UNIVERSITY OF CALIFORNIA, SAN DIEGO UC SAN DIEGO LIBRARY 3 1822 04429 7497 OPTICAL SYSTEMS GROUP TECHNICAL NOTE NO. 223 November 1990 Offsite (Annex-Joi rnals) QC 974.5 T43 no. 223 Factors Influencing the Development of a Short Term CFARC Prediction Technique Based on WSI Imagery T. L. Koehler J. E. Shields UNIVERSITY OF CALIFORNIA SAN DIEGO The material contained in this note is to be considered proprietary in nature and is not authorized for distribution without the prior consent of the Marine Physical Laboratory SITY. SAIN Technical Sponsor, R. W. Endlich HEL Support Branch ut LIG • THE RNIA *1868 Hed Prepared for The U. S. Army Atmospheric Sciences Laboratory White Sands Missile Range, NM 88002-5501 under contract No. N00014-87-C-0127 SCRIPPS INSTITUTION OF OCEANOGRAPHY MARINE PHYSICAL LAB San Diego, CA 92152-6400 UNIVERSITY OF CALIFORNIA, SAN DIEGO 1 1 11 III 11 II 11 LU 3 1822 04429 7497 TABLE OF CONTENTS . . . . . . . . . . ..... 1.0 2.0 3.0 4.0 Introduction ........... ............ ............... Background. . . . . . . . . . . . . . . . ...... Anticipated Sequence of Operations in the Predictive Algorithm ............ Important Considerations in Developing Short-Term CFARC Predictions ... 4.1 Variable Grab Rates................................... 4.2 Multisensor Arrays .. 4.3 Coordination with Other Cloud Information Sources ... 5.0 ...... .......... ............. ... .. ..... Relevant Future Upgrades...... 5.1 Use of Wider Angle Fisheye Lenses . 5.2 Active Ranging Devices .... 5.3 Image Acquisition at Night .......... Concluding Remarks va e cerere een AAN 6.0 Appendix A. List of Figures ....... 1.0 INTRODUCTION TheLSTCIMager Display and Analysis Sub-System (LIMDAS) located at the HELSTF facility at the White Sands Missile Range currently provides near real-time cloud cover imagery for the HEL Support Branch. The system currently has the ability to looppreviousimagery acquired at 5-minute intervals, providing a user with some information concerning the motion of cloud elements in the field of view of the WSI instrument. A more reliable means to predict the short-term future state of the cloud field is needed by those making the decision to fire or not fire the laser. This note describes the initial effort by the Optical Systems Group of the Marine Physical Laboratory to develop prediction techniques that could be incorporated into LIMDAS. 2.0 BACKGROUND field near the target track, but also by the expected state of the cloud field at some specified time in the future. Two important questions to be answered by a predic- tion scheme depend on the current state of the cloud cover along the target track: 1) if the track is currently cloud free, will it be cloud free at the time of firing, or 2) if clouds currently obscure the track, will it become cloud free. 11 During periods of completely clear skies or complete overcast, sky cover is often quite persistent, making a short term prediction (less than 1 hour) almost trivial. The sky cover climatology in Table 1 from Stallion Site in the White Sands Missile Range indicate that, on average, completely clear and overcast situations are present over 40% of the time. On a month-by-month basis, however, this total varies from a minimum of 17.2% during the summer rainy season, to a maximum of 54.7% in February, a period dominated bymidlatitude weather features, such as fronts, low pressure systems and jet streams. The presence of clouds can have adverse effects in many applications, particularly when lasers are used to direct enough energy at a target to damage or destroy it. The presence of cloud along the track of a target is thus an important parameter in the decision of whether or not to fire such a laser weapon. Furthermore, since it takes a finite amount of time and resources to prepare certain lasers for firing, the decision-making process is influenced not only by the current state of the cloud At the other extreme are periods when sky cover changes dramatically in very short periods of time. Fig. 1 illustrates an example of such a situation as observed in a set of daytime images acquired at Co- lumbia, MO, in the spring of 1989. Several times on Table 1 Monthly climatological sky cover amount frequencies (%) from Stallion Site in the White Sands Missile Range, NM. Sky Cover Amount (tenth) 2 3 4 5 0 1 6 7 8 9 10 9.0 7.0 6.5 5.3 6.1 7.3 5.7 3.7 4.8 5.9 4.9 6.8 6.5 3.5 2.8 5.1 4.5 3.8 3.3 3.0 5.1 5.7 8.3 3.6 5.1 6.5 5.3 4.5 4.1 5.1 6.6 6.7 6.9 6.2 6.0 6.7 January February March April May June July August September October November December 8.7 28.4 30.4 26.0 32.0 29.8 29.9 8.3 13.6 27.1 42.0 31.7 28. 