Stereoscopic observations from meteorological satellites

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STEREOSCOPIC OBSERVATIONS FROM METEOROLOGICAL SATELLITES A. F. Hasler*, R. Mack*,**, and A. Negri* 5Goddard Laboratory for Atmospheric Sciences, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. * *G E. /MA TSCO, Beltsville, MD 20705, U.S.A. ABSTRACT The capability of making stereoscopic observations of clouds from meteorological satellites is a new basic analysis tool with a broad spectrum of applications. Stereoscopic observa— tions from satellites were first made using the early vidicon tube weather satellites (e.g., Ondrejka and Conover [1]). However, the only high quality meteorological stereoscopy from low orbit has been done from Apollo and Skylab, (e.g., Shenk etal. [2] and Black [31,[4]). Stereoscopy from geosynchronous satellites was proposed by Shenk [51 and Bristor and Pichel [6] in 1974 which allowed Minzner et al. [71to demonstrate the first quantitative cloud height analysis. In 1978 Bryson [8] and desJardins [91independently developed digital processing techniques to remap stereo images which made possible precision height measurement and spectacular display of stereogranis (Hasler et al. LiO], and Hasler [11]). In 1980 the Japanese Geosynchronous Satellite (CMS) and the U.S. GOES—West satellite were synchronized to obtain stereo over the central Pacific as described by Fujita and Dodge [12] and in this paper. Recently the authors have remapped images from a Low Earth Orbiter (LEO) to the coordinate system of a Geosynchronous Earth Orbiter (GEO) and obtained stereoscopic cloud height measurements which promise to have quality comparable to previous all CEO stereo. It has also been determined that the north—south imaging scan rate of some GEOs can be slowed or reversed. Therefore the feasibility of obtaining stereoscopic observations world wide from combinations of operational CEO and LEO satellites has been demonstrated. Stereoscopy from satellites has many advantages over infrared techniques for the observation of cloud structure because it depends only on basic geometric relationships. Digital remap— ping of GEO and LEO satellite images is imperative for precision stereo height measurement and high quality displays because of the curvature of the earth and the large angular separation of the two satellites. A general solution for accurate height computation depends on precise navigation of the two satellites. Validation of the geosynchronous satellite stereo using high altitude mountain lakes and vertically pointing aircraft lidar leads to a height accuracy estimate of + 500 m for typical clouds which have been studied. Applications of the satellite stereo include: 1) cloud top and base height measurements, 2) cloud—wind height assignment, 3) vertical motion estimates for convective clouds (Mack etal. [13], [14]), 4) temperature vs. height measurements when stereo is used together with infrared observations and 5) cloud elnissivity measurements when stereo, infrared and temperature sounding are used together (see Szejwach etal. [15]). When true satellite stereo image pairs are not available, synthetic stereo may be generated. The combination of multispectral satellite data using computer produced stereo image pairs is a dramatic example of synthetic stereoscopic display. The classic case uses the combination of infrared and visible data as first demonstrated by Picheletal. [16]. ilasleretat. [17), Mosher and Young [18] and Lorenz [191, have expanded this concept to display many channels of data from various radiometers as well as real and simulated data fields. A future system of stereoscopic satellites would be comprised of both low orbiters (as suggested by Lorenz and Schmidt [20], [19]) and a global system of geosynchronous satellites. The low earth orbiters would provide stereo coverage day and night and include the poles. An optimum global system of stereoscopic geosynchronous satellites would require international standarization of scan rate and direction, and scan times (synchronization) and resolution of at least 1 km in all imaging channels. A stereoscopic satellite system as suggested here would make an extremely important contribution to the understanding and prediction of the atmosphere. INTRODUCTION Satellite images of the earth from different viewing angles can be combined to make stero— scopic pairs. If the heights of moving clouds are desired, the images must also be taken at the same time. This means that more than one satellite must be used. The best horizontal resolution of present weather satellite images is about 1 km; therefore the two satellites should be separated by 30°or more to obtain good stereo height resolution. This large angular separation requires remapping of images to produce effective 3—D displays and accurate height measurements.

