Structure of the surface wind field from the Seasat SAR

An intensive analysis of the vector wind field of one Seasat data set: pass 1339 is described. Wind speed and direction signatures are found in Seasat SAR (synthetic aperture radar) images, and the resulting estimates are compared with the Seasat-A scatterometer system (SASS) and simultaneous NOAA P-3 aircraft measurements. A power law is presented to relate SAR-measured backscatter to SASS-estimated wind speed, and the SAR estimates are shown to agree with the SASS estimates to within a standard error of 0.7 m/s over a range of wind speeds from 3 m/s to 13 m/s. The surface expressions of atmospheric roll vortices are apparent in several of the SAR images and may be responsible for the wind-direction signature in these cases. Wind field estimates averaged over regions of variable sizes are possible because of the high resolution of SAR imagery. A method for extracting low wave number directionality and its variability from SAR spectra is described, and SAR direction estimates obtained from spectra of 6.4-km-square images are shown to have a precision of approximately 10°. Although a comparison data set that could validate these higher resolution estimates is lacking, averages over 40-km-square regions are in good agreement with the other wind field information. The SAR wind direction estimates yield a more complete interpretation in a region with a turning wind field near a front that is ambiguous with only SASS observations. In this region the flow patterns of the high-resolution estimates appear consistent with our knowledge of the overall circulation, considering all the observations. However, in another region the small-scale variability is too large and random to represent real wind variability, although averages derived from these estimates are still reliable.

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