0 9.6 10.6 13.2 10.1 12.2 13.8 10.5 9.1 1 1.7 7.9 8.2 9.5 7.4 7.2 5.7 8.6 8.4 5.5 9.3 9.1 10.6 11.5 8.6 6.7 5.3 5.6 8.2 10.3 11.2 7.2 5.9 6.4 4.9 4.2 5.1 7.2 6.9 4.3 4.5 9.7 7.5 4.6 5.2 4.4 8.7 6.3 6.4 3.8 4.5 5.5 19.0 24.3 20.3 11.8 10.7 6.1 8.9 6.3 8.9 8.7 15.3 18.6 5.1 8.3 7.2 7.0 9.0 6.7 4.9 5.6 4.8 3.5 3.7 5.3 5.2 4.4 3.5 3.7 2.9 7.3 6.5 6.9 8.0 Combined 27.3 10.4 7.4 7.1 6.2 4.8 4.4 5.4 6.4 7.4 13.2 this day, the sky cover went from nearly clear to nearly overcast in a matter of minutes. These dynamic situ- ations are often associated with strong tropospheric winds that propagate the cloud elements rapidly across the whole sky images, presenting quite a challenge to any short term prediction technique. T he current LIMDAS runs as a slave off of the WSI field unit that grabs images at a fixed time interval of 1 minute. As specified by the user, LIMDAS currently performs a grab every 5 minutes. The additional time is allocated to the ratio and cloud decision processing, and the looping display. Straightforward upgrades in software would allow more frequent grabs, but the master-slave relationship with the standard WSI im- poses an upper limit on the grab rate at once perminute. Grabs at faster rates would also be possible ifLIMDAS were to drive its own WSI, and if the downstream ratio processing were delayed until a sequence of grabs were completed. Step 2: Compute the Red/Blue Ratios Ratio processing now takes roughly 30 seconds for a full resolution image. Using an accelerator board, the ratio computations take only 8 seconds. Step 3: Perform the Cloud - No Cloud Decision The following sections describe by means of ex- ample, some of the important factors that must be considered in developing a cloud element prediction scheme, and the work completed thus far by our group. We assume that a wide range of other cloud informa- tion from radar and satellite imagery is also available to the decision maker, and that such information may eventually be incorporated into the short-term cloud- free arc predictive technique. 3.0 ANTICIPATED SEQUENCE OF OPERA- TIONS IN THE PREDICTIVE ALGORITHM At lead times of more than a few minutes, the predictive algorithm will most likely be based on the extrapolation of the translation of cloud element edges into the future. At shorter lead times, an alarm system might also be employed to warn of the approach of cloud elements into the target region. Since the ex- trapolation forward in time is the more complicated procedure, the discussion here will concentrate on that technique. The sequence of operations is as follows: Step 1 - Grab the current red and blue image pairs. Step 2 - Compute the red/blue ratios. Step 3 - Perform the cloud-no cloud decision on the ratio image Step 4 - Transform the fisheye view to a pseudo- Cartesian coordinate Step 5 - Detect cloud edges and elements on a single image Step 6 - Combine with recent previous images to detect translations Step 7 - Extrapolate current cloud elements into the future Step 8 - Validate the prediction made for the current time LIMDAS employs our preliminary cloud decision algorithm that uses fixed ratio values as thresholds in delineating thin and opaque clouds, and computes the percent sky coverage. Work is currently underway to develop a new algorithm that allows the background blue sky ratio to vary as a function of scattering angle from the sun and distance from the horizon. The newer technique would require more computing time than the current version. Step 4: Transform the Fisheye Image into Pseudo- Cartesian Coordinates 11 Detecting the motion of cloud elements is an inte- gral part of the prediction scheme. Consider a simple case in which a single cloud element is passing directly overhead, travelling at some constant velocity without changing its shape. In the WSI fisheye view, radial distance from the center of the image is roughly pro- portional to zenith angle. As the cloud enters the image at high zenith angles, a fixed displacement in space over a fixed time interval will result in a smaller zenith angle change than would be produced by the same displacement later when the cloud is directly overhead. Therefore, in the fisheye view, the cloud will first appear