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The simplest way to observe clouds stereoSCOpically is with two identical scan—synchronized Geosynchronous Earth Orbiters (GEOs 1/). In this case, height is a simple function of parallax over a large part of the common field of view of the two satellites. Hasler [11], flack etal. [131 [14], Negri [21], Adler etal. [22], Fujita [23] [12], and others have shown that there are many important meteorological applications for the stereo data. Fig. 1 shows the GOES—East 2/ visible image of a tornadir thunderstorm at 2345 GIlT on April 10, 1979. The

Fig. 1 A GOES—East visible image of a Texas—Oklahoma tornadic thunderstorm at 2345 GIlT on April 10, 1979. Cloud height contours at 500 m intervals derived from a GOES—Cast/Lest stereo pair are shown. 500 m interval height contours in Fig. 1 are derived from images from the two U.S. operational geosynchronous weather satellites (GOES—East and GOES—West). The highest cloud tower (15.6 km) Is the storm top associated with a tornado which killed 45 people and injured 850 people in Wichita Falls 5 mm. later. Another example of a stereo contour analysis of a tornadic thunderstorm is shown in Fig. 2. In this case, the tornado, observed at Muihall, OK at 0050

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GEOS is defined here as the plural of Geosynchronous Earth Orbiter (GEO). GOES is the U.S. Ceostationary Operational Environmental Satellite. The words geosynchronous and geostationary are often used interchangeably, but geosynchronous more accurately describes the orbit of a real sat~lljte.

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GIlT May 3, 1979 is within 2 km of the easterr,—isost tower whfth has .1 ton of 16.8 km. flack St al. [131,[14] describe how time sequences of stereo an.-dvses like th~s~ can be used to measure cloud top ascent rates and estimate the intensity of convectlon. The only place where this kind of stereo can be done at present is during daylight hours in the common field of view of the two U.S. GOES satellites, which is the largest croashatched area in Fig. 3.

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NON—IDENTICAL CEO STEREO U.S.—Japan Joint Stereo Project Even though the Japanese GMS and the U.S. Goes satellites have different resolutions and scan rates, it is possible to obtain nearly scan—synchronized stereo observations over a fairly large area. The Japanese Meteorological Agency started the GMS image such that GHS and GOES scanned 7.5°Nlatitude at the sane time for several test cases on different days. The cross—hatched area in the central Pacific in Fig. 3 was scanned by both satellites within 30 s. Figs. 4a and 4b show a remapped GMS image and a GOES image of a large cloud cluster in the Intertropical Convergence Zone (ITCA). This stereo pair may be viewed in 3—D by using

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Fig. 4a Japanese GMS image remapped to a GOES—West image (see Fig. 4b). ITCZ cloud cluster at 0115 GMT November 26, 1980

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a mirror stereoscope. A stereo cloud top height contour analysis of part of the ares shown in Figs. 4a and 4b is presented in Fig. 5. Deep convective towers reach up to nearly 14 km in the middle of the cloud cluster. Further information on and results of the experiment are given by Fujits and Dodge [12].

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Fig. 5 Cloud top height contours from a scan synchronized GOES—WEST and GMS stereo pair (see Figs. 4a and 4b).

North—South Scan Rate Matchi~ Although the north—south scan rates of GHS and GOES are different in the normal mode of operation, it is possible to slow down the scan of the faster satellite. This can be done by scanning some lines more than once before stepping the mirror such that the scan0W times in Fig. are matched over the entire common field of view (see hatched area centered at 180 3). South to North Scanning Meteosat, the European GEO, scans south to north in its operational mode. Since, it is a simple operation to reverse the scan direction of GOES and presumably the other GEOs, stereo image pairs can be obtained in the common fields of view of all the GEOs in the global system. This is represented by the total hatched area in Fig. 3.

