Structure of the surface wind field from the Seasat SAR

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Abstract
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.