory Project Report ATC-181 , Lexington, MA, 164 pp. Sherretz, L., 1991: Developing the Aviation Gridded Forecast Sys¬ tem, Preprints, Fourth Interna¬ tional Conference on Aviation Weather Systems (Paris, France), Amer. Meteor. Soc., pp. 102-105. Turnbull, D., J. McCarthy, J. Evans, and D. Zrnic: The FAA Terminal Doppler Weather Radar (TDWR) Program, Preprints, Third In¬ ternational Conference on the Aviation Weather System (Anahiem, CA), Amer. Meteor. Soc., pp. 414-419. 192 Session 7.1 Vasiloff, S.V., 1991: The TDWR Tornadic Vortex Signature De¬ tection Algorithm, Preprints, Fourth International Conference on Aviation Heather Systems (Paris, France), Amer. Meteor. Soc., pp. J43-J48. Witt, A., 1990a: A Hail Core Aloft Detection Algorithm, Preprints, 16— Conference on Severe Local Storms (Alberta, Canada), Amer. Meteor. Soc., pp. 256-259. _, 1990b: The New NSSL Mesocyclone Detection Algo¬ rithm, Report to the NEXRAD OSF, Available from the author at NSSL, 1313 Halley Cir., Norman, OK, 73069, 9 pp. _, 1991: An Enhanced Version of the NEXRAD Storm Cell Tracking Algorithm, Report to the NEXRAD OSF, Available from the author at NSSL, 1313 Halley Cir., Norman, OK, 73069, 18 pp. 193 Session 7.2 APPLICATION OF THE COMBINED MOMENT PRODUCT IN AVIATION NOWCASTING Lee C. Anderson NOAA/National Weather Service Forecast Office Des Moines, Iowa G. Douglas Green NOAA/NWS/Operational Support Facility Operations Training Branch Norman, Oklahoma 1. INTRODUCTION In 1989, the implementation of the WSR-88D (Weather Surveillance Radar-1988 Doppler) commenced at NWS (National Weather Service) field sites. Aviation meteorologists in the NWS, for the first time, will be able to generate forecasts, warnings and briefings using this Doppler radar information. Data obtained from the WSR-88D allows the meteo¬ rologist to examine the Doppler spectrum's principal moments (reflectivity, radial velocity, and spectrum width). Usually, these fields have been individually ana¬ lyzed by meteorologists to determine the location of precipitating sys¬ tems, examine storm structure, de¬ termine the degree of storm severi¬ ty, and identify potential areas of turbulence associated with these systems. Only one product, the combined moment (CM), allows the user to view all three principal moments simultaneously on one dis¬ play. This paper describes the CM product and an application of the product in aviation nowcasting. A case study emphasizes the operation¬ al usefulness of the CM product with respect to examination of the three base moments. Strengths and weak¬ nesses of the CM are considered and suggested modifications that could increase the product's practicality in a real-time operational setting are presented. 2. DESCRIPTION OF COMBINED MOMENT PRODUCT The CM product is a presenta¬ tion of Doppler moments in a B-scan format. This type of format is a display of radial data shown as a cartesian-type design. On the CM product, the mean radial velocity is represented by arrow direction (Fig. 1) and the zero radial velocity is a horizontal arrow pointing right. Nonzero radial velocities are pro¬ portional to the angle between the arrow and its zero position. The maximum radial velocity that may be depicted by this product is ± 51 ms" 1 (100 knots), represented by a horizontal arrow pointing left (all velocities are depicted in knots on the WSR-88D system). If the arrow points toward the bottom (top) of the display, the flow depicted pos¬ sesses a component of motion toward (away from) the radar. A 50 knot component of wind toward the radar is indicated if the arrow is shifted by 90 degrees toward the bottom of the display. Information about the spectrum width is displayed by means of ar¬ rowhead size; three arrowhead sizes conform to given ranges of width 194 mmm Session 7.2 (Fig. 1). Arrows are displayed every 0.50 km (0.27 nmi) in range. The azimuthal spacing is at one degree intervals, which is the best resolution the WSR-88D system pro¬ vides. FIG. 1. Combined moment vector construction (Adapted from Unisys Corporation 1990). Reflectivity information is displayed every 1 km (0.54 nmi) in range with one degree azimuthal spacing. A maximum of eight color levels is used in the reflectivity part of the CM display (Fig. 4). Reflectivity levels three to eight coincide with the six D/VIP (digital video integrator and processor) levels used on the WSR-57 and WSR-74 NWS radar displays. Levels one and two on the CM product correspond to reflectivity levels less than VIP 1. (The WSR-88D detects and displays information from targets that cannot be detected by conventional radars.) 3. APPLICATION The NEXRAD Initial Operational Test and Evaluation Phase II (IOT and E(2)) was conducted in Norman, Oklahoma from April through August 1989. During this test, WSR-88D data were periodically collected and archived. The data used for the case described below was acquired from the test. This case features a practical application of the CM product. On the evening of 14 May 1989, thunderstorms developed over the Oklahoma panhandle and moved south¬ east. By early afternoon, these storms had spread into north-central Oklahoma. The 0.5° base reflectivity product at 1838 UTC from the WSR-88D system showed an area of echo (la¬ beled "A" in Fig. 2) of moderate intensity (35 to 54 dbZ). (The data levels located on the side of the base products are lower bound thres¬ holds. The radar is located just below the lower right corner of the data on Fig. 2 at Norman, Oklahoma.) This particular area is near the border of Garfield and Kingfisher counties. From 1725 to 1850 UTC, hail 2.2 cm (0.88 inch) in diameter occurred with these storms in the vicinity of "A" (USDC, 1989). FIG. 2. WSR-88D base reflectivity product (0.5°), 1838 UTC 15 May 1989. Area of high reflectivity is labeled "A". The 0.5° base velocity product at 1838 UTC (Fig. 3) showed an area 195 Session 7.2 of radial velocities as high as 10 ms.., (19 kt) in the area labeled "B". At the same time and in the same location (labeled "C" on Fig. 4), the 0.5° spectrum width product displayed an area of values ranging from up to 10 ms' 1 (19 kt). This value of spectrum width indicates high potential for the presence of moderate to severe turbulence (Na¬ tional Weather Service Operational Support Facility 1991). These val¬ ues seem reliable since they were associated with high reflectivity values. Typically, higher reflectivity values exhibit high signal-to-noise thresholds. Reflec¬ tivity values near the radar's sig¬ nal -to-noise threshold will lead to erratic estimates of spectrum width. A high potential of moderate to severe turbulence exists in this area because the data was derived from reflectivity data associated with thunderstorms. This category of turbulence typically occurs with thunderstorms. FIG. 3. WSR-88D base velocity product (0.5°), 1838 UTC 15 May 1989. Area of high velocities is labeled "B". ties of 10 ms’ 1 (19 kt) near the higher reflectivities and maximum spectrum width values of 10 ms' 1 (19 kt values or large arrows) near the bottom center of the Fig. 5. Ap¬ propriately, these values agree with those values observed on the base products. However, examining the CM product allows the user to view all three moments simultaneously on one display over a small (27 x 27 nm) area. This capability permits the meteorologist to apply the informa¬ tion for smaller areas that may be needed for aviation duties. FIG. 4. HSR-88D spectrum width product (0.5°), 1838 UTC 15 May 1989. Area of high spectrum width is labeled "C". Examination of the 0.5° CM product (Fig. 5) at 1838 UTC, ob¬ tained from area "A" in Fig. 2, indicated reflectivity values as high as 55 dBZ (labeled "D" in Fig. 5), maximum inbound radial veloci- FIG. 5. WSR-88D combined moment product (0.5°), 1838 UTC 15 May 1989 showing area of maximum values of base data labeled "D" associated with data displayed in FIGS. 2-4. 196 Session 7.2 4. STRENGTHS, LIMITATIONS AND SUG¬ GESTED MODIFICATIONS The CM product possesses opera¬ tional strengths. The duty of as¬ similating information from the three principal moments may be dif¬ ficult for a radar meteorologist, especially during operations. At any given time, one or more of the moments may provide important infor¬ mation in potential warning situa¬ tions. If an operational meteorolo¬ gist uses only one of the base mo¬ ments, information regarding an important meteorological feature may not be acquired. Thus, a less-than optimum warning decision may result. Valuable warning lead time may be lost if severe storm indicators are apparent in a base moment that is not examined by the meteorologist. Examining the CM product could alle¬ viate this potential problem. Valuable applications of this product include the detection of vortical-flow fields (Anderson and Green 1990, Unisys Corporation 1990) and the computation of storm-top divergence. Determining the maximum velocity difference (the absolute value of the difference between the maximum outbound and inbound radial velocities) in a region of diver¬ gence near a storm summit allows the meteorologist to estimate the maxi¬ mum hail diameter (Witt and Nelson 1991). Typically, the base velocity product will depict the same color for all outbound velocities greater than 33 ms' 1 (64 kt) and another color for all inbound velocities greater than 33 ms* 1 (64 kt). In contrast, the CM allows the operator to "see" wind speed changes up to + 51 ms* 1 (± 100 kt). Using the base velocity product to compute the maximum velocity difference for a storm top may result in an underes¬ timate of the divergence near the storm summit. Therefore, an under¬ estimate in the maximum hail size may result. Utilizing the CM prod¬ uct could yield more accurate esti¬ mates of storm-top divergence and associated updraft strength. One disadvantage of the CM product is that it is difficult to interpret without practice (Burgess et al. 1990). Filtering out or regrouping reflectivity levels on the CM may simplify the display. However, important reflectivity information may be overlooked if filtering is performed. Another disadvantage of the CM product is that no background maps are avail¬ able to use with the product. How¬ ever, the azimuth (degrees) is dis¬ played on the product's horizontal axis, and the range (km) and height (ft) are displayed on the vertical axis. Additionally, the user must estimate the wind speeds based on vector orientation since numerical radial velocities are not provided. The highest inbound and outbound radial velocities are not displayed on the CM product as is shown on the base velocity product. Finally, the Center Weather Service Units (CWSU), located in all regional Air Traffic Control Cen¬ ters, will be non-associated users of the WSR-88D system (National Weather Service Operational Support Facility 1991). Therefore, the CM must exist on the generation and distribution list at the RPG site in order for the meteorologists at the CWSUs to have access to this prod¬ uct. These users are not capable of generating the CM product at prede¬ termined locations. To make the CM product more practical operationally, some modi¬ fications are suggested. Rather than changing the arrow direction to 197 Session 7.2 display different radial velocities, plot the actual base velocity values numerically. Instead of using dif¬ ferent arrowhead sizes to represent spectrum width variations, use sim¬ ple geometrical figures (e.g., a circle). Plotting velocity values in different colors corresponding to the magnitude of spectrum width while depicting reflectivity infor¬ mation in gray shades would ease product interpretation. 5. SUMMARY The combined moment (CM) prod¬ uct generated by the WSR-88D has been described and applications have been highlighted. The CM allows the user to examine the three principal Doppler moments simultaneously on one display. The product may help aviation meteorologists identify important meteorological features such as areas of potential turbu¬ lence, areas of conver¬ gence/divergence, and vortical flow fields. Without practice, the prod¬ uct may be difficult to interpret in its present form. Certainly, uncom¬ plicated interpretation of the WSR- 88D products is desired by all us¬ ers. However, operational meteorol¬ ogists should attempt to use the CM product to improve their skills at interpreting and to increase the potential to gain information about meteorological features observed during operations. If the CM prod¬ uct is modified in the manner out¬ lined, more operational meteorolo¬ gists will find the product helpful. 6. ACKNOWLEDGEMENTS The authors thank Liz Quoetone, Dave Imy, and Robin Radlein (WSR-88D Operational Support Facility's Oper¬ ations Training Branch) for their help in acquiring the data and the graphics used in this paper. 7. REFERENCES Anderson, L. C. and G. D. Green, 1991: The WSR-88D combined moment product. Preprints , 25th Conf. on Radar Meteorolo¬ gy , 233-236. Burgess, D.W., A. Witt, and D. Forsyth, 1990: NSSL IOT&E II Final Report, Part I: Meteoro¬ logical Overview. Report for the Air Force Operational Test and Evaluation Center (AFOTEC), 31 pp. National Weather Service Operational Support Facility, 1991: WSR- 88D Operations Training Student's Guide. [Available from Operational Support Facility's Training Branch, Norman, OK 73072]. Unisys Corporation, 1990: WSR-88D Operations Training Student's Guide. [Available from Unisys Corporation, One Ivybrook Bou¬ levard, Ivyland, Pennsylvania, 18974]. USDC, 1989: Storm Data - Oklahoma, Vol 31, No. 5. National Clima¬ tic Data Center, Asheville, NC 28801. Witt, A. and S. P. Nelson, 1991: The use of single Doppler radar for estimating maximum hail¬ stone size. J. Appl . Meteor ., 30, 425-431. 198 Session 7.3 THE OKLAHOMA CITY, OK (WILL ROGERS WORLD AIRPORT) SEVERE WET MICROBURST EVENT OF 27 SEP 1986-- USE OF A POTENTIAL GUST FORECAST TECHNIQUE Stacy R. Stewart WSO FAA Academy Oklahoma City, Oklahoma 1. INTRODUCTION Severe wet microbursts emanat¬ ing from summertime, pulse-type thunderstorms can occur at any time of day and at any geographical loca¬ tion. Certain empirical and qualita¬ tive forecast techniques have been developed during the past decade to assist warning forecasters in de¬ termining which thunderstorms are severe. However, a significant amount of subjectivity remains in the preparation of a severe thunder¬ storm warning and many pulse-type thunderstorms are not warned on until after damage reports are re¬ ceived due to the relatively short¬ lived nature of this type of storm. Fortunately in this case, the severe wet microburst occurred at a major airport during a time period when air traffic was at a minimum. Stewart (1991) proposed a sim¬ ple warning technique utilizing Vertically Integrated Liquid (VIL) water content and radar echo heights (TOP), combined with the penetrative downdraft mechanism, to produce maximum potential (downdraft) gust forecasts for individual thunder¬ storms. Warning lead times are usu¬ ally at least 15 minutes prior to the severe weather event and lead times of 30 minutes or more are not uncommon when using this technique. This paper focuses on (1) the rapid development of a pulse-type severe thunderstorm during weak, synoptic-scale forcing and (2) the application of a potential gust forecast technique for pulse-type thunderstorms. 2. SYNOPTIC SITUATION The environment in which pulse- type thunderstorms develop is one characterized by little or no verti¬ cal wind shear and a deep, nearly saturated lower troposphere that is topped by an elevated dry layer of low theta-e (0 e ) air which is the primary source for generating pene¬ trative thermals (downdrafts). Strong evaporational cooling and precipitation loading combine to produce enough negative buoyancy (with respect to the in-cloud lapse rate) to accelerate downward the entrained dry parcel (fig. 1). Caracena, Holle, and Doswell (1989) noted that the low 0 e air must be located high enough above the sur¬ face to allow the negative buoyancy force to accelerate a parcel down¬ ward and reach a severe downdraft velocity. After the parcel reaches the surface, the air diverges and spreads out such that the speed of the horizontal flow is assumed to equal the maximum downward speed the parcel achieved during descent. However, during those times when moderate, unidirectional flow exists in the troposphere, pulse- type thunderstorms can still develop if the storm lifetime is relatively short (< 30 minutes) so as not to allow the updraft to become tilted. Figure 2 is the 1200 UTC Oklahoma City, OK (OKC) sounding on the day of the wet microburst event. Note 199 Session 7.3 the very moist lower troposphere below the 600 mb level topped by ex¬ tremely dry air (dewpoint depres¬ sions >30°). Lifting a well mixed parcel from the lower-troposphere to the 500 mb level would have resulted in a lifted index value of -5 and a moderate area of positive buoyancy. The OKC wind field indicated moder¬ ate unidirectional flow below the 250 mb (approx. 34000 ft) with about 30° of veering above that level. The lapse rate was steep (nearly dry adiabatic) between 850 mb and 680 mb indicating that a parcel would ac¬ celerate upward rapidly once the level of free convection was reached. The mean wind vector in the lower 5000 feet was 225° at 21 kt and the storm motion vector (average wind vector from the condensation level to the equilibrium level) was 244° at 35 kt. It is also important to mention that the actual OKC bal¬ loon launch time was 1100 UTC which means that this sounding was repre¬ sentative of the pre-storm environ¬ ment. 3. DESCRIPTION OF THE POTENTIAL GUST FORECAST The potential gust forecast technique used in this paper only requires VIL and TOP data to calcu¬ late the maximum potential downdraft or peak gust (for a complete description on VIL and how VIL and TOP are calculated by radar, see Greene and Clark, 1972, and DOC Issue No. 19-WSH, 1984, respective¬ ly). VIL is obtained by calculating the rainwater liquid water content (R ) in several layers of a precipi¬ tating system (e.g. a thunderstorm) until the top (TOP) of the radar echo (18 dBz) is reached. The R c of each layer is then summed up over the depth of the system. The spe¬ cific equation for calculating VIL is VIL = 3.44 x 10"* [ VIP 5) and the vertical depth over which the specific inten¬ sity level occurs. In this case, the total VIL (in the center of the cell alona a vertical axis) was 50.5 kgm . Using equation 3 and a VIL value of 50.5 kgm' 2 and a TOP value 203 Session 7.3 Figure 7 of 11500 m, R c is 4.3913 . Substi¬ tuting that value for R c into equa¬ tion 2 yields a potential gust (PG) of 25.1 ms' 1 or 48.8 kt. Adding 10.7 kt for the vector addition of one- third MLW to the PG of 48.8 kt means that a maximum FPG of 59.5 kt could be expected with this storm. TABLE 1 VIP POWER EQUIVALENT LEVEL RETURNED REFLECTIVITY (dBz)(mm 6 m~ 5 ) 1 18-(30) 0-999 2 30-(41) 1000-12588 3 41-(46) 12589-39810 4 46-(50) 39811-99999 5 50-(57) 100000-501186 6 > 57 > 501187 Oklahoma City, Oklahoma Figure 8 is a map of the Will Rogers World Airport (OKC) runway complex. The north-south runways are almost 10000 ft long and the National Weather Service Office (WSO OKC) is located in the north-central part of the runway complex. The FAA anemometer #1 (FAA #1) was located beside the NWS anemometer and both were situated near the south-central part of the runway complex. However, FAA #1 was inoperable that day. FAA anemometer #2 (FAA #2) was located in the center of runway 35R-17L (the easternmost runway) and was in oper¬ ating that day. The apparent track of the wet microburst is depicted by 204 Session 7.3 the dashed arrow extending north- east-southwest across the south- central part of the runway complex. This track is based on visible observations from the roof of the WSO and from the control tower, and the surface wind directions recorded by the NWS and FAA #2 anemometers. The following OKC aviation surface reports will help to give a better understanding of the sequence of events leading up to, during, and after the severe wet microburst event: OKLAHOMA CITY, OK (0KC-- WILL ROGERS WORLD AIRPORT) SURFACE WEATHER OBSERVATIONS, 27 SEP 1986 OKC SA 0848 CLR 20 128/73/69/2007/ 995/CB FQT LTGICCC DSNT SE-S MOVG NE/103 1900 OKC SA 0948 CLR 20 128/73/68/1908/ 995/CB FQT LTGICCC DSNT SE MOVG NE CB OCNL LTGIC SW MOVG NE FEW CU AC SW OKC SA 1048 300 SCT 20 129/73/68/ 1909/996/CB OCNL LTGICCC SW MOVG NE CB FQT LTGICCC E-SE MOVG NE MDT CU SW-W OKC SP 1136 M38 BKN 75 BKN 300 OVC 20T 1911/998/TB35 SW MOVG NE FQT LTGICCC RB26E32 OKC SA 1148 20 SCT 38 SCT 75 SCT E300 BKN 20T 136/73/70/2013/998/ TB35 SW-S MOVG NE FQT LTGICCCCG TCU W RB26E32/30200 1963 68 20144 RADAT 22136 OKC SP 1155 E20 BKN 75 BKN 300 OVC 15T+RW+ 2145G58/996/T S-SW AND OVHD MOVG NE FQT LTGICCCCG RB54 PRESFR OKC SP 1219 20 SCT E75 BKN 300 BKN 15 1910/999/TE18 DSIPTD CB SE OCNL LTGIC MOVG NE RE05 PRESRR PK WND 2158/1155 By 1048 UTC, some of the anvil cir¬ rus from the newly developed storms to the southwest began to spread across the area and thunder was reported on station by 1136 UTC. The main core of the thunderstorm passed approximately 1 nm south of WSO OKC and only 0.01 inch of rainfall was recorded in the raingage located on the north side of the weather office. The brief heavy rainfall lasted less than 30 seconds (i.e. the "machine-gun bullet" effect), with the heaviest rainfall occurring on the southside of the WSO. The maximum wind gust recorded by the NWS anemometer was 58 kt from a direction of 210° at 1155 UTC while surface winds at WSO OKC gusted to only 20 kt based on visual observations. The very sharp surface wind and rainfall gradients attest to the very small diameter of the thunderstorm. The control tower was immediately informed of the severe wind gust and the likelihood of additional microburst activity for the airport complex. At 1157 UTC, the control tower advised WSO OKC that the FAA #2 anemometer recorded a surface wind gust of 240° at 61 kt. The predicted FPG prior to the occurrence of the event was 241° at 59.5 kt ! Based on the two anemometer reports, the FPG gust error was only ± 1.5 kt. Surface temperatures at WSO OKC remained at 73°F prior to and during the wet microburst event. However, a weak rainshower had passed over the northern portion of the runway com¬ plex prior to the event between 1126 UTC and 1132 UTC the resulting evaporation caused the surface dew- 205 Session 7.3 point to increase from 68°F to 70°F. The slight increase in the surface virtual temperature and the result¬ ing decrease in the density may be a possible explanation for the higher than predicted wind gust (61 kt vs. 59.5 kt) observed. Less than 10 minutes after the occurrence of the last severe wet microburst report, visual observa¬ tions indicated the thunderstorm was rapidly dissipating. A well-defined, classical cumulonimbus cloud was located approximately 5 miles east of OKC, but was very translucent with the rising sun dimly visible through the middle portion of the cloud. The storm was dissipating as quickly as it had formed and appar¬ ently had unloaded all of its liquid water during the severe wet micro¬ burst event. The lack of any signif¬ icant liquid water within the re¬ mains of the storm resulted in its thin, translucent appearance and the rapid evaporation of the remaining cloud water (recall the extremely dry air above the 600 mb level). 