to move slowly, accelerating as it passes over- head, then decelerating as it approaches the horizon. Extrapolation of cloud element positions forward in time is far simpler if the elements appear to translate in a linear fashion. It is impossible to map a single fisheye image into a true three-dimensional Cartesian coordinate, because Steps 1 - 3 are already available on LIMDAS. Software for Step 4 has been completed, but is not yet incorporated into LIMDAS. Some samples of identi- fying specific cloud features on consecutive images and detecting their motion (Steps 6 and 7) are presented in the following sections. The prediction and valida- tion steps await further progress on the previous steps. Step 1: Grab the Red and Blue Image Pairs 1 move more slowly than low clouds moving at the same speed. Figures 3 and 4 compare a sequence of 4 WSI images before and after the transformation has been performed. The transformed images only include points within the 65° zenith angle, outlined by a blue circle in the fisheye images (Fig. 3). Note how the the long, thin nature of the cumulus near the occultor is changed in the transformation process. The transfor- mation also enlarges the narrow cloud band along the bottom edge of the 65° region. Step 5: Detecting Cloud Edges and Elements the fisheye projection preserves only the zenith and azimuth angle for a particular target, and provides no range information. A pseudo-Cartesian coordinate must then be developed that can be defined from the two geometric parameters available from the fisheye projection, and, under most circumstances, preserves the linear translation of cloud motion. The simplest method is to rescale distance from the center of the image from the roughly linear dependence on zenith angle in the fisheye projection, to a dependence that varies linearly with the tangent of the zenith angle. As illustrated in Fig. 2, the tangent of the zenith angle is proportional to the horizontal distance of the target from the imager divided by the elevation difference. Assuming a cloud remains at a fixed elevation during its motion, the distance travelled in the transformed coordinate will the be proportional to its horizontal displacement in Cartesian coordinates. The transfor- mation equations used in preparing this report are provided in Appendix A. The transformation does have some noteworthy limitations. Table 2 presents the incremental area ratios for the fisheye and transformation images (see Appendix A). A pixel of unit area in the fisheye image would thus have an area of 0.4 in the transformed image at the center of the image, and an area of 5.02 units at 65°. In practical terms, the transposed image would use only 2 out of 5 pixels at the center, but would repeat the same fisheye pixel 5 times at the edge of the transformed image. If the transformation were extended to the outer edge of the fisheye images (near 80°), the distortion near the outer edge would be even more pronounced. Given a single cloud decision image, boundaries between regions with and without cloud must be de- termined. During this process, pixel groupings that form individual cloud and clear elements will also be defined. That is, the cloud image indicates whether each picture element (pixel) is cloud or no cloud; the next step would be to automatically determine which pixels belong to the same cloud element. The detection of cloud edges and elements for the examples in this report were performed interactively by a human ob- server. Objective methods for performing these rela- tively straightforward tasks can either be developed or possibly extracted from standard image processing programs. Step 6: Combine with Recent Previous Images to Detect Translation Table 2 Incremental area ratios (transformed/fisheye) Zenith Angle Ratio 0° 0.40 10° 20° 30° 400 500 0.43 0.50 0.63 0.91 1.50 600 650 3.09 5.02 The motion of cloud edges becomes quite apparent when a sequence of WSI images are viewed in a standard loop. In order to quantify the motion, the more difficult task of identifying the same cloudy or clear element on consecutive images becomes impor- tant. To further complicate matters, several processes are responsible for the observed motion, including: 1) bulk movement of the clouds, 2) their growth and decay, or 3) some combination of both. Considering the broad spectrum of cloud field combinations, and rates of motion, the development