GEO/LEO STEREO

As reported above, there has been considerable experience with stereo from GEOs. Several proposals have also been made to build dedicated stereo Low Earth Orbiters (LEOs). However, to the authors’ knowledge it has not been previously suggested to combine CEO and LEO Images for this purpose. Fortuitous Stereo On each orbit of a LEO underflying a CEO there will be a region where nearly scan—sychroaized data are taken. Fig. 6a shows a TIROS—N (LEO) image of Hurricane Allen on August 8, 1980 remapped to a GOES—East image (Fig. 6b) such that sea—level features are superimposed. The resulting Stereo pair (Figs. 6a and 6b) can also be viewed with a mirror stereoscope by tilting the figures 450 as indicated by the arrows labled TOP”. This severe Gulf of Mexico hurricane had maximum winds of over 165 kts at the time of this observation. By coincidence the eye of the storm was scanned by both satellites within a few seconds of 1336 GMT August 8, 1980. Fig. 7 shows an artist’s conception of how the satellites observed the storm. The resulting stereo is more complex than that using two GEOs, because the parallax varies greatly over the 500 x 500 km area shown in Figs. 6a and 6b. The diagram in Fig. 8 shows the positions of the two satellites and how the parallax varies over an area which includes that shown by Fig. 6 (solid box) and an area 4 times larger (dashed box). The arrows indicate the direction of offset or parallax for features which are above sea level. In the upper left hand corner of the dashed box the parallax vector is oriented SW—NE with a magnitude of 34 picture elements (pixels) and —14 lines for the highest clouds. Moving across the diagram

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towards the lower right hand corner of the dashed box the parallax vector turns to due south with a magnitude of 0 pixels and 14 lines for the highest clouds. In spite of these complications accurate heights can be obtained if good navigation solutions are available for both images. For the exampl~ shown in Figs. 6a and 6b height accuracies of 500—1000 m are expected. The TIROS—Il satellite scans at a rate of 360 lines per minute while GOES scans 800 lines/ mm, both with resolutions of about 1 km. That means that with perfect synchronization over

the eye of the storm, the synchronization error at the top and bottom of Figs. 6a and 6b will not exceed 20 s. Cloud top height contour analyses have not been made yet since the software for accurate computations is still under development. Scan time and scan rate matching By adjusting

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world—wide between 60° N and 600 S latitude. There will, however, be singular points near the equator where the parallax will be very small. Even Meteosat which scans south to north can be used with LEOs with daylight ascending nodes (north—bound scanning). The north—south scan rate of the CEO can also be matched to the LEO by repeating scans as described previously. This will permit CEO/LEO synchronization within a few seconds over large areas for GEOs with programable scanning. Applications GEO/LEO stereo makes It possible for the first time to include the cloud top height fields in studies of many types of meteorological phenomena equatorward of 60°latitude. Furthermore, all (J.S. LEOs have infrared image resolution of better than 1 km. Thus 3 the researcher has the opportunity to study both the height and temperature structure of the clouds at high resolution. Possible case studies. Rodgers et al., [24] (also see Mack et al. 114]) have developed a technique for estimating tropical cyclone maximum surface wind speeds using stereo cloud heights and infrared cloud top temperatures. The CEO/LEO stereo will allow this technique to be applied to the typhoons of the Western Pacific as well as all Atlantic hurricanes. The structure of these storms may also be studied. Cloud top stereo height measurements will give valuable additional Information for studies of the Asian Monsoon circulation, extratropical cyclones wotld—wide, and mountain cloud systems (eg. Alps, Mimalayas, Andes). Another important application will be for climatology. All climate radiation balance models are heavily dependent on cloud altitude and emissivity estimates. The CEO/LEO stereo will give the most accurate practical measurements of these quantities. ALL LEO STEREO becomes increasingly worse poleward of 600 latitude. Therefore an all LEO system is desirable for high latitude stereoscopy and to fill any gaps in the GEO stereo system. It is the only operational system which presently has sufficient infrared resolution to do high quality nighttime stereo. CEO image quality