6. SUMMARY AND CONCLUSIONS Fortunately there was no sig¬ nificant damage and no aviation accident reported with this micro¬ burst event. However, had this event occurred during a time of the day (e.g. late afternoon) when air traf¬ fic would have been at a maximum, the results could have been devas¬ tating. Despite the relatively small size of this storm (both vertically and horizontally), a severe wet microburst occurred and did so dur¬ ing the most unlikely (climatologi- cally speaking) time of day. This example should further strengthen the fact that severe wet microbursts can occur at any time of day or time of the year if favorable atmospheric conditions exist. During periods of weak synop¬ tic-scale forcing, strong low-level forcing due to enhanced convergence resulting from gust front interac¬ tion can produce strong upward ver¬ tical motion which can lead to a sudden and rapid release of condi¬ tional instability. Strong low-level moisture convergence can result in extremely large amounts of liquid water (and ice) being suspended in the mid-levels of a storm where the entrainment of dry, low 0 can re¬ sult in the development of a severe penetrative downdraft. An objective warning forecast technique has been presented to help eliminate much of the subjectivity and "guesswork" involved in issuing severe thunderstorm warnings on short-lived thunderstorms which are capable of producing severe wet microbursts. The typical gust pre¬ diction error associated with this technique is ±2 kt and the average lead time is 15 minutes with occa¬ sional lead times of 30 minutes or more (the lead time in this case was only 10 minutes due to explosive storm development). The only vari¬ ables required are radar derived values of VIL and TOP. This tech¬ nique could easily be employed in the existing RADAP II system and in the oncoming NEXRAD doppler radars. For those stations still using con¬ ventional weather radar data, a slightly less accurate form of the technique can be used by developing a simple nomogram (or computer pro¬ gram) containing specific VIL values associated with various equivalent reflectivities and vertical depths. That data would then be incorporated into another nomogram of total VIL and maximum TOP to generate a poten¬ tial gust forecast. The final poten- 206 Session 7.3 tial gust would then be determined by adding one-third of the mean layer winds in the lower 5000 ft. REFERENCES Caracena, F., R.L. Holle and C.A. Doswell III, 1989: Microbursts: A handbook for visual Identifi¬ cation. NOAA/ERL NSSFC . 35 pp. Department of Commerce, 1984: Opera¬ ting instructions for RADAP II, Weather Radar Manual, National Weather Service., Issue No. 19- WSH. Emanuel, K.A., 1981: A similarity theory for unsaturated down- drafts within clouds. J. Atmos. Sci., 36, 2462-2478. Greene, D.R., and R.A. Clark, 1972: Vertically integrated liquid water-- A new analysis tool. Mon. Wea. Rev. . 100, 548-552. Miller, R.C., 1967: Notes on analy¬ sis and severe storm forecast¬ ing procedures of the Military Weather Warning Center. USAF Air Weather Service Manual No. 200 . Stewart, S.R., 1991: The prediction of pulse-type thunderstorm gusts using vertically inte¬ grated liquid water content (VIL) and the cloud top pene¬ trative downdraft mechanism. NOAA Technical Memo . NWS, SR;. 136 . 20 pp. 207 Session 7.4 AN EXAMINATION OF DOWNBURSTS IN THE EASTERN GREAT PLAINS ASSOCIATED WITH A VERY WARM MID-LEVEL ENVIRONMENT Stephen F. Byrd National Weather Service Forecast Office Omaha, Nebraska 1. Introduction It has long been suggested that in the eastern part of the Great Plains the chances for thunderstorms becomes small with a 700 mb tempera¬ ture greater than +10°C. It has also been suggested that the areas of thunderstorms occur on the cool side of the 700 mb +10°C isotherm (Fig. 1). Since generated 700 mb temperatures are not available, a field forecaster relies on the 5760 meter 1000-500 mb thickness contour which closely corresponds to the 700 mb +10°C isotherm (Fig. 2). Schaefer (1986) mentions this rule and also notes that in forecasting nocturnal Mesoscale Convective Com¬ plex (MCC) events that a 700 mb temperature greater than 12°C sup¬ presses organized convection. This 700 mb isotherm rule of thumb for the Eastern Plains would not have the same application in the Western Plains due to the higher surface elevations. Rules of thumb are convenient empirical tools that can be of great use to a forecaster "under the gun". There are scientifically based rea¬ sons why the rules work. Sometimes, however, they will fail. Doswell (1986) has pointed out that it is the responsibility of all meteorolo¬ gists to not only be empiricists, but also to understand the founda¬ tions of the rule. Otherwise it is impossible to know in advance when the rule will fail. The author has found the "+10 rule" very useful and also has helped to locate the greatest proba¬ bility of thunderstorms. The author has also observed that when the rule fails and thunderstorms do occur with very warm mid-level tempera¬ tures in the Omaha area, that down- bursts or microbursts are possible. One example of when the rule failed was July 15, 1988 when strong forc¬ ing overcame 700 mb temperatures +12°Celsius or better. The strong forcing mechanisms were a Mesoscale Vortex Center (MVC) (as described by Johnston (1982)), along with differ¬ ential heating and moisture conver¬ gence. The MVC developed with a flash flood producing MCC the previ¬ ous night in southwest Nebraska and moved into east central Nebraska in the afternoon. The resulting severe thunderstorm produced a tornado at Council Bluffs, Iowa, along with several downbursts and microbursts. This paper investigates the situation in which thunderstorms occur with 700 mb temperatures greater than +10°C or +12°C. Fore¬ casters should be alert to the pos¬ sibility of downbursts or micro¬ bursts under such conditions. 2. Case Studies A. Case I - A Major Downburst Event in Northeast Nebraska During the pre-dawn hours of August 5, 1989 thunderstorms developed in North Central Nebraska. 208 Session 7.4 Figure 1. Radar analysis for 04Z and 700 mb temperature analysis for 00Z August 28, 1989. 209 Figure 2 Session 7.4 The OOz and 12z August 5, 1989 Omaha Nebraska (OMA) showed 700 mb temper¬ atures of +12°C. The +10 # C 700 mb isotherm extended east-west across northern Nebraska. The high based thunderstorms developed in an area of divergence aloft in the right rear quadrant of a jet max. Also there was enough mid level cold advection to break the capping ef¬ fect of the warm mid-level tempera¬ tures (see Fig. 3). A violent thunderstorm produced widespread damage due to downbursts across northeast Nebraska extending to just north of Omaha. Winds gusted to 81 mph when the thunderstorms were in the Norfolk, Nebraska, (OFK) area. The WSFO at Omaha reported wind gusts of 55 mph with the passage of the outflow boundary. Thus, with this case there was strong forcing with a warm mid-level environment, and although low level moisture was not lacking (a 12°C dew point at 850 mb) on the 12Z August 5, 1989 OMA sounding (see Fig. 4), the warm lapse rate below the high cumulonimbus cloud base allowed entrained air, cooled by evapora¬ tion, to become extremely negatively buoyant. In light of this case in which strong forcing was necessary for thunderstorm production due to the warm mid-level environment, it can be seen that even with a high low level mean mixing ratio the thunder¬ storms on the morning of August 5 were high based. Note that the thunderstorms did remain near the +10°C 700 mb isotherm, and that in the warmer air to the south no thun¬ derstorms occurred. The strong outflow boundary generated by the downbursts moved well south of the parent thunderstorms, which remained north of the +12° 700 mb isotherm. The thunderstorms favored the area along the +10°C isotherm at 700 mb. So the "rules of thumb" worked in this case. But it is suggested that the "rules" be expanded so that, if thunderstorms do occur with these warm mid level temperatures, forecasters are aware of the poten¬ tial for downbursts and especially microbursts. The next three cases are exami¬ nations of warm mid-level environ¬ ments in which microbursts occurred in the WSFO Omaha county warning area. In these cases the dynamics and thermodynamics the processes producing the microbursts will be examined. From Haltiner and Martin's Dynamical and Physical Meteorology d s \V"~a 5* \Y\- < • ji ♦ 2^ • 31 \Su_ 21"; V. ♦ "• *.4 \\_ 19 . rr.r».* 2o rr.«.* !•; ♦ 2r~' i® 2_ r r\s 3 26*' 16 W_ 27S/ 18 28©' 21 Vu_ ?? 26S' 29 265' 2? poa » •S OIP/KT? DIP TT? Figure 4. 12Z August 5, 1989 OMA Sounding with CONVECT parameters. Dashed line between the temperature/dew point plot is the wet bulb temperature. 211 Session 7.4 environment. The source of the entrained air that is accelerated downward to cause downbursts or microbursts can either be air cooled at or below the cloud base, or as Kessler (1985) suggests, from areas above the base. Downbursts and microbursts that occur in low based thunderstorms in a very moist low layers most likely originate well up into the cloud layer where some dry air is entrained as stated by Kessler (1985). Kessler based this fact on the evidence of the low wet bulb temperatures observed beneath severe thunderstorms. Kessler pointed out that negatively buoyant air will descend at the moist adia¬ batic lapse rate as long as there is enough precipitation to keep the descending air saturated. Fujita and Black (1986) studied the SST (Small Severe Thunderstorm) and pointed out that the mid level dry air can be drawn into the descending current easily by virtue of the small cloud diameter. Darkow and McCann (1975) show that the relative wind flow from 121 thunderstorms was at a minimum at this level, suggest¬ ing that this is the most likely level for injecting environmental air into the storm. If, however, the entire liquid content of the descending parcel is evaporated (as is the case in some microbursts from high based thunderstorms) the de¬ scent would be dry adiabatic. In the three cases of micro¬ bursts that are examined here the soundings are not of a pre-storm environment, but rather of the microburst environment beneath high based cumulonimbus. It was assumed that the air near the cloud base is cooled to the mean wet bulb tempera¬ ture, then descended moist adiabat- ically for a short time, with the parcel then descending and warming somewhere between the dry and moist adiabatic lapse rate. The energy of the negative parcel is proportional to the areas shown on the curves of Figures 5, 6 and 10. The vertical velocity profile of a descending parcel can be constructed using the equation: a t dv z=g(1 , r -T^/T r ( 2 ) where w = parcel vertical velocity m/sec, g = 9.8 m/sec2, and T v = de¬ grees K rewriting A(-!£>*ff(ri-7v)/(7gAz (3) using z = .5 km and assuming w=0 at cloud base, and w(sfc) diverges out becoming a near surface horizontal velocity. The vertical velocity (i.e., w (sfcl) can be computed by adding each w 2 computed for every .5 km below the cloud base to about .5 km above the ground, then taking the square root of the sum. From the equation it can be seen that having a deep layer below the cloud base contributes to a stronger w (sfc). Also, the warmer the environment the greater the negative buoyancy of the parcel. Finally, the lower the wet bulb temperature of the entrained air the greater the acceleration of the parcel downward. B. Case II - July 14, 1200z Although the 700 mb tem¬ perature at 1200Z (Fig. 5) from the OMA sounding was a very warm 14.8°C, there were widely scattered small 212 Session 7.4 2EP , 2^ c _ Too • C3S ^ nr .• VvU_ S'. i ♦ £70' -X M\\i 270' - 12 *' * 2*0' ’ «:; 3 s£>-; IJni >do/ -• 30 kts; and Area 4 - yields gusts > 40 kts. Eastern Plains offices could develop similar gust potential indi- 216 Session 7.4 Figure 9. 0930Z June 5, 1990 GOES satellite imagery with +10 degree Celsius isotherm at 700 mb at 12Z overlaid. -so 2CO / /V / v w ' v x r ' y \ ' r \ *7 7^7 /X/ 7 " TiX^X! 777 / X/ X / X / XX xxife. xx At V / v 7 7'7 7^7 ^7\7\^/7\7\ x 777\ win/ 277 - / -30 / -20 / -lO / O / lo / c0 / JO / 40 /- ///tty////////#///////. W/.'.V/////////*//////.'/// 280/ 34 + 275/ 32 280 / 35 295/ 54 4il\ 290/ 82 285 / 72 X i»v 6 ? 28*;/ 55 ♦ 280/ 54 fc- 27!'/ 50 ^ Z8tf S? ♦ 290/ 43 285/ 41 1! ♦ 285/ 33 \\\i „ 275/ 34 ♦ 275/ 37 WW 275/ 39 ♦ 275/ 43 \\\\i 275/ 44 + 265/ 48 K _ 255/ 48 250/ 44 235/ 44 185/ S2 v % DIP/KTS DIR/KTS rM.’kl ' 1 *^“7 / IU / CSZZ S un *"’ y KM/KFT Figure 10. 12Z June 5, 1990 OVN sounding with CONVECT parameters and showing negative area for estimating microburst winds. 217 Figure 11 Nomogram for estimating gust potential based on the Upper level Stability Index (UI) and the 700 mb Depression. 218 Session 7.4 ces based on this scheme but lifting from 700 mb and using the depression at 850 mb and the upper level sta¬ bility charts 70s and 7qs from the Upper Air Diagnostics program devel¬ oped by Foster (1989). 4. References Brown, J. M., Knupp, K. R., and Caracena F., 1982: Destructive winds from shallow, high based cumulonimbi. Preprints. 12th Conf. on Severe Local Storms (San Antonio, TX), Amer. Mete¬ or. Soc., 272-275. Darkow, G. L., and D. W. McCann, 1977: Relative environmental winds for 121 tornado bearing thunderstorms. Preprints. 10th Conf. on Severe Local Storms (Omaha, NE), Amer. Meteor. Soc., 413-417. Doswell, C. A. 1986: The human element in weather forecasting. Nat. Wea. Dio. . 6-18. Foster M. P., 1988: Upper-air Analysis and Quasi-geostrophic Diagnosis for Personal Comput¬ ers. Scientific Services Divi¬ sion, Fort Worth, TX. Fujita, T. and Black, P.G. 1987: Monrovia Microburst of 20 July 1986: A Study of SST. N0AA/ Mesoscale and Hurricane Re¬ search Division. Haltiner G.J. and Martin F.L., 1957: Dynamical and Physical Meteo¬ rology . McGraw Hill Book Co. New York, NY. Johnston, E.C., 1982: Mesoscale Vorticity Centers Induced by Mesoscale Convective Complexes. Preprints. 9th Conf. on Weather Forecasting and Analysis (Seattle, WA), Amer. Meteor. Soc., 196-200 Kessler E., 1985: Thunderstorm Morphology and Dynamics Vol. 2 . University of Oklahoma Press, Norman, OK. McDonald A., 1976: Gusty surface winds and high level thunder¬ storms. Western Region Techni¬ cal Attachment 76-14, available from National Weather Service Western Region, Scientific Services Division, Salt Lake City, UT. Schaefer, J.T., 1986: Nocturnal thunderstorms, sometimes known as MCC's. Central Region Tech¬ nical Attachment 86-18, avail¬ able from National Weather Service Central Region, Scien¬ tific Services Division, Kansas City, M0. Stone, H.M., 1986: Convective parameters and hodograph program-Convect (Revised). Eastern Region Computer Program 37, available from National Weather Service Eastern Region, Scientific Services Division, Bohemia, NY. 219 Session 7.5 TOWARD A CLIMATOLOGY OF SOUTH TEXAS DOWNBURSTS Nezette N. Rydell and Judson W. Ladd National Weather Service San Antonio. Texas 1. INTRODUCTION The aviation comnunity has devoted considerable attention in recent years to the role of dovnbursts in aircraft mishaps and accidents. In response to this concern, research and operational meteorologists have conducted various studies in an effort to determine the environmental factors contributing to dovnburst generation. Early efforts described the classic dry environment dovnbursts which occur quite frequently on the High Plains. More recent studies have shown downburst events to occur in a wide variety of meteorological environments with multiple factors playing a role in the production of the winds. The variability of environments and causal factors mandates a need for regional climatologies. In Texas, one such climatology has been developed. Read (1987) and Read and Elmore (1989) documented some 55 cases of downburst occurrence in North Texas for the 1985-87 sunnier seasons. This climatology identified several characteristics thought to be responsible for downburst generation and resulted in the development of a forecast and warning technique currently being utilized operationally. Some work has been done regarding West Texas downbursts as well. Sohl (1987) examined two microburst events in May 1986. Garner (1990) looked at an additional event with particular regard to establishing a forecast and warning technique. Preliminary work toward a climatology for South Texas was done by Ladd (1989) who looked at two summer events in the San Antonio area. Ten events that occurred during the sunmers of 1982*90, including the two previously examined by Ladd, are included in this study. Our aim in producing this climatology is not only to describe downburst events and identify their probable generating mechanisms in South Texas, but also to provide the operational forecaster with a tool in forecasting and possibly warning for downburst occurrence. 2. CONTROLS ON SUMMERTIME CONVECTION Downbursts are a product of the convective process. Therefore, a natural starting point for any climatology of these phenomena is to identify the cannon controls on convection. Thunderstorms occur in South Texas in every month of the year. However, since all of the events examined in this study occurred in the summer months, our discussion of thunder¬ storm development will be limited to that season. The predominant air mass overlying South Texas in summer is tropical. The Azores-Bermuda high pressure system is typically positioned over the southeastern tier of states during that time of year, with the circulation around its western periphery producing prevailing breezes off the Gulf of Mexico and across South Texas. Abundant low-level moisture carried by this flow results in a fair degree of convective (potential) instability. Mixing promoted by the instability continually works to deepen the moist layer and produce a characteristic vertical profile that is quite warm and moist', often to a height of 5 km or more. Thus, two of the necessary conditions for thunderstorm development, abundant moisture and instability, exist for a high percentage of sunrner days in South Texas. However, synoptic- scale triggering mechanisms are generally lacking in the summer season. As a result, thunderstorm activity is typically widely scattered and has its impetus in strong after¬ noon heating. Due to the high precipitable water content of the air, this activity can produce locally heavy rainfall; but severe weather, if any, is brief and marginal. On the rare occasion that a synoptic- scale disturbance in the westerlies finds its way far enough south, severe thunderstorms can and do develop across the area. Since wind and temperature fields aloft are weak and the Wet- Bulb Zero Height is quite high, the severe weather usually takes the form of strong straight-line surface outflow, i.e. gust fronts and downbursts. A more common scenario leading to the production of summertime severe weather involves outflow from thunderstorms, often many miles away, that triggers what can be termed "second- generation" convection in increasingly unstable air. Further interaction with a strong thermal gradient at the surface or a late afternoon seabreeze front off the Gulf of Mexico increases the likelihood of severe weather. 3. ANALYSIS PROCEDURES Ten downburst events that occurred in Southeast Texas, South Central Texas and along the Coastal Plains were examined. Nine events occurred during the afternoon from July to early September. The remaining event occurred at night in early June, before the official start of summer. The analysis techniques used were similar to those employed in Read's (1987) North Texas study. The aim was to provide continuity across the two adjacent areas as well as discern significant differences, if any, that may exist in atmospheric structure and downburst generation mechanisms. Considerable effort was devoted to the examination of upper-air patterns to ensure that subtle synoptic features were not characteristically evident in the pre-downburst environment over South Texas. 220 Session 7.5 Hourly surface charts, from several hours prior to as near the time of downburst occur¬ rence as possible, vere analyzed for notable mesoscale features. Isobars were drawn at 1 mb intervals. Isotherms and isodrosotherms were drawn at 2*F intervals. Three-hour wind vector change charts were constructed to aid in determining areas of convergence. Standard level charts up through 200 mb were analyzed for each event. With the emphasis on the pre-downburst environment, the charts used were generally those produced at 1200 UTC. Height contours were drawn at 20 m intervals. Isotherms were analyzed at 1-2*C intervals in search of weak thermal troughs that might have posed a threat of storm development in an otherwise tranquil environment. Soundings of the pre-storm environment (again, generally those based on 1200 UTC data) were analyzed in great detail. Each sounding was modified to approximate atmospheric conditions as close to the time of actual downburst occurrence as possible. The surface temperature just prior to thunderstorm occurrence and the average mixing ratio in the lowest 100 mb were used to compute the Lifted Condensation Level (LCL), Convective Condensation Level (CCL) and Lifted Index (LI). From these modified soundings, a composite, or mean, sounding was constructed (Figure 1). 200 300 100 i&G 7ul B'.C 1000 P R E S S u R E M B Fig. 1. Composite sounding of South Texas downburst events. Solid line represents dry- bulb temperature and dashed line represents dew point temperature. 4. RESULTS OF THE ANALYSES The results of the analyses are summarized in Table 1. Of some interest is the fact that none of the downbursts studied were directly spawned by the parent thunderstorm complex. Rather they were produced as a result of secondary storm development brought about by the interaction of outflow from the non-severe parent complex and some other low-level forcing feature. The role of this convective-scale interaction in producing second generation severe storms has been described by Purdom and Sinclair (1988). As was the case in the North Texas study, a close surface isobaric and thermal analysis revealed the existence of some type of forcing boundary in each of the events. In most cases, outflow from an upstream thunderstorm complex was sufficient to trigger the downburst- producing storm. However, on two occasions, interaction with a northwest-moving seabreeze front aided development of severe convection. Analysis of 3-h wind vector change fields supported the existence of these boundaries and provided some clues to the extent of surface moisture convergence in the vicinity of the downburst. Dew point pooling was thought to play an important role in the North Texas cases and appears to have been a significant factor in South Texas downbursts as well. No apparent pattern emerged in the upper air analysis that could be successfully linked to downburst production. While in many cases troughing was apparent at some level(s) in the atmosphere, downbursts also occurred at times underneath an extensive ridge (e.g. the events of 24 August 1986 and 24 August 1988). Upper winds were quite weak and in most cases did not support the development of severe weather. The use of the qualifiers "wet" and "dry" in the table to describe the events refers to the structure of the sounding and, by implication, the suspected generating mechanisms driving the downdraft. This is in general agreement with reasoning set forth by Wilson fll. (1984). A detailed examination of the soundings indicated a number of differences from those associated with downbursts in North and West Texas. These differences are most apparent when compared with the sounding classification scheme described by Ellrod (1989). Of the ten events studied, eight were typically wet with deep moisture extending above the 500 mb level and capped by a dry (although not nearly as dry as in other studies) layer. The remaining events were most closely identified as hybrid-type, including one event (2 September 1982) previously thought to be dry (Ladd, 1989). As a result, the composite sounding (Figure 1) is uniformly moist at all levels and therefore only slightly convectively unstable. This would support the need for a mechanical means of lift for storm development and subsequent downburst generation. As in the North Texas study, high surface temperatures did not correlate well with the production of downburst winds. In several of the cases, the estimated surface temperature at the time of the downburst was below seasonal normals. A weak capping inversion was present in only two events. Thus, convection was 221 Session 7.5 Table 1. Sunnary of South Texas downburst events. LCCAi TIRE (CDTi 9-2-62 1700 8-8-8*. 