To measure the radar backscatter from the ocean with enough accuracy to allow determination of the wind vector, the effect of attenuation in precipitating regions over the sea must be considered. The attenuation can be determined reasonably well from a knowledge of the brightness temperature observed by a microwave radiometer. For the SEASAT this radiometer was the SMMR. Two algorithms were developed to relate radiometric measurements and attenuation and hence to correct the measured values of scattering coefficient. The two algorithms were compared with the best available information on precipitation, but no direct information on attenuation was available. Hence, the basic comparisons reported here are between the wind vectors reported by surface observers and those determined by the SASS measurements with and without correction for atmospheric attenuation. In general, the attenuation correction appears to improve the results obtained. However, the 'correction' is not as good as it should be because of the differing size of the 18 GHz radiometer footprint and the SASS footprint, and because the scan patterns are different so that averages from several radiometer footprints may be involved in a single SASS correction. tion: the SEASAT SASS attenuation algorithm developed at The University of Kansas (used in normal processing of SASS data), and the SEASAT SMMR geophysical algorithm (developed by Remote Sensing Systems, Inc.). Both algorithms are briefly described in the paper.Evaluation of the degree of success in correcting the SASS data is presented here. Unfortunately, the evaluation is difficult because of the lack of independent measurements of the attenuation with which the algorithm outputs may be • The University of Kansas Remote Sensing Laboratory, Lawrence, Kansas 66045. compared. Consequently, indirect means must be used for comparing the results. These include comparison with estimates of precipitating regions based on interpretation of meteorological-satellite visible/IR images of cloud cover, comparison of wind estimate results with and without correction for attenuation (this method suffers from poor independent information about the winds themselves), and comparison with hurricane observations, where both precipitation and wind estimates by independent means are more plentiful than in other areas. The results of these comparisons indicate that the attenuation algorithms do indeed result in improvement of the wind estimates, and they do correlate with the crude independent estimates available. The two algorithms differ somewhat in their estimates in hurricane regions, but both appear to estimate the trends reasonably well where gradients are not too steep. The SMMR geophysical algorithm is more susceptible to problems from steep attenuation gradients, but both algorithms have problems under these conditions. Major problems in correctly estimating the SASS attenuation occur because the SASS and SMMR footprints on the surface are of different sizes and shapes, and they do not coincide because of diffe...
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