of a reliable, fully automated cloud motion detection system is no trivial matter. For this report we have selected a few cases that illustrate certain characteristics of cloud motion that must be considered in a general algorithm. Video loops from these cases are available on an accompanying video tape. The first case is from C-Station on 24 March 1989. The period examined started completely clear. Several individual bands of cirrus propagated from south to north over the region, eventually producing a complete overcast. The case was quite suitable for this prelimi- nary investigation for many reasons. As noted in the Two other features of the transformed images also deserve attention. First, the vertical extent of the clouds viewed may lead to confusion in the cloud motion determination if the motion is assumed to be horizontal in nature. Also, the apparent speed of cloud element propagation in the transformed images is in- versely proportioned to cloud height. Thus, high clouds moving at some specified speed will appear to coordinate transformation section, high clouds appear to move more slowly than low clouds, enhancing our ability to identify the same cloud element on full- resolution images spaced 10 minutes apart. It is also easier to identify along cloudedge thanmany individual cloud elements. The case also exhibited several of features relevant to this discussion: large feature (line) motion, small feature (element) motion, and feature evolution (line spreading). period, and the propagation of the main cloud mass from west to east is primarily due to the development of new cells that amalgamate to form the larger cloud mass. Only a slight driſt of the cells toward the north- northeast is evident. Steps 7 & 8: Extrapolate the Cloud Element Dis- tribution into the Future, and Validate Previous Pre- dictions Fig. 5 shows a sequence of 4 images featuring what appears to be two crossing cirrus lines. A discernable edge has been highlighted, with the edge positions at previous times being reproduced on later images. The edge position from the image immediately preceding this sequence (not shown) is included on the first image. As expected, the translation of this line feature is very uniform in the pseudo-Cartesian projection. The average velocity of the line is at 45 pixels per 10 minutes toward the 3° azimuth. North (0° azimuth) is toward the bottom of the images, east (90°) toward the right, west (270°) toward the left and south (180°) toward the top. In contrast, the motion of the crossing point of the lines is much more rapid (85 pixels per 10 minutes toward the 62° azimuth), as shown in Fig. 6. It is not unusual in meteorology to have large scale features that move differently than the winds flowing through them. For example, surface fronts often move at a much slower rate than the winds, and squall lines oſten propagate more slowly and in a much diſferent direction than the individual cells within them. Such is also the case in our example. The crossing point motion is probably a good wind velocity tracer, while the propagation of the cirrus band in general differs considerably. A later sequence is presented in Fig 7. Two individual cirrus bands have been outlined in different colors. Note how the bands spread, showing considerable overlap at the end of the sequence. The predictive scheme used will be highly depen- dent on the required lead time, the method used to identify individual cloud elements, and the dominant dynamic process for the current situation (such as line vs. element motion). During its development, the technique will evolve from requiring a high level of user interaction at the beginning, to less and less interaction in later versions. Growth and decay pro- cesses will probably be modeled using some statistical techniques. The accuracy of the statistical forecast can be improved if growth and decay rates are continually updated using current observations. These rates can either be climatological in nature, or computed from an earlier period from the same day or last couple of days. 4.0 IMPORTANT CONSIDERATIONS IN DE- VELOPING SHORT-TERM CFARC PREDIC- TIONS II Developing a predictive algorithm that can provide useful cloud field predictions over the wide range of meteorological conditions possible is not a trivial op- eration. The motion of cloud elements within the WSI field of view is the result of several different dynamic processes. The discussion that follows outlines some of the features that must be incorporated into a final working technique. 