1«30 E-2-B5 1830 NEAREST NWS OFFICE SAT VCT BFT ESTIMATED HAT WIND IRFD 61 70 70 CLASSIFICATION HYBRID HYBRID WEI SFC TENP l*F AT TIRE OF Dd) 97 93 96 SFC DEWPOINT (*F AT TIRE OF DB) 66 72 76 SFC BOUNDARY 0/S T 0 LIFTED INDEX -6 -6 -6 r. INDEX 23 3! 39 650-7CC RB LAPSE RATE («C/r-> -4.9 -5.3 -6.5 CAPPING INVERSION WEAR NONE NONE CCL !FTi 4800 3200 4800 AV6 RII/RATIO (6/r.G LWR 100 RBI 16.8 17.0 18.5 400-500 RE R01STURE NO YES YES DRYING ABOVE 500 RB NO YES NO L0« level CONVERGENCE YES YES YES SFC TEMPERATURE RIDGE OVER OVER TO WEST SFC MOISTURE RIDGE TO EAST OVER OVER B50 RB PATTERN T/R T/R R 700 RB PATTERN T I/R R 500 RB PATTERN T T/R R/T 3C0 RE PATTERN T T R 500 HE WIND (US! 10 10 15 300 RB WIND (US) 1C 10 20 8-21-86 E-c4-6t t-c-67 9-7-67 7-3-88 7-27-88 8-cn-E: 1800 tstc 0345 1700 1356 1800 ie*.5 CRP BFT SAT SAT AU H0J ST 65 70 70 68 s7 V WET WET WET WET WET WET WET 91 69 75 89 93 ?t 100 71 7x 71 69 76 69 tt 0 0 0 0/S 0 0 0 -10 -5 -7 -3 -10 -6 -t 27 22 28 34 35 32 35 -6.0 -5.4 -5.5 -5.5 -6.7 -7.7 -4.2 NONE NONE NONE WEAK NONE NONE NONE 3200 4800 2600 5800 4400 5200 5800 17.5 15.0 15.0 15.0 18.0 16.0 17.5 NO YES NO YES YES YES YES YES /E3 YES YES YES NO YES YES YES YES YES YES YES YES OVER OVER OVER OVER OVER OVER OVER OvER OVER OVER TC EAST OVER OVER TO EAST T R T/R I/R T/R R R R/T R 1 T T/R R R R/T R T T T T R R/T R 1 T T 7 R *.0 10 30 25 10 5 5 40 c 5l 40 30 1C 2C D-IHUNDERSTORH OUTFLOW BOUNDARY T=TH£RRAL BOUNDARY Q/S=THUNBERSTDRfl OUTFLOW BOUNDARY INTERACT 1N6 WITH SEABREEZE FRONT T/R=TRCuSH TO UEST/RID6E TO EAST R/T=R1DGE TO WEST/TROUGH TO EAST T=UNDERNEATH TROUGH RHC'ErNESTs ridge allowed to progress virtually unimpeded, often at a Convective Temperature less than the afternoon maximum surface temperature. Steep subcloud lapse rates (28*CAn) have been noted by several researchers as a precursor to downburst occurrence. For dry environments at high elevations, such as Denver, this would translate to a layer extending from 700-S00 mb. At lower and more moist environments, e.g. Dallas-Ft. North, the comparable layer would extend from 850-700 mb. This lower layer was examined for the South Texas events. However, none of the cases exhibited a lapse rate as steep as 8*C/km, giving a preliminary indication that more than evaporative cooling must be at work in producing the South Texas downburst. S. POSSIBLE GENERATING MECHANISMS It is likely that a number of different processes were at work in generating the South Texas downbursts. Evaporative cooling in the subcloud layer or through the penetrative downdraft process, water loading and the form of precipitation within the cloud, and vertical moamntum transport have been suggested as generating mechanisms. The role of each in the production of South Texas downbursts was considered. Research has pointed to a threshold value of 8*C/km for lapse rates from 850-700 mb for downburst occurrence (Caracena fit gj,., 1986). This layer was taken to represent the subcloud layer. In the South Texas events, this was not always the case. Analysis of the soundings indicated cloud bases were well below 700 mb and often below 850 mb. However, the lapse rate from the surface to the CCL was dry or near dry adiabatic in most of the cases despite high moisture content and so probably approached the threshold value in those cases. In those events where the subclood lapse rate was less than dry adiabatic, we cannot rule out evaporative oooling as a generating mec h a n ism. Srivastavt (1985) has shown that as lapse rates become more stable, higher rainwater content may oompeneate for the increasing stability, providing energy to the downburst through the evaporative oooling process. With the high surface dev points, subcloud mixing ratios (Table 1) and precipitable water content in the events studied, evaporative cooling may 222 Session 7.5 also play a much more significant role in these events than previously thought (Ladd, 1989). The penetrative downdraft process probably plays only a minor role, if any, in South Texas. In a high percentage of the events examined, moisture extended well above the 500 mb level where it was capped by a somewhat drier layer. This dry layer was not nearly as impressive as in the North Texas environment. Water or precipitation loading certainly would be a major contributor to downburst generation in South Texas. As was mentioned, precipitable water contents were high and, according to Table 1, K Index values were often above 30*C. Further, instability values and the extent of positive areas on the soundings suggest the existence of updrafts that could have supported high rainwater content. Indeed, heavy rain was reported with many of the events. An additive effect to the water loading process would be the rapid melting of precip¬ itation in the form of ice. Caracena and Maier (1987) have pointed out that the melting of large quantities of ice particles could add to the negative buoyant energy in the downdraft. Large hail was reported in two of the South Texas events, with small hail reported in another. As suggested in the preliminary study by Ladd (1989), it is unlikely that vertical momentum transport contributes much to downburst generation. The additional events examined bore this out. Upper-level wind speeds in a high percentage of the events were generally quite weak. 6. OPERATIONAL CONSIDERATIONS The initial aim of this climatology was to identify conroon characteristics of downburst- Preducing thunderstorms across South Texas. Experience has shown that many of these characteristics exist to a certain degree on a large number of summer days. No comparisons were made to atmospheric conditions that either were not conducive to thunderstorm development or did produce storms without reported downbursts. Such a comparison would be a natural extension to this study. Nevertheless, a first attempt at an operational checklist for downburst forecasting can be made. Such a checklist is given in Figure 2. It is important to emphasize that this checklist is based only on characteristics identified in South Texas downbursts and may not prove useful in other areas of the country. In addition, independent testing to determine the validity and uniqueness of the parameters cited is needed. Such testing and subsequent refinement are planned beginning in the suntner of 1991. The checklist is quite subjective in nature and is intended for use once a potential for thunderstorm activity has been determined. It focuses on two phases of the forecast problem; assessing the potential for downburst development and determining the most likely location for downburst occurrence. In assessing the potential for downburst occurrence, the upper-air sounding is most significant, since the South Texas downburst appears to be highly thermodynamic-driven. However, close attention should be given the surface chart, since the triggers are mechanical in nature. Once it is judged that the potential is high, close monitoring of convection across the area and for some distance outside the area is required. Satellite and, to a lesser degree, radar and hourly surface analyses are useful in tracking outflow boundaries that could act as triggers of second-generation convection. Of primary concern is the movement of outflow boundaries into an area of increasing instability and moisture convergence. AFOS Data Analysis Program (ADAP) charts (Bothwell, 1988) are particularly useful in assessing these parameters. If the outflow moves into these areas and collides with an existing temperature ridge and/or another boundary, such as a sea- breeze front, the likelihood of downburst occurrence dramatically increases. Hopefully, as additional cases are collected and examined in forthcoming summer seasons, an attempt can be made to quantify the checklist. 7. REFERENCES Bothwell, P. D., 1988: Forecasting Convection with the AFOS Data Analysis Programs (ADAP-Version 2.0). NOAA Tech Memo NWS SR-122, 92 pp. Caracena, F. and M. W. Maier, 1987: Analysis of a microburst in the FACE meteorological mesonetwork in South Florida. Mon . Mfia- Rev.. 115 . 969-985. _, R. Ortiz and J. A. Augustine, 1986: The Crash of Delta Flight 191 at Dallas-Fort Worth International Airport on 2 August 1985: Multiscale Analysis of Weather Conditions. NOAA Tech Report ERL 430-ESG 2. 33 pp. Ellrod, G., 1989: Dallas microburst storm environmental conditions determined from satellite soundings. AMS Third Interna¬ tional Conference on Aviation, Anaheim, CA. 15-20. Gamer, T., 1990: Operational Problems Associ¬ ated with West Texas Dry Microbursts. NOAA Tech Attach NWS WR-90-14, 7 pp. Ladd, J. W., 1989: An Introductory Look at the South Texas Downburst. NOAA Tech Memo NWS SR-123, 19 pp. Purdom, F. W. and P. C. Sinclair, 1988: Dynamics of convective scale interaction. AMS 15th Severe Storms Conference, Baltimore, MD, 354-359. Read, W. L. and J. T. Elmore, 1989: Sumner season severe downbursts in North Texas: Forecast and warning techniques using current NWS technology. AMS 12th Weather Analysis and Forecasting Conference, Monterrey, CA, 142-147. _, 1987: Forecasting Potential Severe Downburst Days in North Texas. NOAA Tech Memo NWS SR-121, 45 pp. Sohl, C. J., 1987: West Texas Dry Microbursts of 21 May 1986. NOAA Tech Memo NWS SR-121. 45 pp. Srivastava, R. C.. 1985: A simple model of evap- oratively driven downdraft: Application to microburst downdraft. J. Atmos . Sci .. ii, 1004-1023. Wilson, J. W., R. D. Roberts, C. Massinger, and J. McCarthy, 1984: Microburst wind struc¬ ture and evaluation of Doppler radar for airport wind shear detection. J. Cliaaifi AppI . Meteor .. 22. 898-915. 223 Session 7.5 SOUTH TEXAS DOWNBURST CHECKLIST DATE: FORECASTER: NOTE: This checklist is not designed to assess the potential for convective storm development across South Texas. Rather, it should be used to assess the potential for dovnburst production from any storm that does develop. PART 1 -- ASSESSING THE POTENTIAL FOR DOWNBURSTS MODIFY the most recent sounding by using the expected afternoon maximum temperature and changes in the boundary layer mixing ratio: YES NO Will the subcloud layer approach dry-adiabatic? _ _ 5re be only a weak cap at best? _ _ Will the instability increase? _ _ Will precipitable water remain high? _ _ From an analysis of hourly surface and wind vector change charts, satellite, and radar: 1. Will 2. Will 3. Will 4. Will YES NO 5. Is upstream convection producing outflow? 6. Is moisture increasing over a particular area? 7. Is a temperature ridge developing? 8. Is wind field convergence increasing? PART 2 — DETERMINING THE LOCATION OF PROBABLE DOWNBURST OCCURRENCE From a continued analysis of hourly surface charts, ADAP moisture convergence and instability charts, and wind vector change charts: YES NO 1. Is instability increasing in a particular area? 2. Is moisture pooling into that area? 3. Is the temperature ridge persisting? 4. Will outflow interact with other boundaries? 5. Will outflow (or the interaction) impinge on the area of increasing instability, pooling of moisture and temperature ridge? THE DOWNBURST WILL HOST LIKELY OCCUR FROM A STORM TRIGGERED BY OUTFLOW THAT IMPINGES ON THE TEMPERATURE RIDGE IN THE VICINITY OF THE GREATEST MOISTURE POOLING (CONVERGENCE). Fig. 2. Prelimi n a r y checklist for forecasting South Texas downbursts. 224 Session 8.1 USING PROFILER DATA IN AVIATION FORECASTING Eric R. Thaler National Weather Service Forecast Office Denver, Colorado 1. Introduction The National Weather Service Forecast Office in Denver, Colorado, has been integrating wind profiler data into its daily operations for about ten years. These data have been used in all aspects of the forecast function and have been particularly useful in aviation forecasting. Due to the short term nature of the aviation forecast, hourly winds aloft provided by the profiler are very useful in identi¬ fying many features that have a direct impact on the aviation commu¬ nity. Shown below are several cases where information provided by the wind profiler has had a direct im¬ pact on the aviation forecast or has been used to better understand the physics of a particular situation. This understanding can then be ap¬ plied to similar cases which occur in the future. 2. Case A Figure 1 shows a vertical time section of winds aloft as observed by the wind profiler located at Stapleton Airport in Denver, Colora¬ do. Time runs from right to left. The first profile at 1200 UTC 15 February 1990 (15/12, this date/time group symbol will be used throughout the paper) shows weak upslope (northerly) winds at the lowest level with strong south southwester¬ ly winds aloft ahead of an approach¬ ing upper level low. As time goes on, note the changes in the winds. In the mid levels (around 500 mb) the winds back around to easterly. then to northerly and finally to north northwesterly. This signature depicts the passage of the cyclone to the south of the profiler. No¬ tice that after the cyclone passes the low level winds increase. This stronger low level flow intensifies the upslope which in turn lowers the ceilings and visibilities as the snowfall increases. The trend in the winds at the end of the time section pointed to a rapid improvement in conditions. The westerly winds in the 700-600 mb layer are downslope off the Front Range of the Rockies which leads to drying and warming. Indeed, the snow ended and the ceilings and visibilities rapidly increased in the two hours following the 15/21 observation in the time section. It is important to realize that everything depicted in this figure occurred between the usual upper air observations at 0000 and 1200 UTC. Consequently, the forecaster had detailed knowledge of the upper winds throughout the event, which would not be available using only the standard rawinsonde data. Since this type of pattern is often re¬ peated, knowing what occurred in this case can be translated into timely terminal forecast updates in future cases. 3. Case B Figure 2 shows the profiler data from Stapleton on 23 January 1990. Again time runs from right to left. The concern on this day was 225 Session 8.1 Figure 1. Session 8.1 whether or not high winds would occur. The first profile (23/01) is close to the standard time of rawin- sonde release and shows relatively weak winds in the lower to middle levels. Standard upper air charts (not shown) were equally unimpres¬ sive as to the high wind threat based on upstream data. The pro¬ filer information portrays a differ¬ ent story as winds aloft increase significantly throughout the time period shown. By 23/10, 55 knots are being observed just 5000 feet above the ground. It was decided that a high wind warning was needed based solely on the profiler data. Again all of these changes occurred between rawinsonde releases. The forecaster in this case did not have to wait for the 1200 UTC rawinsonde data to determine the evolution in the winds aloft and was able to provide several hours of lead time on the warning. In a similar high wind case, model data indicated that winds aloft would be increasing within the 0000 UTC to 0600 UTC time period. Comparing the 6-hour winds aloft forecast (gridded data) from the 0000 UTC NGM model run with 0600 UTC profiler data showed that the NGM had overforecast the 700 mb wind speeds by 30 knots. The decision was made to continue the high wind watch rather than upgrade to a warn¬ ing. If the decision to warn had been issued based solely on model data, a false alarm would have re¬ sulted. 4. Case C Figures 3a and b depicts the vertical time sections from both the Stapleton and Platteville (about 30 miles north of Denver) profilers for the 2-3 February 1990 time period. Due to a different frequency of the radar, the lowest range gate avail¬ able from Platteville is just above 700 mb. The Platteville 350 mb winds show an intriguing pattern in this time section. First, note the backing of the flow after 02/00 from north to west and then to southwest as a short wave ridge passes over¬ head. The southwest flow then in¬ creases rapidly as the next short wave trough approaches. The flow decreases again as the trough line nears the profiler and switches to westerly at 03/00. Thereafter, as the trough pushes east, the north¬ west winds behind it increase quite rapidly. Note that the trough at 500 mb passed the Platteville pro¬ filer at 02/18, a good six to eight hours ahead of the higher level system. This nicely depicts the westward tilt with height of the wave. There is also an indication that the trough may have possessed a closed circulation as shown by the easterly winds at 02/20 between 400 and 500 mb. If the sole source of winds aloft data had come from the rawin- sondes, only three profiles would have been available during this time frame. This may have resulted in the omission of pertinent meteoro¬ logical information to the forecast¬ er. The Stapleton profiler data show a similar trend aloft but the lower level winds are even more important. Surface analyses showed no obvious indication of frontal activity in the Denver area and the sea level pressure gradient (not shown) suggested an easterly wind (this is a good example of how mis¬ leading sea level pressure analysis can be in higher elevation areas). Seeing the increase in low level northerly winds on the profiler, forecasters were able to anticipate 227 Session 8.1 } j J j £ j J i j ^ ,,, { j \~y j 'y y ^ ^ i y ^ w, 77777 ^—i N. <7 J J J J ] y V y ^ J S 7 7 / ^ ’; ] ^ y ^ y y y y* U l ^ / ^ r / L << ^ ^ ^ f 7 H r r < ' V y / y y y y / y -/ j / ^ / V -4 i i ®3/M 93/M 03/05 02/22 02/21 03^3 ,2 ' 19 M 02x17 02/16 02/13 02/11 02/09 02/07 03^3 03/01 62/23 «2/18 02/16 02,14 02/12 02/10 62/08 02/06 02/05 STAPL feb 73 / I 99 c? Figure 3a. 7 ^ y i i J J *1 7 n *t *1 7 J 7 7 i 7 ryii’jb 7*2 — / j z/ y y y ■ y y y y y y y y / / / y y ^ ^ / 7 7 7 7 \ \ v \I K < < 7 / / 7 / l\ //<-///> f^f ; \ ^ /* / 7 7/ 7 /• / 7 7 / 7 /* 7 7 7 V\ 7 \ i 7 * 7 7 7 7 v y 7 7 * y - J 03/06 03/0* 03/02 03/00 02/22 02/21 62/15 02/1? 02/16 02/13 02/11 02/09 02/0? 02/0* 02/02 02/0C 03/OS 03/03 03/61 02/23 02/20 02/1* 02/16 02/1* 02/12 02/10 02/08 02/06 02/05 02/03 02/01 01 PLATT ~cb 3-3, /? Session 8.1 Although not documented here, wind profiler data does get heavy use in the warm season as well as the cool season. Two of the most important uses of these data in the warm season include monitoring the vertical wind shear and the steering flow for thunderstorm, tornado and heavy rain forecasting. As in the cool season, profiler data are also used to monitor low level winds to determine the existence of upslope and downslope flow and the develop¬ ment or dissipation of low ceilings. The cases presented here have been based, for the most part, on data from a single wind profiler. As more profilers are installed, forecasters will quickly discover the benefits of a network of such systems and how the data these pro¬ filers provide can improve aviation forecasting. 232 Session 8.2 SOME CONSIDERATIONS ON A DENSITY CURRENT NOSE AND LOW LEVEL JET IN CASE STUDY Jim Johnson National Weather Service Office Dodge City, Kansas 1. INTRODUCTION With the advent of the wind profiler demonstration network across the central United States, meteorologists have been able to obtain hourly vertical wind profiles yielding a temporal resolution hith¬ erto available only occasionally in research projects. The result has been that wind configurations which do not initially seem to meet our present understanding of the atmo¬ sphere have occasionally appeared. The following investigation involves one such event. A 404 Mhz, three beam array wind profiler is located at Haviland, Kansas. On February 23, 1991, a polar airmass invaded the western high plains passing over this profiler. A synoptic composite and an AFOS produced time cross- section from the profiler are of¬ fered in Figures 1 and 2. The front on the leading edge of the polar airmass was preceded by a low level jet event. As seen on the AFOS time cross-section, a sharp wind shift has occurred some 1200 to 1500 me¬ ters above the surface two hours before the surface boundary passed the profiler. Analysis of the data set was limited to what could be gained from the time cross-section data avail¬ able from the AFOS computer. Basic surface analysis was used to locate the front and find the velocity of propagation. Using wind direction and velocity, computations were accomplished for speed and direc¬ tional shears in the vertical and for directional shears in the hori¬ zontal. Vertical motion associated with the wind field was computed via a stream function. Except where x-y plane (plan view) representation is helpful for visualization, the majority of the calculations were performed in two different orientations of the x-z plane. A few possible explanations for the configuration are offered based upon the results of the data analy¬ sis. 2. ANALYSIS SCALES AND PHYSICAL MODELS A brief discussion of the phys¬ ical scale involved is needed. Since we are dealing with a large scale event in plan view, the length scale becomes synoptic. The data represented by Figure 2 (the AFOS time cross-section) is in the range of a few kilometers in the vertical. Therefore, the x-z plane analyses presented are generally in the Meso- B scale (Fujita, 1981). The assump¬ tion was also made, for this study, that the time cross-section was a reasonable approximation of the frontal environment at a mid-point in that time frame. Thus, using the velocity of propagation, time "t" in the abscissa may be converted to length "x". The study of any frontal system must be conducted under certain assumptions as to the physical model 233 Session 8.2 involved. Hoskins and Bretherton (1972) found that inviscid, adiabat¬ ic frontogenesis produces infinites¬ imally narrow fronts within a finite time. Their work was based upon the assumption of cross-front geostrop- hic balance. It is clear from the time cross-section presented in this case that accelerations involved are largely ageostrophic and non-hydro- static. As will be seen in the data analysis, there is also reason to believe that Kelvin-Helmholtz insta¬ bilities are occurring so that mix¬ ing effects may lead to the break¬ down of Hoskins' and Bretherton's (1972) model. The author found two conceptual models which lend themselves to the wind field found on the time cross- section. The first is a density current model as detailed in the laboratory by Simpson (1972, 1982), Simpson and Britter (1979, 1980); in the atmosphere by Charba (1972), Goff (1975) and Shapiro et al . (1985), Young and Johnson (1984), Smith and Reeder (1988); and finally numerically simulated by Sha et al ., (1991). The second is the conceptu¬ al model of a katafront as described by Bergeron (1928). 