Single satellite Lorenz and Schmidt [201, [19] have proposed to build a split—beam stereo scanner for a LEO. Their instrument would look forward and backward along the orbital track making observations of the same point with about 2 mm time separation. A lidar would measure heights of selected clouds so that the dual images could be used to estimate winds from cloud motions. Dual satellites Fortuitous. There are 4 LEOs of two types (TIROS—N and D}ISP) routinely operated by the U.S. In the polar regions where the orbits overlap, the chance of at least a few occurrences of simultaneous coverage both in time and space by two LEOs is high. The only serious obstacle to demonstrating high resolution infrared stereo using two LEOs is the calculation of the proper coincidences and obtaining the necessary digital data tapes. Dedicated system. For a dedicated LEO stereo system it would be necessary to have two satellites following each other in the same orbit. To keep the proper separation would require an accurate station keeping capability like that used on Landsat. The two satellite system is the only one which would have the capability to measure height and motion of all clouds in the field of view. SYNTHETIC STEREO Background 4a and 4b, and 6a and 6b, it is apparent When one views stereo image pairs like those in Figs.

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that a tremendous amount of information is conveyed by a three dimensional (3—U) display of satellite—observed cloud systems. Unfortunately, only a small fraction of the satellite data which is presently archived is available in stereo. A method for combining visible and infrared satellite images to make synthetic stereo pairs was devised first by Pichel etal., (16) and has recently been developed further by several groups. This opens up the vast library of visible/infrared image pairs from both LCOs and GEOs that can be viewed stereoscopically. Figs. 9a and 9b of Hurricane Allen, which are also viewed in 3—D using a mirror stereoscope, provide an example of the synthetic stereo. Description of the techni~~. A synthetic stereoscopic image pair consists of 1) an image and 2) a computer—generated image. Generally, the second image is generated by shifting each pixel of the first image to the right according to its temperature from the corresponding infrared “coding” image. The lowest temperature receives the maximum shift and the highest temperature the minimum. Further information on the technique is given by Hasler et al., (17), Mosher and Young [18] and Lorenz [19]. The “coding’ image may be any scalar field which contains information related to the primary image. The primary inTage may also be used as the “coding” image. Advantages. Using this technique, multispectral data may be combined in a way which allows two or more types of data to be assimilated at the same time. The insights obtained by this merging are often quite unexpected. All data from weather satellites launched since 1974 and data from any multispectral imaging instrument may be displayed in this manner.

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Fig. 6b GOES—East image which together with Fig. 6a makes a stereo pair. Use a mirror stereoscope to view Figs. 6a and 6b in 3—D. Note: the figures should be tilted about 45’ as indicated by the arrows labeled “TOP.” Description of Hurricane Allen stereograph In the stereograph of Hurricane Allen, in Figs. 9a and 9b, the Central Dense Overcast (CDO) surrounding the eye of the storm appears like a huge doughnut. The tropopause penetrating convective domes, like the one south—west of the eye, are the highest features in the image. To the west of the storm center, tall convective towers are surrounded by regions of sup— pressed—low convective clouds. If one compares the synthetic stereograph in Fig. 9 with the CEO/LEO true stereo in Fig. 6 one sees that the thin cirrus clouds north of the CDO appear much lower in the synthetic stereo. This is due to the high transmissivity (low emissivity) of the cirrus and the radiation from the warmer clouds and ocean coning through from below. Visible/infrared synthetic stereo will give heights which are too low for clouds with low emissivity and for clouds smaller than the field of view of the sensor. CONCLUSION AND RECO~~NDATIONS ~

of weather satellite stereo

SLercoscopic cloud top height measurements, accurate to + 500 m, have been made using opera ti()flal weather satellites. The stereo has been demonstrated using various combinations of geosynchronous (CEO) and low earth orbiting (LEO) satellites. It can be done over most of the earth (600 5 to 60°N) during the day. At higher latitudes and at night, stereoscopy can be done using two LEOs if the proper coincidences occur. Tho utility of the stereo cloud