4.1 Variable Grab Rates LUI The movement of cloud elements across the WSI field of view is a function of both the wind motion in the atmospheric layer in which clouds are present, and, as shown earlier, the elevation of those clouds above the instrument. The most difficult situation arises from fast moving low clouds. The rate that images are grabbed to best identify element motion falls within a limited range. If grabs are made at too slow a rate, identifying a particular cloud element is nearly im- possible. For example, the choice of cases for this report was limited to cases in which cloud propagation was slow, so that features could still be identified in consecutive 10 minute full resolution images. Grab rates can also be too high. The individual elements must move farenough to reliably estimate their motion. The second case, from 9 July 1989, had overall qualities similar to the March case, with a practically cloud free sky changing to complete overcast in less than an hour. Since cumulus is the predominant cloud type, identifying particular cloud elements on the 10 minute images in Fig. 8 is difficult. To aid in element identification, 1 minute images were examined. Fig. 9 shows a time series of 1 minute intervals over a 30 minute period for this case. (The time interval between images is 2 minutes.) It is clear from this figure and from the corresponding loop on the video tape, that translation of elements is not the dominant process in the cloud element evolution for this case. In fact, several cumulus cells are seen to develop during the 1 TIS The optimum grab rate for any two situations may thus be quite different. Here user input will probably be helpful. In terms of system design, there will be a limit on how fast grabs can be made in the current WSI, designed to provide multispectral images for ratio processing. Ifmore rapid grab rates become necessary, consideration would have to be given to performing motion detection on raw radiance imagery, nominally available at full video rates (30 frames per second). 4.2 Multisensor Arrays increase in camera chip resolution to 1024 or 2048 pixels, compared to the current 512 pixel arrays, the new lenses could significantly enhance our resolution of cloud elements near the horizon. The image trans- formation to pseudo-Cartesian coordinates could then be performed to greater zenith angles, permitting an earlier detection of cloud elements. 5.2 Active Ranging Devices If the range to a particular cloud element could be determined by an active ranging device, such as a lidar, image transformations could be made to a true Carte- sian coordinate. Map projections of the clouds could then be produced and imagery from multiple sensors could be better coordinated. 5.3 Image Acquisition at Night Placement of the target track relative to the cloud layer wind can have a significant impact on the reliability of forecasts. In a single imager system, forecasts made for tracks on the downwind side of the image will be far more reliable than a track near the upwind edge of the image, because reasonable estimates of cloud element translation can be made before they impinge on the target track. Placing additional imagers upwind of the prevailing winds for the region may provide the in- formation needed to derive better predictions for the upwind target tracks. Additional climatological in- formation regarding the prevailing cloud layer winds, and frequency of particular cloud elevations would affect location selection for additional sites. 4.3 Coordination with Other Cloud Information Sources Work is currently underway to develop a night system. The best alternative employs a slow scan camera. Thus, the lower limit on the grab interval would be greater than that available during the day, decreasing the forecasting ability for fast moving low clouds at night. However, the benefits of acquiring any data at night far outweigh the limitations. 111111 The decision makers already have satellite cloud imagery at their disposal. The satellite images suffer from poor time resolution. However, the satellite data can provide a “heads up" warning in many situations. Forexample, in the frequent cases with eithercompletely clear skies or complete overcast, the satellite images may detect large scale regions of clearing or cloud approaching the site long before the clouds appear on the WSI imagery. This information could then alert the decision makerto pay more attention to the WSI, which would fine tune the propagation rates. 