3. THE DENSITY CURRENT MODEL Although this particular case is of nearly an order of magnitude larger, there is a resemblance to the density current model. In order to equate this case study with the above mentioned work in density currents, several computations were undertaken. A Froude number as computed by Simpson and Britter (1979) and by Charba (1972) was not possible owing to the lack of both temperature and density except for the bottom of the fluid. A hybrid type of Froude number was available however. It was defined to be the ratio of the height of the density current (cold dense air behind the front) to the total depth of the fluid (taken as the troposphere). This ratio has been used by Simpson and Britter in their laboratory models and is sug¬ gested in accepted texts on fluid dynamics (White, 1991). Simpson and Britter (1979) found in laboratory model density currents that this ratio was normally 1/5 if the depth of the Kelvin-Helmholtz billows collapse region was omitted. Assum¬ ing the absence of K-H billows in this case, and a total winter tropo¬ spheric depth of approximately 10 kilometers, a Froude number of 1/5 was found also. Even allowing for the presence of the K-H instabili¬ ties along the top of the cold air boundary (which there is some evi¬ dence for), this hybrid Froude num¬ ber still compares quite favorably to those found by Simpson and Britter (1979) in the laboratory and by Charba (1972) in the atmosphere. Simpson and Britter (1980) also found that ambient flows in the direction of movement of the front (as in this case) increased the frontal speed. They compared their laboratory data to atmospheric data compiled by Miller and Betts (1977), Clarke (1961), and Goff (1976). The results of a ratio of ambient flow, U,, to the speed of the flow behind tne front, U 3 , gave values of be¬ tween positive and negative unity while a ratio of velocity of propa¬ gation of the front, U Q , to the ve¬ locity of the flow behind the front, U,, produced values of just less tnan unity. Computing these same ratios for the present case study, values of just less than positive unity were found for both. 234 Session 8.2 Reynolds and Prandtl number computations were also not possible due to the lack of temperature and density profiles of the two air- masses (Rawinsondes were available in both airmasses but with height scales in the Meso-B range it is hardly appropriate to mix in temper¬ ature and density profiles of synop¬ tic spacing). It was possible, however, to use surface data in the frontal and pre-frontal zone to approximate virtual temperatures for the boundary layer. Using these values, a velocity of propagation for the leading edge of the boundary was calculated as 9.2 ms' 1 after Shapiro et al . (1985), Carbone, (1982) and von Karman (1940). This is exceptionally close to the veloc¬ ity of propagation of the boundary calculated from 12 hours of surface synoptic data as 9.17 ms' 1 . In a study of thunderstorm outflow boundaries treated as densi¬ ty currents, Goff (1975) included two cases where there was a pro¬ nounced "overhang" at approximately 1 kilometer above the ground. In fact, the average velocity of propa¬ gation for 17 cases presented by Goff (1975) is found to be 9.72 ms’ 1 which again compares closely to the 9.17 ms' 1 found in this study. Goff (1975) also gave an empir¬ ical equation for the frontal ad¬ vance velocity U , in terms of the wind behind the front U 3 and the ambient flow U 2 , U 0 = .7 U 2 + . 3 U 2 (1) Using "front relative" values of velocity from this case study, a value for U c of 9.25 ms* 1 is arrived at which again compares closely to the manually calculated velocity of propagation of 9.17 ms’ 1 . Considering these factors, there seems to be a reasonable basis for applying density current theory to this particular polar boundary. 4. THE KATAFRONT MODEL The basic conceptual model for the Katafront (Bergeron, 1928) in¬ volves the ambient flow (in the less dense air) being of greater velocity than the propagation velocity of the more dense air comprising the front as was found in this case. This leads to a katabatic wind with es¬ sentially downward motion just ahead of the frontal zone in the lowest levels. Although somewhat compli¬ cated by the presence of the low level jet, this type of a model is somewhat evident from cursory exami¬ nation of the time cross-section winds. The actual calculated mo¬ tions will show that while the ambi¬ ent flow is indeed away from the frontal zone, there is upward motion occurring in the region where the katafront model requires only down¬ ward motion. For this reason, the katafront model was not used. 5. CALCULATED MOTIONS In order to focus upon the area of interest, the wind data from the time cross-section was reduced to a temporal range of 0900 UTC to 2000 UTC and a height scale of 0 to 2500 meters. The rest of the calculations and analysis of data were performed in this range and domain (Figure 3). Initially, the wind data were reduced to their zonal and meridio¬ nal components on a standard polar coordinate axis. Analysis of the zonal wind component (u) in ms’ can be seen in Figure 4. The greatest velocities in the zonal direction are away from the front and located above and to the right of the low 235 Session 8.2 JJJiYW 3 l y sr^ i jjJaihv- «H ^ "'V-t i*? t-I *-l *H H H H H *H ^ H H jjjjj, jjj- \i.-\- 1 li.W © © r\ © © CO © © cn © ■S CO o r 1 \ w JQ ro — Ljj M Q0: S -JUJLJ W <110. in f-U 2 Z f- \ ouo n noz w ^ i CD O \ n ro \ n DJ \ n OJ > 1 c o 6 — T3 C >> >. — »« L. I E ' r ! JJJJ JJJJ JJi) JJ j ny *uu iiiujj JJJJ^jJJj JJJJJH) \^r— JJJJ JJJ J V^T- 'iMJJJJJJi JJJJ JJJ \\V^~ yyjjjjjjjjjjjjj ia\\w^ 3YU. Ull 236 Session 8.2 level jet. The indication is, as expected, that the low level jet of some 65 knots is rapidly extracting mass in this area. It is therefore possible that some horizontal pres¬ sure gradient has established itself between the low level jet and the more dense cold air aloft at the leading edge of the boundary. The existence of the density nose aloft may be the result of these forced mass adjustments in the horizontal. At this point, a new set of axes were constructed with the ab¬ scissa being normal to the front (Figure 5). Figure 6 offers a view of the results with values for u now being "front relative". Again, the greatest velocities are away from the boundary. There is still a maxima that seems to be associated with mass extraction by the low level jet but there is now a second maxima located just ahead of the advanced wind shift aloft. Analysis now proceeded after Charba's (1972) gravity current model analysis of a thunderstorm gust front. The assumption was made that values for dv/dy become an order of magnitude smaller than the values for du/dx when the x axis is normal to the boundary. In the mass continuity equation for incompress¬ ible flow then, dv/dy is assumed negligibly small compared to du/dx everywhere in the vertical plane from the surface to 2500 meters and using the boundary layer assumption du + dw dx dz = 0 ( 2 ) After Clarke (1961), a stream function is developed such that u=- dH dz w= and ar dx (3) (4) It is important to remember that this represents only the rota¬ tional, non-divergent part of the wind. Since u is known as a function of height, the stream function field is computed by H = -fudz (5) After the stream function has been computed, w can be obtained by fi¬ nite differencing with w= ^ (x-Ax) ^ 80% are striped; (c) Same as (b) except that the RH anlaysis is on the 305 K surface. In all three panels, observed values are plotted in small numbers (from Benjamin, 1989). 293 Session 10.1 the West, including a closed circu¬ lation over New Mexico, and a large ridge stretches from the Gulf Coast to the northern Great Plains. A short wave has entered the Pacific Northwest states. Note the 500-mb contours, one crossing the northern U.S. and southern Canada, and the other enclosing the circulation over New Mexico. The relative humidity field at 500 mb is shown in Fig. 8b, with values greater than 80% striped. One patch of moist air is centered near Oklahoma City, Oklahoma; anoth¬ er stretches from Idaho to Minnesota. The connection between these patches and the flow pattern is not obvious. Contrast this with Fig. 8c, the relative humidity anal¬ ysis for the 305 K isentropic sur¬ face. The moist air in the Pacific Northwest is associated with the short wave. The tongue of moist air from the Gulf Coast to North Dakota is associated with a moist conveyor belt and upgliding motion in advance of the western trough. The second example is taken from Benjamin et al . (1991b). The MAPS 250-mb wind analysis for 0000 UTC 20 January 1990 is shown in Fig. 9a. Note that the short and long barbs and flags correspond to 5, 10 and 50 m s' 1 , respectively (not knots). A strong jet stretches from southwest Texas to New England. The jet is particularly strong (more than 90 m s' 1 ) over the northern Great Lakes, having strengthened considerably in the previous 12 hours. The error in the 12-hour NGM forecast (Fig. 9b) verifying at 0000 UTC exceeds 10 m s' 1 from Minnesota to Maine. The corresponding error in the 3-hour MAPS forecast (Fig. 9c) verifying at the same time is less than 5ms' 1 . The improvement lies in the frequent assimilation of ACARS data by the MAPS model and, to a lesser extent, in the use of isen¬ tropic coordinates, which are close¬ ly spaced near the tropopause. 4. How MAPS Compares to NMC's Nested Grid Model Figure 10 gives the RMS vector error of wind forecasts verified for the month of December 1991. These forecasts were interpolated to the locations of rawinsonde sites in the U.S. and southern Canada and dif¬ ferenced with the observed winds at 0000 and 1200 UTC. Forecast accura¬ cy clearly improves with assimila¬ tion frequency. Improvement is greatest in the high troposphere, where ACARS reports are plentiful, but it is also substantial in the lower troposphere, in part, because of the hourly wind profiles now available in the central United States. The models have access to the same data and have comparable resolution, but the NGM has more sophisticated physics. 5. Porting MAPS to NMC and Experi¬ mental Output During 1991, MAPS was ported to computers at NMC. As of this writ¬ ing (mid-February 1992), most prob¬ lems associated with the transfer from a VAX-based system at FSL to a Unix-based system at NMC have been worked out. Modules for data ingest, quality control, and objective anal¬ ysis are running successfully on the NAS 9000 and Cray Y-MP computers at NMC. The complete system is expect¬ ed to be running at NMC by March 1992, at which time it will become known as the Rapid Update Analysis and (short-range) Prediction System (RUAPS). In support of ST0RM-FEST (the STORM Fronts Experiment Systems 294 Session 10.1 u i itj.t i m 1 1 1 1 rirm 1111 V;i rn 11 hi i i i i i i i i i i h i i itl P \ \ r ^ ^ ) it .. V “ C-Vy* A ^ J • i * J vjyj-ja** V ^ J : il< S? < V..^-JSY, VT /•<-■ > .i :#-0£ y* / — f 1 / , v .. ,sr /'-t r x.;v\n^|A ;• ■•„ (_ ’ '.• ;.•• o _ -1 I I I I I I I I I I I I'V.I? I I I f.l I I I I I I I I I I I I I 1:1 I I I I I I I I I I I 1 I 1 I I I I I I I 1 1 I r (b) («0 Fig. 9. (a) the MAPS 250-hPa wind analysis for 0000 UTC 20 January 1990. The next two panels illustrate vector differences in m s" (analysis minus forecast) at 250 hPa between wind forecasts and the verifying analysis in (a), (b) For NGM 12-hour forecast, (c) For MAPS 3-hour forecast (from Benjamin et al., 1991b). 295 Session 10.1 Cj P-. 100 . 200 . 300 . 400 . 500 . 600 . 700 . 800 . 900 . RMS DIFF - (FCST-RflOB) HIND VECTOR 4. 5. 6. 7. 8. m s - 1 Fig. 10. The root-mean-square vector error (m s" 1 ) in wind forecasts produced by MAPS and the NGM. The error was estimated by comparing the predicted winds with winds measured by rawinsonde in the U.S. and southern Canada during December 1991. The symbols labelling the curves have the following meanings: 3 - MAPS 3-hour forecast; 6 - MAPS 6-hour forecast; N - NGM 12-hour forecast. 296 Session 10.1 Test) from 1 February through 15 March 1992, FSL is sending output fields from MAPS to NMC for experi¬ mental dissemination. The output is interpolated to the 80-km C-grid (the standard grid used for NMC model output), converted to GRIB (GRIdded Binary) Version 1 format, and stored in "common" (COM.) files on disks at NMC for access by autho¬ rized users. During 1992, these users are located in Kansas City, Missouri (National Severe Storms Forecast Center and the National Aviation Weather Advisory Unit); Norman, Oklahoma (National Severe Storms Laboratory and the National Weather Service Forecast Office); and Denver/Boulder, Colorado (Fore¬ cast Systems Laboratory and the Weather Service Forecast Office). Kansas City receives the output via its VDUC line to NMC. Other sites receive it via Internet. Once MAPS/RUAPS runs regularly and reliably on NMC's computers, the transmission of output by FSL to NMC will cease, and dissemination will occur directly from NMC. If tests of the rapid update cycle are suc¬ cessful in 1992, national dissemina¬ tion is possible by 1993, but it should be noted that the current communications network (AFOS) cannot accommodate the required bandwidth. ACKNOWLEDGMENTS I thank my colleagues, Stan Benjamin, Patty Miller, Tracy Smith, Kevin Brundage, Dongsoo Kim, Pan Zaitao, and Dezso Devenyi, who helped develop MAPS. Special thanks to Stan Benjamin, who supplied many of the figures used here. From the beginning, the National Meteoro¬ logical Center has supported efforts to incorporate MAPS into its opera¬ tional cycle. The Facilities Divi¬ sion of FSL has assisted on numerous occasions by decoding incoming data. REFERENCES Benjamin, S.G., 1989: An isentropic meso-alpha-scale analysis sys¬ tem and its sensitivity to aircraft and surface observa¬ tions. Mon. Wea. Rev. . 117 . 1586-1603. _, 1991: Short-range forecasts from a 3-h isentropic-sigma assimilation system using ACARS data. Pre¬ prints. Fourth International Conference on Aviation Weather Systems (Paris, France), Amer. Meteor. Soc., Boston, MA, 329-334. _, T.L. Smith, P.A. Miller, D. Kim, T.W. Schlatter, and R. Bleck 1991a: Recent improvements in the MAPS isentropic-sigma data assimila¬ tion system. Preprints. Ninth Conference on Numerical Weather Prediction (Denver, CO), Amer. Meteor. Soc., Boston, MA, 118-121. _, K.A. Brewster, R. Brummer, B.F. Jewett, T.W. Schlatter, T.L. Smith, and P.A. Stamus, 1991b: An isentropic three-hourly data assimilation system using ACARS aircraft observations. Mon. Wea. Rev. . 119 . 888-906. Bleck, R., 1984: An isentropic coordinate model suitable for lee cyclogenesis simulation. Estratto dalla Revista di Meteoroloqia Aeronautica . 44, 189-194. 297 Gandin, L.S., 1963: Objective Analysis of Meteorological Fields . Gidro- meteorologicheskoe Izdatel'stvo, Leningrad. Translated from Russian by Israel Program for Scientifi Translations, Jerusalem, 242 pp. Session 10.2 THUNDERSTORM FORECASTING USING GRIDDED MODEL OUTPUT AND THE FAA'S METEOROLOGIST WEATHER PROCESSOR (MWP) Thomas M. Hicks and James R. Ott Center Weather Service Unit Fort Worth, Texas 1. INTRODUCTION One of the primary concerns of the Center Weather Service Unit (CWSU) is that of forecasting thun¬ derstorms. This paper presents an aid for thunderstorm forecasting that uses the gridded model output and the FAA's Meteorologist Weather Processor, or MWP. Before we begin, let us briefly introduce the MWP. 2. MWP BASICS The MWP is part of the FAA modernization plan and represents a major step forward in technology for the CWSU. In terms of technology, the CWSU has always been near the bottom of the scale. Teletypes and facsimile machines have continued in the CWSU, while the rest of the Weather Service has been using AFOS. But with the MWP, technology in the CWSU is now considerably more ad¬ vanced than that offered by AFOS. The MWP is basically a communi¬ cation and processing system con¬ sisting of a satellite dish, two minicomputers, a meteorologist work¬ station, and nine briefing termi¬ nals. Data is transmitted by satel¬ lite to the CWSU and consists of satellite, radar, alphanumerics, and graphic products. Graphics can be AFOS products, manually generated products produced in the CWSU, or GRIB (gridded binary) products. 3. GRIB (Gridded Binary Products) GRIB data is output from the NGM and AVN models and is available twice daily. Forecast data is avail¬ able for six-hour intervals, and some of the forecast parameters include: height temperature relative humidity u & v components vertical velocity precipitation pressure lifted index. 4. OBJECTIVE Our objective was to use the gridded data on the MWP to produce a graphic product that displays thunderstorm-related parameters--but only those that meet certain thresh¬ old values. By combining several parameters on a single composite chart, the most favorable location for convection should be the point where the most "bullseyes" converge. Such a chart could be evaluated much more quickly than could a number of individual charts of different parameters. From the available GRIB data, we chose the following parameters to be related to thunderstorm develop¬ ment: • Surface moisture convergence • 850 mb moisture • 850 mb convergence 299 Session 10.2 • 700 mb Omega (vertical motion) • 250 mb divergence. This very basic model simply states that convection is most like¬ ly where we have low level (surface and 850 mb) moisture and conver¬ gence, vertical motion, and upper- level divergence. The intensity and areal coverage of the convection should be related to the magnitude of the parameters. 5. EXAMPLE 1: November 29, 1991 Figure 1 is an example of our composite chart for November 29, 1991. This chart at first glance appears quite busy, with many lines and colors. But after becoming familiar with the parameters and colors, the chart can be quickly evaluated. (Note: The charts in this paper may be difficult to evaluate properly due to printing limita¬ tions. The MWP workstation, however, produces these charts with outstanding resolution and quality. The MWP also permits toggling off selected colors for easier interpre¬ tation.) 12-hr Relative Humidity . The first thing to look at is the legend at the bottom of the chart. The bottom-most legend indicates that NGM 12-hr 850 mb relative humidity is depicted in brown. The threshold value for this parameter is of 60%. Any lower value will not be printed at all. The second legend from the bottom is also NGM 12-hr 850 mb relative humidity, but in this case humidity values of 90% or more will be shown in white for quicker inter¬ pretation. On this day 850 mb rela¬ tive humidity was forecast to exceed 90% over a large part of the country from Utah to New England and south¬ ward to northeast Texas. 12-hr 850 mb Convergence . The next legend up from the bottom is 850 mb divergence. But our threshold value was chosen so that contours would be drawn (in blue) only for areas of convergence . In this case, the strongest 850 mb convergence was forecast over an area extending from the southwestern U.S. across the northern plains and southward to Missouri and northern Oklahoma. 12-hr 700 mb Omega . The legend indicates that 700 mb Omega is shown in yellow. Our threshold value was chosen to indicate only areas of upward vertical motion. In this case, vertical motion was forecast to be most pronounced over the southwestern U.S. and from the northern plains southward to Oklahoma and Missouri. 12-hr 250 mb Divergence . The legend indicates that 250 mb diver¬ gence is shown in violet. On this day, strong upper divergence was forecast over the southwestern U.S. and over the northern plains with an axis extending southward to Missouri. 12-hr Surface Moisture Conver¬ gence . There is no legend for sur¬ face moisture convergence because this is an AFOS graphic that is overlaid onto the other data. (There is no GRIB data for this parameter available through the Family of Services, which is the data source for the MWP). Surface moisture convergence is indicated in green; and on this day, an axis of particu¬ larly strong values was indicated from southeastern Nebraska to north¬ eastern Texas. The Bio Picture . To reach a conclusion about what the data is suggesting, we must consider the 300 Session 10.2 l \ •-V * I / •... * _^V 1 ■ *- -—• i ' T ’ ■ < _-■•• .•'/*■-’••<“ Q.&l'.Ksif \ ; ■'• • 7 / » •: : ; .<,:•./• =," » \ -f ; v i ■ -.. .. t-0> fc•■;«’•: : ■. | '•.•■' .■ ; • {• i t. I ... ■ ‘ ;• t V. '•! 1/ ■ft. r v > -t'> L. \V' : ■•--..•MV V./-V ' J i l i Jff'r? _ V -v.._ K • '.■■• i -■ r \ ~ l ( ‘VW. 1 i . ; l 1 i?Cv i r V-- • / J • > • / / * r; / ■ l' * i 'i I < / /•ft, MU.. \ /.'/ 1 V:l * -t •• 7 r' ' • 4 \ V .. f / \ ‘ \ I -u : / , .'-y i 00*1 V. // • 1 ,1 V ^ ,/v ■-/ i 4’ r 1 ! .= / S ■■ I “jt.mi. ••> ■ i. : ■ i rSit- v /' <■ V f _ ’r>* ■ ‘“J f i ; « ,i * 1 * • < • V. ■ * J \ * /- '■"■■• v •■ rf?''-'i. ,< y * i2*friR' fcst/.sv^-« x coriF _2 FROM 1200ZI / ‘ - V 29r-W0V-193i / I if 7 •: i ^ 4 . Sl.. « ncii Wifi ■> liHF .’f.C'llt: OI.iCOh II. ."h . ••> 6 ; f:$0ns p tLHi 1 V'E. r:!.inl£i rs • k-2 ’ 002 « • vii; Figure 1 301 Session 10.2 locations of the bullseyes and how they coincide with one another. In this case, the bullseyes converge over the northern plains with an axis of bullseyes extending south¬ ward to northeastern Texas. Surface moisture convergence values over this area are especially high. Bullseyes are also indicated over the southwestern U.S., but one important ingredient, 850 mb mois¬ ture, is missing over Arizona. Verification . The forecast was verified by evaluating the corre¬ sponding radar summary (AFOS prod¬ ucts 90R and 90S) which was valid at 2335Z. Figure 2 shows this chart and indicates first of all that several severe thunderstorm an/or tornado watches were in effect at the time. The chart also shows lines of thunderstorms from north¬ western Arkansas to northeastern Texas. Additional reports indicated that a devastating tornado occurred during the afternoon over Missouri. The composite chart performed ex¬ tremely well for this event. 