A.F. Ilasler, R. Mack and A. Negri

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Fig. B Diagram of the TIROS—N and GOES—East scan pattern over Hurricane Allen indicating how the parallax varies (see arrows). top height measurements has been demonstrated by many scientists studying convective storm cloud systems. Other important research and operational applications have yet to be fully exploited. Some examples are: cloud wind height assignment, thunderstorm and hurricane intensity estimation, temperature sounding, and cloud climatology algorithm calibration. Recommendations Short—term. There lite data archived satellites at high start times should worldwide.

are large amounts of planned and fortuitous stereoscopic weather satelon digital tape, which should be exploited. Stereoscopy from two LEO latitudes and at night needs to be demonstrated. Adjustment of CEO image be routinely scheduled to make stereo measurements of important phenomena

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Medium term. Minor modification of ground systems should be made to allow routine scan rate matching of all CEO and LEO combinations.

Long—term. All future planning of operational weather satellites should consider the goal of a global system of high resolution scan—synchronized visible and infrared stereo satellites. ACKNOWLEDGEMENTS The authors would like to recognize the work of Tim Sletten for the analysis in Fig. 2 and Roy Teagle for the cloud top height contours in Fig. 5. Joan Wentz typed the manuscript in time to meet a very tight deadline. References 1. 2. 3.

H. J. Ondrejka, and J.H. Conover, Mon. Wea. Rev. 94, 611—614 (1966) W. E. Shenk, R. J. Holub and H. A. Neff, Bull. Am. Meteor. Soc. 56, 4—15 (1975) P. C. Black, Skylab Explores the Earth NASA, Washington, D. C. 417—461 (1977)

4.

P. C. Black, this volume

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

W. E. Shenk, Private communication to R. Wirth (1974) C. L. Bristor, and W. Pichel, Bull. Am. Meteor. Soc. 55, 1353—1355 (1974) H. A. Minzner, W. E. Shenk, J. Steranka and R. U. Teagle, EOS, 57, 593 (1976) W. Bryson, Ph. D. thesis, University of Wisconsin 91 pp (1978) M. desJardins, NASA/GSFC PIP Presentation 10 July 1978 (unpublished manuscript) (1978) A. F. Hasler, M. desJardins, and W. E. Shenk, Fourth National Aeronautics and Space Administration Weather and Climate Review NASA/GSFC Greenbelt, MD 67—72 (1979) A. F. Ilasler, Bull. Am. Meteor. Soc., 62, 194212 (1981) T. T. Fujita and J. C. Dodge, this volume R. A. Mack, A. F. Hasler and R. F. Adler, Preprints, ANS 12th Conf. on Severe Local Storms, San Antonio, TX (1982) R. A. Mack, A. F. Hasler and E. B. Rodgers, this volume G. Szejwach, T. Sletten and A. F. Hasler, this volume W. Pichel, C. L. Brister, and R. Brower, Bull. Am. Meteor. Soc., 54, 688—691 (1973) A. F. Hasler, M. desJardins and A. Negri, Bull Am. Meteor. Soc., 62, 970—973 (1981) F. H. Mosher and J. T. Young, this volume D. Loreox, this volume D. Lorenz, and E. Schmidt, Bildmessung u. Luftbildwesen, 47, 1—14 (1979) A. J. Negri, Bull. Am. Meteor. Soc., in press (1982) H. F. Adler, M. J. Marcus, 0. U. Fenn and W. E. Shenk, Preprints, AIlS 12th Conf. on Severe Local Storms, San Antonio, TX (1982) T. T. Fujita, Preprints, AIlS 12th Conf. on Severe Local Strons, San Antonio, TX (1982) E. H. Rodgers, H. A. Mack, and A. F. Hasler, NASA Tech. Memo. Greenbelt, MD In press (1982)