5.0 RELEVANT FUTURE UPGRADES 6.0 CONCLUDING REMARKS The results presented here are reasonably encour- aging. We have demonstrated that the translation of cloud elements can be reliably tracked on consecutive images after projecting the WSI fisheye view into a pseudo-Cartesian coordinate. While development of short-term prediction capability not trivial, it does appear tractable if sufficient long-term support is available, and if a clear set of requirements are speci- fied. Future upgrades to both the hardware and the software of the system could significantly enhance the forecasting capability. The development of a highly automated version of the predictive algorithm is not a simple task that can be solved by finding the right off- the-shelf software. The feasibility of producing a fully-automated predictive scheme, able to handle all possible scenarios without any human interaction, is still open to question. Several of the following changes in our image acquisition ability are currently under consideration. Some of these could enhance our forecasting ability. 5.1 Use of Wider Angle Fisheye Lenses The fisheye lenses currently used in our WSI sys- tems have a nominal 160° field of view and are no longer available. Possible replacements include 180° and 220° prime lenses, with better focusing charac- teristics than the current fisheye. Coupled with an There are limits to a single WSI being able to provide reliable predictions, particularly for longer lead times. If longer and longer lead times are needed, data from other sources, such as satellite images, be- comes more important. Eventually, developing pro- cedures for incorporating non-WSI information into the decision-making process will have to be addressed. Appendix A Transformation from Fisheye to Pseudo-Cartesian Coordinates The coordinate transformations used for this note are based on the following expressions. Angles are measured in degrees. Fisheye: R= [1.25(X, – 255)| +(Y, –240)? = 0(3.032–.002590) The only real difference between the two coordinate systems is the R vs O relationship. The procedure for constructing the pseudo-Carte- sian image from the fisheye image begins with iden- tifying a pixel (Xp, Yp) on the transformed image. A corresponding (0, 0) pair is then determined. From these the corresponding fisheye pixel (Xf, Yf) is computed, and the value for the fisheye pixel is entered into the transformed array at (Xp, Yp). Near the outer edge of the image, a single value from (Xf, Yf) will be used at several (Xp, Yp) locations. A parameter called the incremental area ratio quali- fies the ratio of the area mapped into the transformed array from the fisheye array. TU 0 = 585 (1-V1-.001128R,) © = tan"" [1.25(x,-255)/(Y, –240)] or inversely, (AR)RE 208/ 285 / sin 0 ) cose) (173.72 -8.50250) (173.72–17.050) tan 65 X, = 255+0.8 R, sino Y, = 240+ R, cosº where 0 is expressed in radians. Results from this formulation are given in Table 2 of the text. Pseudo-Cartesian: R2 = \[1.25(x,-255)* + (Y, –240)* = 235(tan® / tan 65°) R, tan 650 7 -1 0 = tan- P 235 0 = tan-1 X, = 255+0.8 R, sino Y, = 240+ R, coso List of Figures Fig. # Figure Description A sample time series of sky cover based on 1-minute WSI images from Columbia, MO. A comparison with hourly meteorological observations is included. A graphic depicting the relationships between zenith angle, cloud height and horizontal distance from the imager used in developing the pseudo-Cartesian coordinate. A time series of 4 WSI red/blue ratio images in the original fisheye view. The time interval between images is 10 minutes. The blue circle is drawn at the 65º zenith angle, the outer edge of the transformed imagery. Same as Fig. 3, except it displays images transformed into the pseudo-Cartesian coordinate. An example of cirrus band motion. An identifiable linear feature is outlined in blue. Previous positions of the feature are also drawn, with the corresponding minutes appearing on the left edge of the image. The same set of images used in Fig. 6, only the crossing point in the X pattern is tracked using green X's. An example of cirrus band spreading from the same day as Figs. 6 and 7. 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