6. EXAMPLE 2: August 29, 1991 Figure 3 is an example of the 18-hr composite chart from the 12Z model output of August 29, 1991. In this case, the 850 mb relative hu¬ midity contours of 60-80% had been switched off at the MWP workstation to facilitate viewing the other parameters. The bullseyes especial¬ ly converged over southern Missis¬ sippi and southern Alabama, and convection did indeed occur over this area. But this example will deal with the bullseyes over south¬ ern Oklahoma and northern Texas. Figure 3 shows high forecast values of 850 mb and surface moisture con¬ vergence, along with quite strong vertical motion. The only ingredi¬ ent which was not depicted was 250 mb divergence. Verification . Figure 4 shows the MWP radar mosaic which was valid at 0630Z on August 30, corresponding to the 18-hr forecast from 12Z on August 29th. The radar reflectivity (and ground clutter) is shown for Amarillo, Oklahoma City, Little Rock, Longview, Stephenville, and Midland. An area of strong thunder¬ storms (levels 4 to 6) is shown over southern Oklahoma near the Texas border. This area corresponds ex¬ tremely well with the 18-hr fore¬ cast. This example was especially interesting, because the composite chart actually performed better than the Weather Service forecasts of August 29th. 7. CONCLUSIONS This paper has presented only two examples. But since this com¬ posite chart was first developed, many cases have verified extremely well. The gridded model output has shown itself to be an excellent forecast tool, and it has proven to be particularly helpful for fore¬ casting thunderstorms. Also, the capability of producing composite charts of gridded data that meet threshold values is very important and should be made available on future meteorologist workstations (AWIPS). 302 Session 10.2 Figure 3 ■j 'BHT TCC j~ ;.-U LEB bw i -—- - i 7~T* .... PEQ .rt .i' - - . , <<*f. - V I'M'' :k T !JL ^ V. 4 K - ... 45* • r r» i <••—. A._- * V r : r« c ■* %r. •.v;V* '** * f •-•r'.-O if >■! ■-> ■ ML C CD* - __ _p SP'E- Sfc 5 ■ c ^ ij".. •*««■ j.. - □rsfi-'s * (:TT-« > '?** .- i-* •*• *r i , si'y ■'■■*>•'*• ■J **•..* • me; •* y rV.T i.ri. * 7 T K T RRDRR MOSftIC Valid 0G3OZ . HU* LEi’END < NM> UE ML „ , 1 MH l 30-RUG-1991 / -•i c* roon r 5 •• ? 5 1 •: n 1{* . * — I - - -1 - •» * ■« ^ 0 <0 * * / l «0 Figure 4 ■CD. ftv. ML '. 1 '■ Ht.:. LCH 303 Session 10.2 APPENDIX MWP COMMAND STRING FOR COMPOSITE CHART * H12TSTMCOMP * composite chart of key tstm-related parameters * can be adapted for 6 and 18 hrs by changing 12 to 06 or 18 * command string should be scheduled to run around 0445z and 1645z * MAP,A_US * any suitable map will do * 850 MB RELATIVE HUMIDITY IS0,G,N,L85,F12,RH,60,100,S,E,N "C:BROWN" LEG,OFF * turns off legend IS0,G,N,L85,F12,RH,70,100,S,E,N "C:BROWN" ISO,G,N,L85,F12,RH,80,100,S,E,N "C:BROWN" LEG,ON * legend back on IS0,G,N,L85,F12,RH,90,100,S,E,N "C:WHITE" * 850 MB CONVERGENCE IS0,G,N,L85,F12,DIV,-0.4,100,S,E,N "C:BLUE" LEG,OFF ISO,G,N,L85,F12,DIV,-0.8,100,S,E,N "C:BLUE" IS0,G,N,L85,F12,DIV,-1.2,100,S,E,N "C:BLUE" IS0,G,N,L85,F12,DIV,-1.6,100,S,E,N "C:BLUE" IS0,G,N,L85,F12,DIV,-2.0,100,S,E,N "C:BLUE" IS0,G,N,L85,F12,DIV,-2.4,100,S,E,N "C:BLUE" LEG,ON * 700 MB OMEGA (vertical motion) ISO,G,N,L70,F12,0MG,-.002,100,S,E,N "C:YELLOW" LEG,OFF IS0,G,N,L70,F12,0MG,-.004,100,S,E,N "C:YELLOW" ISO,G,N,L70,F12,OMG,-.006,100,S,E,N "C:YELLOW" IS0,G,N,L70,F12,0MG,-.008,100,S,E,N "C:YELLOW" ISO,G,N,L70,F12,OMG,-.010,100,S,E,N "C:YELLOW" IS0,G,N,L70,F12,0MG,-.012,100,S,E,N "C:YELLOW" ISO,G,N,L70,F12,OMG,-.014,100,S,E,N "C:YELLOW" ISO,G,N,L70,F12,OMG,-.016,100,S,E,N "C:YELLOW" ISO,G,N,L70,F12,0MG,-.018,100,S,E,N "C:YELLOW" IS0,G,N,L70,F12,0MG,-.020,100,S,E,N "C:YELLOW" ISO,G,N,L70,F12,OMG,-.022,100,S,E,N "C:YELLOW" LEG,ON * 250 MB DIVERGENCE ISO,G,N,L25,F12,DIV,0.4,100,S,E,N "C:PURPLE" LEG,OFF ISO,G,N,L25,F12,DIV,0.8,100,S,E,N "C:PURPLE" ISO,G,N,L25,F12,DIV,1.2,100,S,E,N "C:PURPLE" ISO,G,N,L25,F12,DIV,1.6,100,S,E,N "C:PURPLE" ISO,G,N,L25,F12,DIV,2.0,100,S,E,N "C:PURPLE" IS0,G,N,L25,F12,DIV,2.4,100,S,E,N "C:PURPLE" ISO,G,N,L25,F12,DIV,2.8,100,S,E,N "C:PURPLE" IS0,G,N,L25,F12,DIV,3.2,100,S,E,N "C:PURPLE" ISO,G,N,L25,F12,DIV,3,6,100,S,E,N "C:PURPLE" IS0,G,N,L25,F12,DIV,4.0,100,S,E,N "C:PURPLE" LEG,ON * SFC MSTR CONVERGENCE 304 Session 10.2 RET,A,A,EXACTLY,NMCGPHL2Z ★ LAB,SCR,8,33,MB,"12-HR NGM FCST...TSTM COMPOSITE" "C:WHITE" LAB,SCR,8,18,MB,"FROM SVTIME2Z" "C:WHITE" LAB,SCR,8,3,MB,"$VDATE1" "C:WHITE" LAB,SCR,600,3,SB,"CTIME2Z" "C:GRAY" SHP,AATSTMCOMP,C0MP12HR * or your choice of names CLR, P 305 Session 10.3 ON THE POSSIBILITY OF USING THE GLOBAL SPECTRAL MODEL’S NORMAL MODES TO FORECAST HIGH ALTITUDE TURBULENCE Valerie J. Thompson National Weather Service Forecast Office Washington, DC ABSTRACT Previous studies have linked CAT to enhanced gravity wave activity. This study indicates that the forecast fields may contain gravity wave information that can be used as an aid in forecasting potential high altitude turbulence areas. A subset of permissible oscillations as modeled by the National Meteorological Center’s (NMC) Global Spectral Model (GSM), namely gravity modes with periods less than 36 hours, are examined as a high-altitude turbulence forecasting tool. Preliminary results show a low false alarm ratio of 9.1 and critical success index of 38.5 for a winter regime set of forecasts. The area of study covers most of the Northern Hemisphere with verifying observations of turbulence from Pilot reports (PIREPs). 1. INTRODUCTION Turbulence can be encountered in varying degrees depending on atmospheric conditions. Aircraft size and structure also influence the sensitivity to turbulence. Turbulence is both an economic and physical hazard that constitutes a serious threat to all vehicles that must fly in or traverse the atmosphere. Generally turbulence, including clear air turbulence (CAT) and turbulence associated with mountain waves, has been referred to as a mesoscale phenomenon. However, areas of moderate or greater turbulence sometimes extend over a synoptic scale horizontal domain with a vertical thickness of a few thousand feet. Certain satellite signatures within developing baroclinic disturbances have been associated with observed high-altitude turbulence (Ellrod 1985). It also has long been recognized that high-altitude turbulence (including jet cirrus associated turbulence) ahead of a developing low is common, especially in winter, and particularly in cases of jet stream associated explosive cyclogenesis (Bosart and Cussen 1973; Rammer 1973). CAT may arise from various effects, not all of which are well understood, and may be manifested through many scales, some of which are very difficult to measure (Baumgardner et al. 1990; Shapiro 1974). The interaction of mesoscale jet streaks within synoptic scale weather features as a generator of gravity wave activity is receiving substantial attention, especially in association with cyclone development near warm ocean currents such as the Gulf Stream and Kuroshio Current. Several studies have linked gravity wave activity with cyclogenesis or the enhancement of a synoptic scale or mesoscale weather feature associated with a synoptic scale weather disturbance. See Appendix A for examples. Internal gravity (or buoyancy) waves exist only when the atmosphere is stably stratified, and they are responsible for the occurrence of mountain lee waves. Thus, gravity waves are often associated with the formation of CAT (Holton 1979). Similarly, mountain waves have their maximum amplitude in the layer of strongest stability, which is usually at the tropopause. 306 Session 10.3 A favorable place for mountain lee wave formation is under an upper level ridge. The middle and upper level wind direction has a westerly component, which often is normal to the north-south oriented topographic features common across the continental United States, and wind speeds are generally 40 to 80 knots. With such velocities, wave motions fit an example of mountain forced vertical propagation of stationary waves discussed by Charney and Drazin (1961). At least one in every twenty-seven weather related aircraft accidents during the period from 1964 to 1982 was related to a mountain wave condition according to Blake (1988). In operations, routine dynamical model output and satellite signatures have been used to develop conceptual models relating synoptic scale patterns with the incidence of aircraft observed turbulence (Hopkins 1977). These conceptual models are part of the techniques applied by NMC’s Monitoring and Aviation Branch (MAB) meteorologists to produce forecasts of turbulence. In this paper, a new turbulence forecasting technique, based on dynamical model output, is presented. The objective of this technique is to use the numerical output from the GSM to assess the potential for high altitude turbulence associated with gravity wave activity. This was achieved by normal modes decomposition to extract gravity wave information from the model’s output. The decomposition was done in the model’s sigma domain. The gravity mode oscillation periods were those with periods faster than 36 hours. Relatively high frequency gravity wave content in the temperature and winds fields on the sigma surfaces were chosen as model output parameters possibly indicative of turbulence. The horizontal domain of the experiment covered most of the Northern Hemisphere. A summary of the study, verification results, and suggestions for further work will be presented. 2. METHODOLOGY 2.1. Extracting Gravity Wave Activity from the Dynamic Model Forecast The GSM utilized for this experiment had 18 non-equally spaced levels and a triangular truncation of 80 waves. Levels 11 through 14 were packed in the region of the expected jet stream, from approximately 400 to 200 mb while levels 5 through 9 were between 850 and 500 mb. Excessive gravity wave activity can be eliminated from a model’s initial conditions by projecting the initial fields on the model’s gravity modes. The undesired fast components are then removed and the data are then transformed back into the model’s variables. Further improvement in the balance of these data can be achieved if this process is also applied to the tendencies of the model’s variables. When both steps are performed the method is known as the Baer-Tribbia non-linear normal modes initialization. In this study, the first step of the initialization technique was used to filter out the gravity waves contained in the forecast from the Medium Range Forecast (MRF) model. The operational MRF forecasts at 24 hours were projected on the MRF model’s first four vertical modes and the associated horizontal modes. The gravity wave components with wave periods of less than 36 hours were zeroed out and then returned to the model’s spectral expansion. The difference between the original forecast and the filtered forecast represents the "fast" gravity wave content of the forecast. In this experiment the first four vertical modes employed in operations were used to project the data. More vertical modes, or a subset of internal vertical modes, should be used in future experiments. 307 Session 10.3 2.2. Examining Gravity Wave Activity on Siama Surfaces The method of extracting gravity wave activity as described in the previous section was applied to a total of 20 cases. During two, 30-day periods (summer 1988 and winter 1988-89), gravity wave content in some of the 0000 UTC MRF model forecast fields was calculated, mapped and compared to manual turbulence forecasts and to observations every second, third, or fourth day. Additionally, for one winter case and one summer case, the gravity wave content in the temperature and wind speed fields on various sigma levels from 5 through 16 were examined. While the model’s sigma surfaces are not the usual quasi-horizontal surfaces operational meteorologists typically study to forecast weather related phenomena, they are closest to the true model forecast and the least contaminated (by, sigma to pressure interpolation) numerical output available. Sigma level maps were made for each of two levels, usually 7 and 12, on the Gaussian grid, for the entire globe. Three charts were made for both temperature and wind fields. For each, the forecast parameter is depicted, then the parameter with the gravity wave content removed, and finally the difference between the two, which is the gravity wave content of the displayed field. International and domestic pilot reports (PIREPs) are among the data collected at NMC. From these PIREPs, turbulence maps are made four times a day covering a 6-hour period with a geographic area covering most of the Northern Hemisphere. Groups of moderate or greater turbulence are encircled. These charts were compared to previous 24-hour prognoses for verification of manual turbulence forecasts. Similarly the gravity wave content of the displayed field was compared to the encircled observations to assess the performance of this scheme. Of course, the reliance on PIREPs is a weakness of this high altitude turbulence verification scheme (and others), mainly due to the limited coverage of PIREPS which are concentrated along flight routes. Advances in radar and lidar techniques may be useful in the future for turbulence verification. For example, a weak gravity current has been observed by Raman Lidar (Koch et al. 1991). 3. RESULTS FROM SUMMER AND WINTER CASE STUDIES In this study, an area of turbulence was considered predicted by the gravity wave scheme if the wind speed difference was greater than 3 m/s (6 kts) or if the temperature difference was greater than 2°C at level 12. Usually, only the former criterion was met. The gravity wave maps were subjectively evaluated as in the manual forecast scheme by using PIREPs with the following criteria: HIT = forecast area of turbulence touching or overlapping any observed area of moderate or greater turbulence. MISS = forecast area of turbulence in (or near) an observed area of light or no turbulence. UN FORECAST = no forecast area of turbulence in (or near) an observed area of moderate or greater turbulence. UNVERIFIABLE = forecast area of turbulence with no observations in or near the area. "Near an area" was defined to be within 300 nmi of the particular location measured locally by 5° of latitude on the Polar Stereographic map projection. When an area would have been unverifiable due to lack of data, but 308 Session 10.3 where observations were available within 300 nmi, those nearby observations were assumed to be quasi-representative of the conditions at the normal modes scheme forecasted location. In the winter case for which many vertical levels were examined it was found that most of the gravity wave amplitude in temperature and wind speed occurred over mountainous regions above level 13. It was also found that wind speed amplitude was highest near the elevated polar regions of Antarctica and Greenland. The amplitude of the temperature difference in levels 13 to 16 was nearly double that in level 5 to 9, and was greatest over mountain ranges. Because of this difference in behavior at middle troposphere levels and near tropopause levels, for each of the cases in the study, at least two levels were examined, mainly level 7 and level 12. The choice of a level near the tropopause was done to reflect the capping effect on mountain wave energy transport. The lowest internal mode used in the projection had an equivalent depth near a subtropical or summer tropopause (200 mb). There have been recent studies of undulations in the tropopause (Hirschberg and Fritsch 1991a,b) which support the choice of levels near the tropopause as especially relevant to turbulence generation. The first series of experiments used Northern Hemisphere summer data separated 2 to 4 days apart during June 1988. Preliminary verifications were similar to the manually produced turbulence forecasts. The next set of experiments were conducted 6 months later. The purpose of the winter experiments was to determine whether meteorological phenomenon associated with strong baroclinic systems, or rapidly developing extratropical cyclones, such as merging jets or strong baroclinic instability, produce more predictable turbulence. Figures la-c show a summer case, and Figures 2a-c depict a winter case. These examples are among the better ones in terms of verifiability and diversity of turbulence generating mechanisms. The verification area covered the Northern Hemisphere westward from 30°E to 120°E. Solid lines in Figure 2c enclose areas of gravity wave scheme forecast turbulence. Turbulence was forecast when the gravity wave content exceeded 3.0 m s' 1 as defined by the mark on the unit vector depicted in the lower left corner of Figure 2c. Table 1 summarizes the overall results of the gravity wave content forecast scheme. SUMMER WINTER CASES CASES # Forecast 10 16 HITS 1 10 UNFORECAST 7 15 MISSES 1 1 UNVERIFIABLE 3 8 POD 0.13 0.40 FAR 0.50 0.09 CSI 0.11 0.38 where: POD = HITS HITS + UNFORECAST FAR = MISSES HITS + MISSES CSI = HITS HTS+UNFORECAST+MISSES Table 1. Results of the gravity wave content forecast scheme. The winter results show a low false alarm ratio of 9% with a somewhat encouraging probability of detection of 40%. While these values are promising, more cases 309 Session 10.3 $ LEVEL=12 1988 17 JUN. 90N 60N- 30N 180W 150W 1 20W 51.2 M/S 90W 60W 30W Figure la. Summer case - 24-hour wind forecast on sigma level 12 depicted on a Gaussian grid. 90N $ LEVEL= 1 2 1988 1 7 JUN r 60W r r 1 ' « * < » 1 < < < ‘ k * < E *■ *• *■ 1 &-»»• > ^~rr ^ v r- * a * 30N E -i + * * * * * * x tc * * i /JVV53^ ji-z. J , /m*-* * + V » i 5, » f * if * ,f W ^ » ... ■» * V^V-}/* jr + +1l+.r*< * * A r > * r *•* A T * * 1* * * k| /.,«*«.« ^4 *« k<» <* * + * T «- ♦ » <->«- * ♦ f * i » 4 » .» A >* V ^ *- * 4 * * * * * -A^i<3|==*--»^r4'» Si !■* a i Figure 1b. Summer case - Same as Figure la except the "fast" gravity wave content has been removed. 310 Session 10.3 $ LEVEL= 1 2 GW . 4 4 4 *1 1 1 > 1 -7 A / * S\ A n -'I r V < < *'» c < < < < < A ^ t : a ^ A U > A. w 'y* r ^T‘ I ■ * t ■ -r t*V h * 1 r r ► ^ ] «r *" «- K \ 1 ■ * 4T 4r * ^ ' . i. C 4 4- 4 A ■* 3T»n *4^t,tV 4 v*'* * rrrf ^ * 41 ■ T * ^ 4 » t t> + -C■«* + + + 4 £ * * ► *-« r -i -i - ?>»*»ir«-V, ! ,nr 4 -» " * ^ I i +* ■* T ■ ■< ■* l t ? 1 r t > t t \ \v uu a -r 1 ► r* A ► *■ - * t ft? f » rki r- • * t * / 7 t \ ♦ r- r»|» rk^“* |-rrf f<* Jf r «» < ^ *■ ? -r * , L --- 1 ' 4 4 4 ± + 4 f * ’ H V 4 «- )< 4r -» -* 4 * £* *y \ v »5 *^//U v L\ \ 4- ■» |v/ * t * ^ ^ * * / *-i*»_^r , 7 ' ♦ | a * f fy.’f 4 - 4 v * / f 1 - 4 *- . . _ _ ^ . . t. » 1 >* A _-A- ^ 60S.: 90St ■*— ▼* / r T ♦ * 4 * '•U—*-* * * * t *- < 4 - ' : r- <- *-4- r - • < - 4 -“f i «-*- ♦ <■ «- 4 ► ► < *. 1 80W Figure 1c. Summer case - The fast gravity wave content in the forecast winds (e.g., Figure 1b subtracted from Figure la). $ LEVEL=12 1988 20 DEC. _.• ■* •» * * • , , f < < 1 t i l. .1. [• v 180W 150W 120W - 51.3 M/S 90W 60W 30W Figure 2a. Winter case - 24-hour wind forecast on sigma level 12 depicted on a Gaussian grid. 311 Session 10.3 $ LEVEL= 1 2 1988 20 DEC 150H 120W 54.4 M/S Figure 2b. Winter case been removed. Same as Figure 2a except the "fast" gravity wave content has $ LEVEL-12 GW 90N 60N 30N L 30S 60 s; 90S Figure 2c. Winter case - The fast gravity wave content in the forecast winds (e.g., Figure 2b subtracted from Figure 2a). 312 Session 10.3 are needed to accurately evaluate the performance of gravity wave content as a turbulence forecast parameter. These results may be compared to another index used in MAB operations during 1988, the TIGE" index, which has given very good results over the continental United States. TIGE" is derived from an objective deformation and shear-based scheme (Mancuso and Endlich 1966) that was evaluated from NGM output on the VAS Data Utilization Center (VDUC) by NESDIS and MAB personnel in real-time during April, May, and September of 1988. The POD was 70%, the FAR was 20%, and the CSI was 60% (G. Ellrod, personal communication). The forecast area for this test was the continental United States where data sparsity is much less of a problem. The gravity wave scheme also performed well over the Rocky Mountains. In real-time operations, the availability of more than one scheme is useful, especially if the different methods corroborate each other by highlighting the same geographic areas for enhanced high altitude turbulence potential. 4. SUMMARY It was demonstrated that numerical model forecasts of gravity wave activity, especially in the winter can be used to identify potential regions of turbulence. The origin of this turbulence is usually orographically induced, or associated with developing baroclinic systems. 5. ACKNOWLEDGEMENTS I would like to express my gratitude to all of the NMC personnel who helped me in this study. In particular I would like to thank Joseph Sela for adapting the initialization codes to extract gravity wave information from the global model. Thanks are also due to management of NMC’s Meteorological Operations Division and Monitoring and Aviation Branch for their support during this study. Thank you also to Eastern Region management and staff members for their encouragement and support in the continuation of this study. I am also grateful to Gary Ellrod of NESDIS for many helpful discussions. REFERENCES Baumgardner, D., Fimpel, H. and Jochum, A. 1990: International Workshop on the airborne measurement of wind, turbulence, and position workshop summary. Meeting review. Bulletin of the American Meteorological Society. 71 (4), 538-541. Blake, N. A. 1988: FAA engineering programs for aviation. Bulletin of the American Meteorological Society. 69 (6), 639-640. Bosart, L. F. and J. P. Cussen, Jr., 1973: Gravity wave phenomena accompanying east coast cyclogenesis. Mon. Wea. Rev., 101, 446-454. Charney, J. G., and Drazin, P. G. 1961: Propagation of planetary scale disturbances from the lower into the upper atmosphere. J. Geophys. Res. 66, 83-109. Ellrod, G., 1985: Indicators of high altitude, non-convective turbulence observed in satellite images. Proceedings of the 2nd International Conference on the Aviation Weather Systems, June IQ- 24, 1985, Montreal, PQ, Canada. Amer. Meteor. Soc., Boston, MA, 277-284. Hirschberg, P. A., and Fritsch, J. M., 1991a: Tropopause undulations and the development of Extratropical cyclones. Part I: Overview and observation from a cyclone event. Mon. Wea. Rev. 119, 496- 517. 313 Session 10.3 Hirschberg, P. A., and Fritsch, J. M., 1991b: Tropopause undulations and the development of Extratropical cyclones. Part II: Diagnostic analysis and conceptual model. Mon. Wea. Rev. 119, 518-550. Holton, J. R., 1979: An Introduction to Dynamic Meteorology. Second Edition. Academic Press, New York. 391 pp. Hopkins, Robert H., 1977: Forecasting techniques of clear air turbulence, including that associated with mountain waves. World Meteorological Organization, Geneva, Technical Note Number 155. 31 pp. Koch, S., Dorian, P., Ferrare, R., Melfi, S., Skillman, W., and Whiteman, D., 1991: Structure of an internal bore and dissipating gravity current as revealed by Raman Lidar. Mon. Wea. Rev., 119, 857- 887. Mancuso, R. L., and R. M. Endlich, 1966: Clear air turbulence frequency as a function of wind shear and deformation. Mon. Wea. Rev., 94, 581-585. National Weather Service, 1986: Weather Service Forecasting Handbook No. 6 - Satellite Meteorology. Chapter 8, Winds and Turbulence. 8.1-8.E.6. Ninomiya, K., 1980: Mesoscale aspects of heavy rainfalls in the Japan Islands. Proceedings of CIMMS Symposium - Collection of lecture notes on dynamics of mesometeorological disturbances. 144- 145. Rammer, W. A., 1973: Model relationships of CAT to upper winds flow patterns. National Meteorological Center’s Aviation Weather Forecast Branch Note. August 23, 1973, 14 pp. Shapiro, M. A., 1974: A Multiple- structured frontal zone-jet stream as revealed by meteorologically instrumented aircraft. Mon. Wea. Rev. 102, 244-253. Uccellini, L. W., 1975: A Case study of apparent gravity wave initiation of severe convective storms. Mon. Wea. Rev., 103, 497-513. 314 Session 10.3 APPENDIX A - SOME EXAMPLES OF GRAVITY WAVE ASSOCIATED WEATHER FEATURES WEATHER FEATURE Enhancement of convection due to a gravity wave. LOCATION United States REFERENCE Uccellini (1975) Amplification of a gravity wave related to mesoscale heavy rainfall. Japan Ninomiya (1980) Mountain induced orographic cirrus associated with turbulence. United States NWS (1986) Tropopause undulations with strong cyclone development. United States Hirschberg and Fritsch (1991a,b) Dissipating gravity current United States Koch et al. (1991) revealed by Raman Lidar. 315 Session 10.4 VERIFICATION OF THE EXPERIMENTAL AVIATION PACKAGE BY THE NEWFOUNDLAND WEATHER CENTRE Claude Cote, Michael Webber, and Dave Brown Newfoundland Weather Centre Atmospheric Environment Service Gander Newfoundland 1. INTRODUCTION Early in 1989 an Experimental Aviation Package (Jones, 1989) was developed at the Canadian Meteoro¬ logical Centre (CMC) to provide forecasts of various meteorological elements considered important to high and low level flight opera¬ tions. The transmitted forecast charts were for an area encompassing eastern Canada and the western North Atlantic Ocean. The Newfoundland Weather Centre (NWC) carried out a systematic evaluation of the prod¬ ucts for the winter months of 1989-1990. After the evaluation process was completed, a report was to be produced specifying the strengths and weaknesses of the package so that it could be modified and later adapted for operational use. During July and August of 1989, the package was transmitted twice daily for a period of preliminary evaluation. In late October, a few suggestions for improvement in pack¬ age format and content were communi¬ cated to CMC. After the suggested changes were implemented in Novem¬ ber, three NWC staff meteorologists began to verify several components of the package on a systematic and regular basis. The process continued until the end of April 1990. Pre¬ liminary results of the evaluation process were summarized and present¬ ed at the Third Workshop on Opera¬ tional Meteorology held in Montreal in May 1990 (Cote et a/., 1990). The Experimental Aviation Pack¬ age is based on the CMC Regional Finite Element (RFE) model run at 00Z and 12Z daily. Charts compris¬ ing the package were transmitted via the regular facsimile circuits. Twenty-four hour forecasts, at 6-hour steps, were provided for each of isobaric pressure fields, wind fields, surface stress (mechanical turbulence), cloud cover, rime ic¬ ing, freezing level, wind shear, buoyant energy and clear air turbu¬ lence. Each panel provided coverage between 40 degrees N and 65 degrees N latitude and 25 degrees W and 80 degrees W longitude. The results of the evaluation indicated that the Experimental Aviation Package has considerable skill in forecasting a number of meteorological parameters important for flight operations. It was con¬ cluded that if these forecasts were available routinely to the aviation forecaster, it is likely that a more accurate forecast product could be produced for terminal and enroute flight operations. 2. VERIFICATION The evaluation period from November 1989 to April 1990 corre¬ sponded to the time of peak occur¬ rence for some of the more hazardous phenomena for low and high-level aircraft operations over eastern Canada and the North Atlantic Ocean. During these winter months most weather systems are strong and well 316 Session 10.4 defined, especially as they approach the east coast of Canada and move out to sea. High level clear air turbulence (CAT) is a common occur¬ rence in the winter over these areas as high velocity jet streams develop in the strongly baroclinic atmos¬ phere. Since the controlled air¬ space traffic corridors between North America and western Europe are affected by these weather systems, hundreds of pilot reports (PIREPS) may be generated each day giving locations and magnitudes of CAT. At the same time, the tight surface pressure gradients generate strong low level wind fields. Their interaction with local topographical features can produce wind shear and mechanical turbulence which may affect aircraft on approach or de¬ parture from airports. Although not as numerous as high altitude PIREPS, a few local PIREPS of wind shear and mechanical turbulence are received each day depending on the synoptic situation existing at the time. Most flights in Newfoundland and the Maritimes involve short haul routes of only a few hundred kilom¬ etres. The aircraft frequently spend a significant amount of time in cloud and may report the occur¬ rence and magnitude of rime icing when it occurs. Because of the large number of PIREPS available, they could easily form the basis of confirming or negating a forecast for mechanical turbulence, rime icing or CAT. As a result, almost every PIREP received at the NWC between November 1989 and April 1990 was examined and details noted about the location and magni¬ tude of any of the weather elements mentioned in it. These details then were compared with those extracted from the aviation package charts noting differences and similarities between observed and forecast ele¬ ments. The process was laborious but yielded a large data file from which some basic conclusions were eventually drawn. The forecasts for cloud heights and extent were verified using representative atmospheric soundings and satellite pictures. Buoyant energy was not evaluated. The wind shear forecast did not warrant much attention as the shear forecast was for the lowest - 6000 feet and was considered to be of little value to aviation forecasters. The results of these evaluation techniques follow. 2.1 Mean Sea Level Pressure, Wind Field and Surface Stress Fore¬ cast A 4-panel chart containing the contoured sea level pressure pattern forecast at 6-hour steps for the following 24 hours was one of the components of the package. Superim¬ posed on each forecast isobaric pattern were surface wind isotachs drawn at intervals of 10 kt and a measure of surface stress (related to wind speed) contoured at incre¬ ments of 0.25 pascals. Each 6-hour forecast panel was verified. The isobaric pressure field forecasts were not verified since they followed directly from the RFE model output. The isotach forecasts were verified qualitatively by com¬ paring them with hand drawn isotach analyses produced at the NWC using real winds. It was found that fore¬ casts of wind speed were underesti¬ mated by about 5 kt prior to the new RFE model becoming operational on April 1, 1990. After April 1, it appeared that the wind speed 317 Session 10.4 forecasts agreed more closely with the reported winds. The forecast of mechanical turbulence (a result of surface stress) was verified using PIREPS. Figure 1 indicates that about 80 per cent of the low level mechanical turbulence reports were correctly forecast in terms of surface stress values. As Figure 2 indicates, the magnitude of reported mechanical turbulence appeared to correspond to particular levels of surface stress (a function of wind speed forecast by the model). Thus, it was possi¬ ble to associate a particular cate¬ gory of reported mechanical turbu¬ lence with a particular level of surface stress. There were 145 PIREPS reporting LGT-MDT mechanical turbulence or greater. Of these, 116 had a sur¬ face stress contour 0.50 PA or greater. The distribution was as follows: Table 1: Distribution of Successful Predictions per Category of Mechanical Turbulence Category # of Hits Events/ Percent Range of Predicted Indices for 70% Hits Average Value LGT-MDT 39/34% 0.25-1.5 0.69 MDT 50/43% 0.25-1.5 0.90 MDT-SVR 20/17% 0.75-1.5 1.04 SVR 7/ 6% 0.50-1.0 0.82 For all but the extreme catego¬ ry of turbulence, the forecast in¬ tensity was slightly higher at T+12 hrs than for T+24 hrs. In cases where turbulence categories did not correspond to the appropriate sur¬ face stress forecast values, wind direction (as well as speed) may have played a part. The RFE model utilizes a rather incomplete repre¬ sentation of topography with which wind speed is allowed to interact. However, it was the experience of the meteorologists evaluating the package that wind direction played an important part in initiating local occurrences of mechanical turbulence. This was especially true for aircraft operating from airfields in western Newfoundland where topographical features affect wind from certain directions more than others. It was felt that the surface stress forecast would be helpful in identifying only general areas of low level mechanical turbulence. The forecast was not site specific and sometimes failed to predict correctly the occurrence and/or magnitude of turbulence. Wind di¬ rections were considered to be an important consideration for generat¬ ing turbulence at sites such as Deer Lake, which is located in a northeast-southwest oriented valley. Furthermore, reporting turbulence is a subjective matter depending on the experience of the aircrew and the size of the aircraft. In any case, a liberal mix of forecaster experi¬ ence and model output would be es¬ sential for a proper diagnosis and forecast of mechanical turbulence. 2.2 Cloud Forecast The cloud forecast charts de¬ picted the horizontal and vertical extent of synoptic scale broken low and middle cloud areas. Two differ¬ ent shades were used to differenti¬ ate the two layers. Sets of two numbers scattered about the depic¬ tions represented the forecast bases and tops of the layers in thousands of feet. Tephigrams representing verti¬ cal profiles of temperature and moisture at Sable Island, St. John's, Stephenville, Goose Bay, Keflavik (Iceland), and Narssarssuaq 318 Session 10.4 MECHANICAL TURBULENCE TOTAL: 145 REPORTS FIGURE 1. 319 Session 10.4 (Greenland), were used to verify the bases and tops of broken layers. Satellite pictures were used to assess, qualitatively, the horizon¬ tal extent of the cloud systems. Only cloud forecasts valid for 00Z and 12Z were verified to coincide with the availability of upper air data. In general, there was good agreement between the forecasts and real time data depicting middle cloud systems. There appeared to be a slight bias to overforecast the tops of mid-cloud layers by includ¬ ing cirrus layers above. The dif¬ ferences were not large. Low cloud forecasts, however, were more want¬ ing. Topographically forced low cloud was almost never forecast, nor was stratiform cloud over water, but low cloud formed during cold air outbreaks across open water was depicted only when water-air thermal contrasts were strong. The forecasts provided valuable assistance to public forecasters whose main concerns lie in predict¬ ing motions of large scale synoptic mid cloud systems and their associ¬ ated precipitation areas. But as it stands, the forecast charts are of little utility to the aviation fore¬ caster whose concerns lie mainly in predicting cloud bases below 3000 feet. In Newfoundland, for example, success in writing terminal fore¬ casts is achieved by knowing when to forecast onshore stratus or strato- cumulus since most of the terminals are located close to the coast. However, the model's greatest utili¬ ty lies in its ability to adequately forecast synoptic scale cloud sys¬ tems over the water. This will prove to be especially beneficial to weather centres whose problems in¬ volve synoptic systems moving on¬ shore from data-sparse ocean areas. 2.3 Icing and Freezing Level Fore¬ cast Another important component of the Experimental Aviation Package was the rime icing and freezing level forecast. Forecast freezing levels were contoured at intervals of 2500 feet. Categories of forecast rime icing were depicted by shading with scattered pairs of numbers indicating the bottom and tops of the icing layer. The forecast freezing levels were verified using the same sound¬ ings as in the cloud forecast veri¬ fication scheme. Only cases where no strong thermal gradients existed near the stations were used. This would eliminate interpolation diffi¬ culties associated with frontal slopes. From Table 2 it is apparent that the forecast freezing levels lie within about 1000 feet of the observed freezing levels. Table 2: Freezing Level Verification for Four Upper Air Sites, August 1989 to April 1990 USA YYT YYR Ship CHARLIE Number of Cases 130 101 82 32 Maximum Underforecast -4200 -3700 -5100 -2000 Maximum Overforecast 5700 2100 4200 1000 Mean Forecast Error -280 22 93 -103 Std Dev of Fcst Error 1090 900 1213 788 Values are in feet. Where rime icing was forecast there were frequent PIREPS indicat¬ ing same. However, success in fore¬ casting rime icing is very dependent on the ability of the model to fore¬ cast cloud. There was a high success rate at forecasting rime icing in synoptic scale mid cloud as revealed by Figure 3. But results were poor where low cloud was involved. In the winter month it is low cloud with which rime icing is most fre¬ quently associated. Since low cloud 320 Session 10.4 RIME ICING VERIFICATION PER SYNOPTIC SITUATION N U M B E R O F R E P O R T S CATEGORY □ NEAR MISSES MISSES HITS VVLY FLO COLD FNT WRM FNT WRM SCTR VCNTY LO SYNOPTIC SITUATION OTHER TOTAL: 55 REPORTS FIGURE 3. RIME ICING HITS 45% MISSES 55% TOTAL: 55 REPORTS FIGURE 4. 321 Session 10.4 was not resolved adequately by the model, there were many PIREPS of rime icing which were not supported by a positive forecast. Figure 4 indicates only a 45 percent total success rate in identifying areas of reported rime icing regardless of category. Freezing levels warrant consid¬ eration only in the presence of clouds. Hence, it would seem more appropriate to have the freezing levels superimposed on the cloud depictions rather than have the features depicted separately.In this way, there would be an immediate visual impact indicating areas of potential rime icing in cloud. 2.4 Buoyant Energy and Wind Shear Forecast The ability to forecast wind shear is often critical when, for example, strong low level tempera¬ ture inversions tend to damp out wind flow near the earth's surface. Shear of 10-20 kt per 1000 feet can be observed under certain condi¬ tions. Shears of that magnitude can have significant impact on aircraft during ascent or descent. There¬ fore, a good forecast for low level wind shear would be very valuable to an aviation forecaster. The forecast wind shear (kt per 1000 ft) as provided in the Experi¬ mental Aviation package represented the average shear anticipated in the lowest -6000 feet. The model could not resolve the strong shears close to the surface, a knowledge of which is important for low level flight operations. Consequently the forecast shear values were of little interest to the aviation forecaster. Buoyant energy was not verified as no adequate means to do so were achieved. 2.5 Clear Air Turbulence Forecast Two indices, Ellrod and Empiri¬ cal, were used to provide forecasts of clear air turbulence. Two sepa¬ rate charts were produced, one for each index. Both sets of charts featured contoured indices of CAT at the high levels. The Empirical and Ellrod indices were contoured at increments of 10 and 30 respective¬ ly. Each index increases as the horizontal and/or vertical shear increases. Figure 5 shows the number of North Atlantic ocean-crossing flights for 1989. The numbers jus¬ tify the need for accurate forecast information along these busy corri¬ dors. During the same period the NWC issued 267 SIGMETS for CAT over the Northwestern Atlantic as indi¬ cated in Figure 6. These SIGMETS represent at least 534 h of forecast CAT in one year assuming that each SIGMET has a life span of 4 h but must be updated every 2 h. PIREPS of clear air turbulence, mostly over the North Atlantic Ocean, were used to confirm or ne¬ gate a forecast for same. Both indices had similar success rates in predicting the various categories of CAT. Figure 7 indicates that the Ellrod Index had the higher number of total successes. Tables 3 and 4 indicate the number of prediction successes for each category of CAT for the Ellrod and Empirical indices, respectively. Obviously, there is no clear winner. Both indices had nearly identical prediction success scores for each category of CAT. 322 Session 10.4 SIGMETS ISSUED IN 1989 BY NWC PHENOMENA TOTAL: 505 SIGMETS ISSUED FIGURE 6. 323 Session 10.4 CLEAR AIR TURBULENCE EMPIRICAL INDEX ELLROD INDEX HITS 70S REJECTED 2S TOTAL: 1066 REPORTS FIGURE 7. MISSES 21N NEAR MISSES 7% HITS 71S REJECTED 2 % NEAR MISSES 4% MISSES 22S TOTAL: 1026 REPORTS 324 Session 10.4 Table 3: Distribution of Successful predictions per Category of CAT - Ellrod Index Category * of Events/ Percent Range of Predicted Indices for 70% Hits Average Predicted Index LGT-MDT 283/39% 30- 90 55 MDT 334/45% 30- 90 65 MDT-SVR 95/13% 60-210 111 SVR 20/ 3% 90-330 179 Table 4: Distribution of Successful Predictions per Category of CAT - Empirical Index Range of # of Predicted Average Events/ Indices Predicted Category Percent for 70% Hits Index LGT-MDT 295/40% 20-40 26 MDT 335/45% 20-40 30 MDT-SVR 100/13% 20-50 39 SVR 16/ 2% 50-80 60 When it came to overpredicting the incidence of CAT, namely, gener¬ ating false alarms, the Ell rod Index was somewhat more reliable. Figures 8 and 9 show relative performance in terms of false alarm forecasts. ing. Under these conditions, the Ell rod Index performed somewhat better. But both indices tended to predict CAT through too deep a lay¬ er. Theory would suggest that CAT associated with upper ridging is limited to a relatively thin layer from just above the tropopause to a couple of thousand feet below it. Consultations with aviation forecasters, weather briefers, and potential users produced a unanimous agreement for the need of a good forecast for CAT. In addition, the Canadian Pilots Association (Avia¬ tion Weather Services meeting, Janu¬ ary 1990) indicated that the chart should also include the forecast height of the tropopause to replace the discontinued TVWS chart. But despite the minimal shortcomings of the experimental forecasts, aviation briefers at the NWC considered CAT forecasts to be very valuable when assessing its potential over the Western North Atlantic and else¬ where. The modified false alarm ratios presented in Figures 8 and 9 were determined using 500 PIREPS with no turbulence selected randomly between November 1989 and March 1990. In the case of the Ell rod Index a threshold value of 55 (from Figure 8) was associated with the category of light-moderate CAT. But the nearest contoured value of CAT on the forecast chart was 60. There¬ fore, a false alarm event was con¬ sidered to have occurred every time a PIREP with no reported turbulence came from a location where the fore¬ cast Ell rod Index was 60 or greater. The same procedure was followed for the other categories of CAT for both the Ellrod and Empirical Indices. Many of the major episodes of CAT were associated with upper ridg- 3.0 CONCLUSIONS The Experimental Aviation Pack¬ age shows considerable skill in forecasting some aviation-related weather phenomena. Most of the charts in the package were able to provide the necessary additional information if the quality of aviation-related forecast products is to improve. It was felt that, whatever adjustments are necessary to correct some of the forecast shortcomings they be made quickly so that the package can become opera¬ tional with minimum delay. Two recommendations are made with regard to changes in format. The first recommendation is that the cloud charts have the freezing lev¬ els superimposed. The second is 325 Session 10.4 326 Session 10.4 that the tropopause heights be con¬ toured on the Ell rod Index chart. The NWC verification team real¬ ize that not all elements in the package have been verified in depth. But the verification process was conducted in as complete a fashion as was possible given the time and means available. It was recognized that these forecasts, like all others, can never be perfect because all parame¬ ters were developed from the numeri¬ cal models. However, as the RFE is improved over time, so should the precision of its products. Most of the aviation package products, how¬ ever, cannot by themselves represent the "final forecast." Forecaster experience is a necessary ingredient in preparing the final forecast. The additional forecast information provided by an operational aviation package will prove most valuable to all concerned. 4.0 REFERENCES Jones, R., 1989: Aviation Package. CMC document. Cote, C., D. Brown and M. Webber, 1990: Verification of Experimental Aviation Package 1989/90. Preprints, Third Workshop on Operational Meteo¬ rology, Montreal, CMOS. Cornwall, Ont., Aviation Weather Services Meeting, January 29-30, 1990. 327 Session 10.5 APPLICATIONS OF GOES SATELLITE DATA IN THE ANALYSIS OF NON-CONVECTIVE AVIATION HAZARDS Gary P. Ell rod Satellite Applications Laboratory (NOAA/NESDIS) Washington, DC 1. INTRODUCTION Imagery from geostationary satellites can be an invaluable tool in the analysis and short range prediction of aviation weather haz¬ ards. Even where dense observation¬ al networks are present such as over the United States, knowledge of conditions above the surface is limited, and remote sensing tech¬ niques must be used to estimate the extent and severity of weather con¬ ditions. This paper will describe some of the ways that satellite imagery can assist in the detection of non-convective aviation weather hazards such as: (1) fog and stra¬ tus, (2) icing, (3) clear air turbu¬ lence (CAT), and (4) mountain waves. 2. FOG AND STRATUS DETECTION Fog and stratus clouds result in low ceilings and visiblities that are a significant contributor to weather-related aircraft accidents each year. Fog is easily detected during daylight hours using high resolution (1 km) visible imagery from the GOES (Geostationary Opera¬ tional Environmental Satellite). However, it is much more important to observe the presence of fog at night, when only low resolution (7 km) infrared (IR) is available. When fog forms in relatively warm conditions, and in coastal or moun¬ tainous regions, there is often insufficient thermal contrast for detection of fog in IR images. In such cases, the use of two IR chan¬ nels has shown to be superior. The use of multispectral IR imagery in nighttime fog detection relies on the fact that low level stratiform clouds appear colder in the near-IR window channels (3.5- 4.0pm wavelength) than in the long¬ wave window (11pm) due to a differ¬ ence in the emissivities (Hunt, 1973). The temperature difference is typically small (3-7°C) but de¬ tectable. In cloud-free regions, temperatures are similar in both IR channels. When the difference image of the two channels is enhanced, the low clouds stand out very clearly. The presence of thin cirrus clouds will counteract this temperature difference, however, and obscure the presence of fog. This technique was first used in Europe with imagery from the NOAA Advanced Very High Resolution Radiometer (AVHRR) imager (Eyre et al ., 1984), and was later adapted for use with GOES data (Ellrod et al ., 1989). Figure 1 shows a case where fog and low ceilings occurred in Missouri and western Illinois on 25 September 1991. GOES imagery was obtained at 0920 UTC, when multi¬ spectral IR data was available. The 11pm IR channel (Figure la) (rou¬ tinely received at all weather of¬ fices via the GOES-Fax network) revealed a broad, dark grey area from western Illinois to northern Arkansas, suggesting a region of low level moisture. Obscuration of terrain features such as rivers and lakes within this region indicated that fog could be present, but there 328 Session 10,5 FOG IDENTIFICATION WITH GOES INFRARED IMAGERY 25 September 1991 - 0920 UTC a 11 im Infrared C Difference of 3.9-11/^m IR b 3.9/an Infrared Figure 1. GOES infrared images at 0920 UTC, 25 September 1991 for (a) 11/Ltm wavelength, (b) 3.9jum wavelength and (c) the smoothed, enhanced difference of 11/m and 3.9/xm channels. Stations reporting surface weather conditions at 0900 UTC are shown in (d). 329 Session 10.5 was no clearly defined border. The 3.9/jn channel image at the same time (Fig. lb) showed a brighter area with distinct edges (arrows) embed¬ ded within the moist region. The lower resolution of this channel is evident from the "blocky" appearance of this image. The enhanced, smoothed difference in the two imag¬ es (Fig. lc) clearly marks the sus¬ pected fog layer. Significant fog was not reported anywhere in Missou¬ ri at 0900 UTC because of the sparse reporting network (Fig. Id). Light westerly winds advected the low ceilings and visibilities into St. Louis (STL) and Peoria, Illinois (PIA) areas shortly before 1200 UTC. The use of multichannel IR imagery would have been a great help to the aviation forecaster in this situa¬ tion. Recently, a technique has been developed to estimate the thickness of fog and stratus from the tempera¬ ture difference in the 3.9nm and 11/zm IR channels (Ellrod, 1991). The depth of fog is useful in esti¬ mating the time that clearing will occur. The fog thickness was deter¬ mined from aircraft cloud top re¬ ports within several hours of the image time. A linear regression relationship based on 60 cases showed that for every 1°C change in the temperature difference, the thickness increased by about 100 meters (314 ft). Image enhancements have been developed to show spatial variations in fog depth, after smoothing the imagery to remove the effects of instrument noise. Com¬ parisons of satellite-derived fog depth to aircraft reports for more than two dozen independent cases indicate an average error of less than 100 meters. 3. AIRCRAFT ICING The occurrence of aircraft icing requires the presence of supercooled water clouds, preferably with large drop sizes and high liq¬ uid water contents. At temperatures around -13*C and colder, ice crys¬ tals grow rapidly at the expense of the supercooled water (e.g., Byers, 1965), fall out as precipitation, and the potential for icing is di¬ minished. One simple way to use IR satellite imagery in icing analysis is to enhance cloud top temperatures to show the optimum 0°C to -15°C range. This technique is effective for stratiform cloud systems such as cold advection stratocumulus and warm frontal cloud systems that lack extensive cirrus. This enhancement scheme is less reliable when embed¬ ded convection is present, since convective clouds often contain supercooled water at temperatures of -30°C or colder. Also, mid-level layered clouds originating in the subtropics can produce extensive icing at relatively cold tempera¬ tures. 4. CLEAR AIR TURBULENCE In the same way that turbulence intensity is now measured subjec¬ tively, satellite imagery can pro¬ vide valuable qualitative informa¬ tion on the presence of non-convec- tive turbulence, also known as clear air turbulence (CAT). Most large outbreaks of CAT are accompanied by various signatures in IR, visible (VIS) or water vapor (WV) imagery (Ellrod, 1989). Water vapor imagery is perhaps the most useful tool in turbulence analysis because it pro¬ vides information on both cirrus cloud features (albeit at low reso¬ lution) and dynamic processes in cloud-free regions, such as sinking and cold advection associated with 330 Session 10.5 upper level fronts. Animation of the imagery is preferred because trends can be seen much more clear¬ ly. 4.1. Mesoscale signatures One of the most common CAT signatures is transverse cirrus banding often seen on the anticy- clonic side of the subtropical jet stream. These bands are accompanied by moderate or strong vertical shear (at least 5-6 kt/lOOOft). The tur¬ bulence intensity is typically light to moderate. Cases with severe CAT occur when wide, thick transverse bands are present that sometimes resemble convective cloud plumes. This sometimes occurs in diffluent flow patterns. Inspection of radio¬ sonde temperature profiles near such features frequently reveals nearly dry adiabatic lapse rates just below the tropopause. Jet cirrus that has sharply curved (in an anticyclonic sense) segments with ragged or scalloped edges may contain moderate or severe CAT. These ripples (or bulges) are believed to be related to mesoscale speed maxima along the jet, referred to as "jet streaks." When the curved portion of the cirrus is relatively short (<500 nm), the likelihood of severe turbulence is greater, provided that upper winds are >75 kt. Sharply curved jet cirrus is often found along the jet in advance of an upper trough when convective systems are present that strengthen the upper winds by latent heat release. The GOES water vapor image in Figure 2 shows an example of a this type of jet cirrus boundary. Many curved cirrus segments can be seen east of an upper trough from eastern Texas to Missouri at both 0800 and 1600 UTC on 20 November 1991. Note the bright, cold clouds associated with thunderstorms along the leading edge of the cloud system. Moderate to locally extreme turbulence was reported along the edge of this jet cirrus during the day. 4.2. Synoptic scale signatures Col regions in the upper atmo¬ sphere, often called "deformation zones," can be significant CAT-pro- ducers. Cloud or moisture edges are often seen in satellite images along the stretching axis of deformation zones (Anderson et al ., 1982). The same process that stretches and compacts the cloud edge (or moisture boundary) in these regions contrib¬ utes to upper level frontogenesis, and thus an increase in vertical wind shear necessary for CAT. The intensity of CAT varies with the synoptic situation, becoming espe¬ cially strong in cases of cyclo¬ genesis with a building upper ridge, the confluence of the polar and subtropical jet streams, or the interaction of a strong jet with a closed upper low. A useful CAT signature often observed in WV imagery is a progres¬ sive darkening with time that is related to enhanced convergence, cold advection and sinking in the middle and upper troposphere. Al¬ though not all CAT episodes are accompanied by WV darkening, 80% of all darkening events have moderate or greater turbulence associated with them (Ellrod, 1989). The syn¬ optic situations that seem to be especially conducive to WV darkening are deformation zones with positive¬ ly-tilted (NE-SW) troughs. An exam¬ ple of such a case is shown in Fig¬ ure 2, with the accompanying 300mb plot at 1200 UTC, 20 November 1991 in Figure 3. The WV imagery became 331 Session 10.5 Figure 2. GOES 6.7^m water vapor images at 0800 UTC (left) and 1600 UTC (right) on 20 November, 1991. 10 ° -4s q 934 . .. h^z **> :. f —f / : S® ; _ic L, ■ 4S X 918 - &8 n 930 •. -to 946 / y S*94> ' . V .\ e >^ S1 : ^ *?* "X 9 * 2 " 4< fe f. >!!?/.it --.J.. ~ 3 5o 962 i &QS \lH ... 959 +00 ! -38. 95? \ O • 30 ;' 933 320 \ . jw 300 .yi22U*20NOSt ; • • ' Figure 3. 300mb upper air plot at 1200 332 Session 10.5 darker from 0800 to 1600 UTC in a band from upper Michigan southwest- ward to Kansas. This dark band appears to be related to the upper trough axis. The most pronounced darkening was from Iowa northeast¬ ward, where the the convergence in the upper winds was strongest. Moderate turbulence was reported with this band. Darkening also occurred in central Texas at the base of the upper trough. 5. HIGH LEVEL MOUNTAIN WAVES When strong winds cross moun¬ tains in stable conditions, the flow is disturbed, often to high alti¬ tudes. Thick, cold, orographic cloud plumes can form that become anchored along the mountain ridges and persist for many hours. The majority of these cirrus cloud plumes seem to be non-turbulent, however. In the cases where moder¬ ate or greater high level turbulence is reported, a subtle but important characteristic has been noted about the orographic cirrus. In these situations, the upstream edge of the cirrus is displaced slightly down¬ stream from the mountain ridges (Ellrod, 1987). The result is often a gap or trench in the high clouds along the lee slopes. Figure 4 is an example of this feature over northern Montana on 25 January 1990. These cloud-free trenches often occur during strong Chinook wind events along the lee slopes of the Rockies from New Mexico to Montana. High amplitude mountain waves have been observed by aircraft during these windstorms (Lilly, 1978). The strongest turbulence occurs just downstream from the clear zone. 6. LOW LEVEL MOUNTAIN WAVES AND WIND SHEAR If there is an absence of cloudiness over and west of the mountains, the water vapor image will sometimes show a distinct dark (warm) zone along the lee slopes. This suggests that pronounced drying and katabatic warming is occurring due to strong downs!ope winds. When there is no cirrus present, the occurrence of these dark zones in WV images usually indicates moderate to strong low level winds and possible wind shear. The WV image in Figure 5 shows this type of signature along the east slopes of the Front Range in Colorado on 3 March 1991 near the time a passenger jet crashed while approaching the Colorado Springs airport. Low level wind shear and turbulence were believed to be fac¬ tors in the disaster. The 1200 UTC radiosonde profile from Denver showed low level westerly winds of 20-30 kt increasing to 50 kt near the mountain top level. When sufficient low level mois¬ ture is present, transverse wave clouds are often seen downwind of mountain ranges. In general, the longer the wavelength of these clouds, the more intense the turbu¬ lence encountered, since wavelength is proportional to wind speeds. The observation of these waves in low resolution IR imagery usually indi¬ cates the presence of turbulence. Wave cloud patterns are also seen in visible imagery in areas of strong low level wind shear caused by low level jet streams and near frontal boundaries. 7. SUMMARY AND CONCLUSIONS Satellite imagery has proven to be a valuable supplement to other types of information available to 333 Session 10.5 Figure 4. GOES infrared image at 2130 UTC, 25 January 1990 showing a clear zone caused by a mountain wave east of the Rockies in western Montana (arrows). Upper winds are for 300 mb at 0000 UTC, 26 January 1990. Figure 5. GOES water vapor image at 1600 UTC, 3 March 1991. Arrow refers to a north-south oriented dark band caused by katabatic warming from downslope winds east of the Rockies. 334 Session 10.5 the aviation meteorologist. In remote or data-sparse regions, sat¬ ellite imagery may be the only in¬ formation available. The use of multichannel IR imagery appears to have great potential in fog detec¬ tion at night. Inferences can also be made about the presence of icing, clear air turbulence, mountain waves and in some cases, low level wind shear. 8. REFERENCES Anderson, R. A., J. J. Gurka and S. J. Steinmetz, 1982: Applica¬ tions of VAS multispectral imagery to aviation forecast¬ ing. Preprints, Ninth Conf. on Weather Forecasting and Analy¬ sis (Seattle, WA), Amer. Mete¬ or. Soc., 227-234. Byers, H. R., 1965: Elements of Cloud Physics . University of Chicago Press, 191 pp. Ellrod, G. P., 1987: Identifying high altitude mountain wave turbulence and strong Chinook wind events with satellite imagery. AIAA 25th Aerospace Sciences Meeting, Paper AIAA- 87-0183, Reno, NV, January 12- 15, 1987. CA), Amer. Meteor. Soc., Bos¬ ton, 515-520. _, 1991: Nighttime fog detection with bi-spectral GOES-VAS imagery. Preprints, Fourth International Conf. on Aviation Weather Systems (Par¬ is, France), Amer. Meteor. Soc., 71-75. Eyre, J. R., J. L. Brownscombe and R. J. Allam, 1984: Detection of fog at night using Advanced Very High Resolution Radiometer (AVHRR) imagery. Meteor. Mag. , 113 , 266-271. Hunt, G. E., 1973: Radiative properties of terrestrial clouds at visible and infrared thermal window wavelengths. Quart. J. Royal Meteor. Soc. . 99, 346-369. Lilly, D. K., 1978: A severe down- slope windstorm and aircraft turbulence event induced by a mountain wave. J. Atmos. Sci. , 34, 59-77. _, 1989: A decision tree approach to clear air turbu¬ lence analysis using satellite and upper air data. NOAA Tech. Memo. NESDIS 23 , Satellite Applications Laboratory, Wash¬ ington, DC, 20 pp. _, E. M. Maturi and J. Steger, 1989: Detection of fog at night using dual channel GOES-VAS imagery. Preprints, Twelfth Conf. on Weather Analy¬ sis and Forecasting (Monterey, 335 Session 10.6 FORECASTING FOR AVIATION WEATHER HAZARDS IN THE WESTERN NORTH PACIFIC Thomas S. Yoshida National Weather Service Office Guam, Pacific MANUSCRIPT WAS NOT AVAILABLE AT THE TIME OF PUBLICATION 336 Session 11.1 ADVANCES IN MESO-SCALE MODELING AT NMC John Ward National Meteorological Center, Development Division Camp Springs, Maryland MANUSCRIPT WAS NOT AVAILABLE AT THE TIME OF PUBLICATION 337 Session 11.2 A MICROCOMPUTER-BASED CLIMATOLOGICAL INFORMATION SYSTEM FOR TERMINAL FORECAST PRODUCTION Blaine Jelley Canadian Forces Forecast Centre Canadian Forces Base Trenton Astra, Ontario 1. INTRODUCTION Quicklimate permits the opera¬ tional meteorologist to easily and quickly access over 55,000 graphs and associated data sheets on the climatology of individual airports. This proves especially useful in training new meteorologists or in forecasting for new locations. 2. DISCUSSION Hourly observations for air¬ ports for up to 35 years have been accessed from the Canadian Climate Centre. The Quicklimate program, produced under contract for the Canadian Forces Weather Services, then produces a wide range of sta¬ tistics. Annual and seasonal over¬ views and diurnal variations are available for all parameters. Sta¬ tistics are provided for each month and for three 10-day periods in each month. Weather elements include: ceiling visibility temperature wind speed wind direction relative humidity total cloud opacity rain drizzle freezing rain freezing drizzle snow ice pellets hail/thunderstorms All calendar periods and weath¬ er element combinations may be stra¬ tified by time of day or wind direc¬ tion. Ceiling and visibility can be further stratified by the occurrence of either liquid or solid precipita¬ tion. 3. OPERATIONAL UTILIZATION Especially for the new fore¬ caster, or as a quick reminder for more experienced forecasters as the seasons change, Quicklimate can quickly provide answers to questions like: * What is the probability of ceiling height with winds from a given direction? * When will the sea breeze begin? * What is the probability of ceiling/visibility given rain is expected with winds from the southeast? * What is the frequency of fog this month? (10-day period)? 4. OUTPUT Graphs are generated based on the nature of the variable: continu¬ ous variables - probability distri¬ bution functions (fig. 1) and per¬ centile plots (fig. 2); discrete 338 Tempeialuio (Celsius) CurnulatrvePfobability Session 11.2 variables - stacked bar charts (fig. 3) ; and, duration statistics (fig. 4) . 5. INTERPRETATION In Figure 1 the variability of ceiling height at Gander, NF in January is depicted. It is quickly seen that winds from the west bring higher ceiling heights than average while north and northeast winds are associated with much lower than average ceiling heights. Ceilings are less than 1000 feet approximate¬ ly 5% of the time with westerly winds as compared to about 60% for northerly and northeasterly winds. Probability by Wind Direction - Ceiling Height Gander A 1953-1990 January 1-31 igure 1 Percentile by Wind Direction - Temperature Edmonton Namao A 1955-1989 January 1-31 Figure 2 Figure 2 shows the January temperature percentile plot for Edmonton, AB stratified by wind direction. Temperatures are normal¬ ly distributed (relatively straight lines) with calm, southeast and south winds. There is pronounced curvature, the effect of Chinooks, with southwest and west winds. Figure 3 shows the relative frequency of winds from different directions by time of day for Tren¬ ton, ON in July. The onset of the sea breeze between 15 and 17 UTC is clearly evident as is the predomi¬ nance of calm conditions at night. Relative Frequency by Time of Day - Wind Direction Trenton A 1957-1989 July 1-31 Number ol observations pet houily data point 1023 Figure 3 Duration of Ceiling/Visibility Conditions Cold Lake A 1954-1989 March 1-31 Figure 4 339 Session 11.2 In Figure 4 duration of opera¬ tionally significant ceiling/ visi¬ bility categories are indicated for Cold Lake, AB in March. Information of the percentage occurrence of each of the categories is also shown. Conditions less than 1000 feet/3 miles last greater than 6 hours almost one sequence in four whereas conditions below 300 feet/1 mile last beyond 6 hours on about one sequence in 10. 6. EQUIPMENT REQUIREMENTS Quicklimate runs on any micro¬ computer with an MS-DOS 3.2 or high¬ er operating system. A hard disk is required to store the main program (200 K) and the data files for each airport of interest (1300 K per station). Colour graphs are pro¬ duced on EGA or VGA monitors and can easily be printed. 7. FUTURE DEVELOPMENT Since forecaster/briefer work¬ stations at both military and civil¬ ian weather offices in Canada are UNIX-based, work is currently under¬ way to adapt Quicklimate to operate in X-windows. This will permit quicker, easier access to climato¬ logical information to assist in the development of terminal forecasts. Many desired enhancements have been identified with this first release of the program. These will be addressed over the next year and will, among other things, provide more display options and increase the definition of ceiling/ visibili¬ ty in the lower categories. 340 ■ Session 11.3 STRATUS: A PROTOTYPE EXPERT SYSTEM FOR LOW CLOUD FORECASTING Denis Jacob 1 2 , Michel C. Desmarais ajid Frances de Verteuil , Peter Zwack 3 1. Introduction Stratus is a 26-month project aimed at developing a prototype expert advisory system, based on physical principles, to assist the meteorologist in the production of airport terminal weather forecasts. This million dollar project began in July 1989. The multi-disciplinary team is made up of personnel from the private, university and govern¬ ment sectors. OASIS, the feasibility expert system prototype, developed by CRIM and Meteoglobe Canada Inc. during a 5-month joint project was used as a starting point [4]. 2. Overview of the Forecasting Process The complexity of the weather forecasting process is well known. It involves numerous data sources which consist not only of quantita¬ tive and qualitative observations, such as temperature and cloud type, but also include a large number of analyses, forecasts from numerical models and climatological informa¬ tion. To this data, the meteorolo¬ gist is expected to apply complex physical laws and empirically de¬ rived rules to produce a forecast. Because of time constraints and the complexity of the physical process¬ es, the forecaster rarely uses ana¬ lytical reasoning. Instead they use the same approach as do experts in other fields working with similar constraints, namely, Recognition Primed Decision, [2],[3]. The expe¬ rienced forecaster, using a personal choice of parameters among those available, rapidly categorizes or prototypes the meteorological situa¬ tion according to his experience and generates future weather events that correspond to this prototype. Expert systems based on such empirical and pattern matching techniques repli¬ cate the ability of a particular meteorologist for a specific loca¬ tion. Such systems link specific conditions to specific conclusions without requiring a clear chain of causal links. 3. Objectives The long term objective of STRATUS is to develop an operational model of the fundamental physical rules that govern the existence and evolution of clouds. Because of its universal nature, this model is, in 1 Environnement Ca,-da, SEA, Centre meteorologique du Quebec, 100 Boul. Alexis Nihon, 3eme etage, Saint-Laurent, Quebec, H4M 2N8 2 Centre de recherche informatique de Montreal(CRIM) 3744 Jean Brillant, Suite 500, Montreal, Quebec, H3T 1P1 3 Physics Dept., Universite du Quebec a Montreal, P.0. 8888, Station A, Montreal, Quebec, H3C 3P8 341 Session 11.3 principle, easily adaptable to new location. The development philosophy behind STRATUS is to try at first to model the physical phenomena mathe¬ matically and, when this is not feasible, qualitative modelling is used. The integration of qualitative modelling with quantitative model¬ ling and the integration of differ¬ ent sources of meteorological data constitute the two major theoretical problems for the project. The problem of forecasting low clouds can be broken down into two totally independent processes. The first simulates the time evolution of atmospheric parameters which are relevant to low cloud formation. The second is the analysis of these parameters, at a specific time, to determine the low cloud coverage. STRATUS addresses these two aspects of meteorological forecasting. The relationship between the evolution and diagnostic processes is shown in Figure 1. In this paper, the empha¬ sis will be on the design and test¬ ing of the diagnostic module and the X-Window visualization tools that were developed to assist the opera¬ tional meteorologist in his valida¬ tion efforts. The preliminary re¬ sults of the forecasting module will be presented as wel1. Forecasting process To •' •■-V . ••• Temporal ■; r';’ • V tT.Vr parameters,'"^* ' \ Physical parameters Physical parameters *1 . :* i 1 T 1 Dciivcd parameters Diagnosis pniantclcis i i * [... I’Ciouds - ] To | Clouds | Figure 1. The diagnosis and cvolulion of physical parnmclcis 4. The Diagnostic Module The following methodology was used to develop the qualitative physical model for the diagnostic module. The physical conditions/ scenarios and relevant parameters which lead to the formation of low clouds were identified. The rules and algorithms that define the rela¬ tionship between the parameters and the formation of low clouds were then established. It must be noted that a considerable amount of work had been done on the qualitative physical model for low clouds prior to the start of the STRATUS pro¬ ject^]. Figure 2 shows the archi¬ tecture of the diagnostic module. STRATUS DIAGNOSIS PROCESS | OBSERVED | | DERIVED | |CLOUD COVER | I PARAMETERS | | PARAMETERS | PRESSURE PRESSURE TENDENCY DEW POINT TEMPERATURE WINDS VERTICAL MOTION MIXING RAI10 STABILITY DEW POIN r DEPRESSION ICING TURBULENCE CLOUD OPACITY CLOUD HEIGHT CEILING HEIGHT | LOW CLOUD | RULES I Figure 2. The diagnostic module PARAMETERS RULES The diagnosis of low clouds involves two sets of parameters. The first set is directly observable. The second set, the "derived parame¬ ters", is obtained from the observ¬ able parameters by using a set of rules/algorithms, the "parameters rules". The "derived parameters" constitute the relevant physical variables involved in the diagnosis of low clouds. These are the parame¬ ters which an atmospheric physicist would use to explain the formation and dissipation of low clouds. To determine cloud cover, the diagnos¬ tic process shown in Figure 2 is applied to the values of each param- 342 Session 11.3 eter at a number of altitudes, de¬ pending on the resolution required for the diagnosis. Specifically, this involves obtaining the five observable parameters from the ap¬ propriate data sources, then apply¬ ing the "parameters rules" followed by the low cloud rules to obtain the cloud opacity at a particular alti¬ tude. This process yields a sequence of cloud opacities from which we derive a total cloud cover, a cei¬ ling and a ceiling height. Those metrics are then used in the valida¬ tion process to compare the diagnos¬ tic module with actual observations. The design philosophy followed in the development of the diagnostic module has been to simplify the physical rules of low clouds with the understanding that as progress was made the rules and the relation¬ ships would be modified, refined and expanded. 5. Architecture of STRATUS The first prototype was imple¬ mented in Knowledge Craft, an expert system shell, on a TI Explor¬ er. Preliminary testing showed that a relatively sophisticated database management utility which provides fast access to the data was essen¬ tial. Besides being extensively used in the expert system module, the data is also required for diagnostic and analytical tools that the meteo¬ rologist would need for evaluating the system. Consequently, it was decided to change the development environment to UNIX and implement an efficient database utility in C. 'X' was chosen as the windowing standard for the interface and graphical tools. The architecture for the fifth prototype is shown in Figure 3. STRATUS ARCHITECTURE STRATUS contour | } tephi| l~ DIAGNOSIS~| | FT | | - f° B I _ | |LOwIclOUD||ENCODE 11 DECODE | GRI-DB] |SQN-DB jOBS-pB METEOROLOGICAL DATA Figure 3. Architecture ol STRATUS The database is composed of a controlling module and three sub- modules, one for each data source. The data sources for STRATUS are numerical model output (RFE, the Canadian Regional Finite Element Mo¬ del), ground observations and verti¬ cal soundings. Each physical parame¬ ter can be indexed in four dimen¬ sions: time, height, latitude and longitude. It now takes only 10 se¬ conds to load a day's worth of data and 10 milliseconds to access a par¬ ticular record. On average, the database manager handles 10 MB of data per day. It is this fast and flexible access to the different data sources that has enabled us to validate the system statistically with a large number of cases. The database utility is written in C, while the diagnostic module is im¬ plemented in PROLOG. Several stand¬ alone analytical tools have been developed like an interactive tephi- gram and most of them have been integrated into a user-interface de¬ scribed hereafter. 6. Tools In order to generate an expert score, the expert would have to evaluate a large number of cases. These evaluations involve examining, modifying and calculating parameters from the vertical soundings of tem¬ perature and humidity on tephigrams 343 Session 11.3 and from horizontal contours of both observations and numerical model forecasts. Using available tools, the expert was only able to complete one or two cases per day. Clearly special tools were needed to accele¬ rate the process in order to have a data set large enough for statisti¬ cal evaluation. Therefore, an inter¬ active tephigram and a contouring program were developed. The interactive graphical teph¬ igram (Figure 4) displays profiles of either observed or numerical model forecast temperatures and dew points, allows for zooming, and can be modified using the mouse. A new set of parameters can be calculated automatically from the modified sounding. Also included in the teph¬ igram tool is a hodograph (Figure 4) that displays the wind profiles. With the interactive tephigram, the expert is able to analyze 25 cases per day. The contouring program (Figure 5) displays surface observations, analyses of either observations or numerical model forecasts. The for¬ mer are analyzed objectively using a Barnes objective analysis scheme[l]. From the menu, the user can choose the date, time, variable and contour interval. Presently over 20 numeri¬ cal model output variables (e.g.,- pressure, temperature, vertical motion and dew point depression) are available at 3 hour intervals in either pressure or sigma coordi¬ nates. The module automatically interpolates numerical model output from sigma coordinates to the de¬ sired level. Also it allows for zooming, overlaying and animation. Included in the contour device is a program that estimates lower tropos¬ pheric vertical motion from the objective analyses of a series of pressure observations using the method suggested by Zwack and Kabi1[5]. This vertical motion, not yet used in STRATUS, is now under¬ going further development and evalu¬ ation. Figure 5. The contouring program Two other display panels com¬ plete the graphical tools. These panels show all the parameters used and the results obtained by the diagnostic and forecasting process¬ es. They help the expert meteoro¬ logist in the evaluation of the strengths and weaknesses of the diagnostic and forecast modules. Underlying the forecast module are an FT encoder to write the forecast 344 Session 11.3 and an FT decoder to check for mis¬ takes. 7. General Performance Evaluation In order to validate the diag¬ nostic module, evaluation criteria had to be established. This is a very important procedure as it sets the performance expectation levels. For the second prototype, the met¬ rics were chosen to facilitate com¬ parison with actual observations. The metrics are the detection of ceiling, height of the ceiling and total cloud cover below 6,500 feet. Climatology which is based on his¬ torical statistics, has a success rate of about 55% for ceiling detec¬ tion. This figure provides a base¬ line for the evaluation of our sys¬ tem's performance. For an upper bound, we expect the system to be able to perform at the level of an expert given the same input data. To accurately assess the per¬ formance of the diagnostic module, the most reliable data sources for the five input parameters were used, namely, vertical soundings and ground observations (SA) at 0Z and I2Z. The output of the diagnostic module is then compared with the ground observation at the corre¬ sponding station and time. The test data set consists of 789 cases spread over 23 stations for 27 days in May 1990. The stations are within a 2,500 km grid centered near Alba¬ ny. The results for the second prototype (V2.4) indicated a 79% success rate (Phi of .51) for ceil¬ ing detection, a correlation of .42 for the ceiling height and a corre¬ lation of .69 for the cloud cover. Our expert examined by hand the cases where the diagnostic module performed badly. He was quickly able to identify several shortcomings in the second prototype. After these corrections were addressed, the success rate in the third prototype (V3.7) became 87% (Phi=.71) with a ceiling height correlation of .50 and a cloud cover correlation of.74. Other shortcomings, these mainly related to below zero conditions, have been corrected and allow for a more stable diagnostic throughout the year. The success rate of our last prototype (V5.4) is now 88% (Phi=.72) with a correlation of .61 for the ceiling height and .78 for the cloud cover, the most improved being the ceiling height score. For 119 different cases during the month of June 1990, clearly not a big enough sample to draw defini¬ tive conclusions, we compared the performance of the fifth prototype to our expert. The success rate for the ceiling detection was 87% (Phi=.64) for the expert and 78% (Phi=.54) for STRATUS while the ceiling height correlation was .79 for the expert and .67 for STRATUS and the cloud cover correlation was .68 for the expert and .63 for STRA¬ TUS. Figure 6 shows these results. Diagnostic Stratus version 5 I I □ Oiling OrtrciHOi «rature for ^nburet end non-downburet day*. (After Atkin* end Wekijeoto. 1991). Figure S. *00 mb eeelyeU ior 2 Aueuet IMS «t 1 w* Session L2 types of downbursts. As a rule, forecasters can predict them with good accuracy by noting a dew point depression of greater than 8°C at 700 mb and less than 8 # C at 500 mb. The Microburst Line These systems have been studied along the front range of the Rocky Mountains. Again, they appear to be associated with cloud systems that are high-based and shallow. They probably are initiated by sustained surface convergence brought about by orographic influences. They can cause major disruption of air traf¬ fic in and around an airport due to their long lifetime (usually on the order of an hour). The Airmass Thunderstorm and Pulse-Type Severe Thunderstorm These storm systems occur in an environment of high convective in¬ stability, high low-level moisture, and little or no vertical wind shear. In many cases, a distinct layer of much drier air is superim¬ posed on the lower moist layer. Downbursts from these systems are of the wet type. The penetrative downdraft pro¬ cess plays a significant role in downburst generation and maintenance in this environment. In addition, water loading appears to be impor¬ tant. As such, intense updrafts would be required which is supported by the large positive energy pro¬ files often noted in many of these storms' soundings. These are the least understood of the downburst-producing systems, yet recent studies and field experi¬ ments have begun to uncover some clues to their development and pre¬ diction. The most promising to date, appears to be a radar detec¬ tion technique developed by Stewart (1991). The technique requires the examination of Vertically-Integrated Liquid (VIL) water content and radar echo heights as provided by the RADAP II system. These parameters are incorporated into a form of a vertical velocity or gust equation by Emmanuel (1981). The final gust estimate takes into account horizon¬ tal momentum transport in the lower 1.5 km (5000 ft) through the addi¬ tion of the layer mean wind speed. The estimate was found to be quite accurate and aided the forecaster in warning for these wind storms that characteristically have lower VILs and echo heights than those of hail- producers. Lead time on predicting downbursts from these pulse-type storms was on the order of 20-30 minutes. Read and Elmore (1989) noted very high reflectivity (VIP 5) aloft in downburst-producing storms. They observed that whenever this elevated VIP 5 core extended to at least 9 km (approximately 30,000 ft), the po¬ tential was very high for downburst occurrence and a warning should be issued. The resultant lead time was 5-10 minutes. Another promising finding emerged from work by Atkins and Wakimoto (1991) in the Southeast U.S. They noted a significant dif¬ ference in the vertical profile of equivalent potential temperature in downburst-producing storms versus non-downburst-producing storms. The different profiles are shown in Figure 4. Most apparent is the layer of rapid decrease in theta-e values for the environment of the downburst-producing storms, indica¬ tive of high convective instability. 383 Session L2 6. AN APPROACH TO FORECASTING DOWNBURSTS With a brief understanding of the types of downburst-producing systems, a systematic approach to downburst forecasting was presented to laboratory participants. The approach was geared toward the hybrid- or true wet-type downburst environments and consequently re¬ sulted from the work done by Read (1987) in North Texas and Rydell and Ladd (1991) in South Texas. Labora¬ tory participants applied the ap¬ proach to the forecasting of a well- documented downburst event; that of the Delta 191 accident at DFW in 1985. The forecasting process can be broken down into two phases: assess¬ ing the potential for downburst development and determining the most likely location for downburst occur¬ rence. Concentrating on the first phase, it too can be broken down into two sub-phases: a thermodynamic assessment of the atmosphere and a kinematic assessment of evolving features. A good thermodynamic assessment of the environment naturally re¬ quires a close examination of sound¬ ing data. However, we first must make an attempt at narrowing the region of suspicion based on upper- level flow regimes and expected advection of properties. So, an analysis of the upper-level charts is a must. Being that many downburst- producing systems are spawned by subtle features in an otherwise tranquil environment, it is impor¬ tant to carry out the height analy¬ sis at a 10-20 meter interval and the temperature and dew point analy¬ sis at a 2-3 °C interval. In the lower levels (i.e. 850 and 700 mb). location of convergence zones, tem¬ perature ridges and moisture axes are important. Higher up, the posi¬ tioning and tracking of dry layers becomes vital. For the DFW event, the most notable upper-air feature was the existence of a large dome of high pressure over North Central Texas which resulted in very weak flow (see Figure 5). High dew points were noted over East Texas in the lower levels with a wedge of dry air in the upper levels extending from West Texas across Oklahoma and into Arkansas. The upper flow pattern, despite being weak, could be expect¬ ed to advect the dry air over the lower moist layer during the day. Thus, a characteristic profile for a wet downburst event was establishing itself over North and East Texas. Laboratory participants, now focusing on this region, shifted their attention to an examination of the Lake Charles (LCH), Longview (GGG), and Stephenville (SEP) sound¬ ing data. It is important to modify the soundings to take into account the expected afternoon maximum tem¬ perature, moisture content in the lowest 100 mb, and changes in the upper temperature and moisture pro¬ files due to advection. Determining the Convective Condensation Level (CCL) and noting the degree of posi¬ tive energy available on these modi¬ fied soundings should follow. It was pointed out that with the advent of the Skew T/Hodograph Analysis and Research Program (SHARP) in the field offices, modification of soundings will become much easier. All the soundings supported the possibility of downburst generation to some degree. However, the fur¬ ther west you went, the more defini¬ tive the sounding data became. The modified SEP sounding (Figure 6) 384 Session L2 exhibited high cloud bases despite high precipitable water, strong instability, little or no capping inversion and very dry potentially colder air aloft. Comparing the SEP sounding with some conceptual models of "dry-type", "hybrid-type" and "wet-type" downburst soundings (Fig¬ ure 7), one can argue that the DFW event most closely resembles that of a hybrid-type environment. An analysis of subcloud lapse rates ended the thermodynamic as¬ sessment and solidified in the fore¬ caster's mind the region of greatest concern. High subcloud lapse rates (700-500 mb) have been positively correlated with downburst potential over the High Plains. Critical values appear to be 8°C/km or greater. However, owing to lower terrain and higher moisture content, subcloud layers across North and East Texas are more aptly represent¬ ed by the 850-700 mb layer. The analysis (Figure 8) showed an axis of 8-9 °C/km lapse rates extending north-to-south across Central Okla¬ homa into North Central and South¬ east Texas. At this point, it was time to shift our attention to the surface chart. Paramount in importance was the detection and tracking of bound¬ aries, development of temperature ridges, and increased moisture pool¬ ing (or convergence) into an area. It has been noted in the South Texas study (Rydell and Ladd, 1991) that downburst occurrence always occurred in what could be termed "second- generation" convection. In other words, outflow from a parent storm complex eventually triggered the downburst-producing thunderstorm. So, an hourly 1-2 mb analysis is necessary, as well as close monitor¬ ing of radar and satellite, to de¬ tect and track these outflows. This proved necessary in the DFW event. A strong and persistent line of thunderstorms was evident along the Red River and into North¬ west Louisiana throughout much of the day. This line was associated with a slow-moving cold front and was producing outflow. The outflow was moving south and west into North and East Texas. A well-defined surface temperature ridge was noted developing ahead of the outflow boundary with some evidence of dew point pooling occurring as well. Noting the coming together of these features constitutes the major task during the second phase of the forecast. A conceptual model of composite surface features and the probable location of downburst oc¬ currence (Figure 9) was presented at the laboratory session to assist the forecaster. The AFOS Data Analysis Programs (ADAP) charts should be ideal in tracking increasing insta¬ bility, temperature and moisture convergence into an area. As an outflow boundary approached this area, the downburst could be expect¬ ed to occur on the warm side in the vicinity of greatest moisture pool¬ ing. In South Texas, we must also note the role that additional bound¬ aries, such as the Seabreeze, may play in increasing the moisture convergence into an area. All in all, the conditions associated with the DFW event fit the conceptual models (both surface and sounding) quite well. Most laboratory participants were able to zero in on the area a couple of hours before the downburst occurred. Thus, it was agreed that some skill in forecasting these type of events could be had given the time and attention to detail needed to accu¬ rately detect and track important features. 385 VLcura t. «S0-700 mt Iipm rata aaalyaU (4a« CA*) «o 2 Aataat IMS at UOt Off. x; \ N \ \ \ \ \ \ V $ T X. \ v \| \ \ v \ V- \ "U \ \ \ II \ \ N \ \ X \ X -J rijvr* 7. *xat-typa- Conceptual aouadlac aadala far Mistype* S k-H O D C -H Ei C >1 O -H JJ O 4-> «—1 (0 •o -H -H jj a e Ifl JJ -H *-> u o U e o jj CP <0 Uj -«H »0 O *w o , cp»a •H o c c>o to <— 1 pj JJ r— { H -H 'O ■ —( U Z3 to o U O.W o 0 CQ D> O jj o > C £ <0 <0 •—1 jC <0 to cu-h ir CO E <1> JJ £ C k -O JJ E > 3 E <0 <0 <0 J-i o 35 kt at mtn top to >75 kt at jet level) and normal to ridges? YES * Wide, thick bands are those easily distinguished in low resol. IR images. Present /Look for narrow, stationary ' clearing in cirrus along east, slopes (IR, WV) See Fig 5. Absent i M-S mountain wave turbulence No sig. CAT likely (80%) (50%) 11 Figure s 403 Session L 3.2 Sharply curved flow Ridge' (on south side of jet) i Look for well definec transverse bands near upper ridge axis./ Absent Present Only #1 present Trough (preferably with N-S or NE-SW tilt and strong upstream jet) Check for: 1. sharp, well-defined edge to cirrus near trough axis. Dark zone in WV image along or upstream from trough which darkens with time. Both present or #2 alone Neither present CAT possible.(60%) Check following subjective chart of convergence (CONV) versus temperature gradients (DELTA-T) from latest upper air data or forecast. 1 CONV 2 DELTA-T CAT Strong Large M-S Strong Small M Moderate Large M Moderate Small L-M Weak Large L-M Weak Small None Strong CONV is defined as a deceleration of about 50 kt (25 m/sec) in 4 degrees latitude. Moderate CONV would occur with about a 35 kt deceleration. 2 Large -DELTA-T is > 4°C/180 nm (3 degrees latitude) 12 Session L3.2 Deformation Zone Patterns Determine cloud feature present Comma cloud Little or no cloudi¬ ness Delta-shaped cirrus (IR/VIS) (W-NW flow) Determine location^, with respect to / vcomma cloud GO TO PAGE 14 Is darkening with time occurring in WV imagery? Look for presence of transverse cirrus cloud bands Present Absent Determine nature of cloud bands. 7 Long and narrow Wide and thick 13 RGoee \o 405: Session L3.2 Location with respect to comma cloud Near poleward edge of comma head In dry- slot Near comma tail GO TO PAGE 15 Is slot forming rapidly, accompanied by darkening in WV imagery? Look for trans-' verse cirrus .cloud bands Present YES NO /Check nature \of bands L-M CAT No sig. likely CAT (80%) (60%) Long, narrow Wide, thick Check table at bottom of page 12 Absent i Is darkening in WV images occurring upstream? NO Is back edge of comma well; defined? NO £ No sig. CAT 80% 14 Figure i\ 406 Session L3.2 Poleward Edge of Comma Determine Synoptic Situation Deepening or steady state upper level 7 Upper low opening into trough or Upper level vorticity ij Z Is downwind cloud edge sharp and well defined ?/ YES / MOGR CAT likely (70%) 15 Figure. \q. 407 Session L3.2 Developing or steady state upper low with surface cyclone Full comma 1 Check for one of the following conditions: 1. Cloud border well defined in IR or is becoming sharper with time. 2. Transverse bands or billows (VIS) near cloud edge. Rapid movement of cloud edge (>25 kt ) toward clear air (occurs with building or rapidly propagating ridge)/ One or more present None present MOGR CAT likely along most of cloud edge, mainly just on clear air side. (2 or more 90% otherwise 70%) Sheared (jet crosses over head cloud) MOGR CAT possible from near crossover point to just east of downstream upper ridge. The presence of any of the fol¬ lowing may be help¬ ful clues: Bands (in IR or VIS) or billows (VIS) near jet (80%) Darkening in WV images north of jet. (80%) Sharp, anticyclon- ically curved jet cirrus segments. (80%) 16 FlOJfce 13 408 Ellcod injoex Session L3.2 18Z RGL 12/11/91 Session L3.2 3. The tropopause versus temperature falls into the crosshatched area of fig¬ ure 15. (Temperature -68 C or colder and tropopause below 150 MB.) SATELLITE DATA Ell rod (SAL, NOAA/NESDIS) has observed a satellite signature that distinguishes turbulent from non- turbulent high level mountain wave activity. He has noted that in some significant turbulence events the mountain enhanced cirrus has moved downstream from the ridge line leav¬ ing a sharply defined nearly sta¬ tionary clearing zone just east of the ridge line with the most signif¬ icant turbulence occurring in the western portion of the enhanced cirrus (figure 16). This differs from the non turbulent situation where the enhanced cirrus is right along the ridge line. Also, when the clearing zone ceases to exist, so does the significant upper level mountain wave turbulence. 410 Tropopause Prtjsur* Lave! (MB) Session L3.2 -70 Tropopatrt. Tcnpcrzture ( C) -55 -60 F \&ue.e \s TURBULENT LEE WAVES NON-TURBULENT Foehn Ridges Gap F\GO££ \