Wind field model‐based estimation of Seasat scatterometer winds

Abstract

A model‐based approach to estimating near‐surface wind fields over the ocean from Seasat scatterometer (SASS) measurements is presented. The approach is a direct assimilation technique in which wind field model parameters are estimated directly from the scatterometer measurements of the radar backscatter of the ocean's surface using maximum likelihood principles. The wind field estimate is then computed from the estimated model parameters. The wind field model used in this approach is based on geostrophic approximation and on simplistic assumptions about the wind field vorticity and divergence but includes ageostrophic winds. Nine days of SASS data were processed to obtain unique wind estimates. Comparisons in performance to the traditional two‐step (point‐wise wind retrieval followed by ambiguity removal) wind estimate method and the model‐based method are provided using both simulated radar backscatter measurements and actual SASS measurements. In the latter case the results are compared to wind fields determined using subjective ambiguity removal. While the traditional approach results in missing measurements and reduced effective swath width due to fore/aft beam cell coregistration problems, the model‐based approach uses all available measurements to increase the effective swath width and to reduce data gaps. The results reveal that the model‐based wind estimates have accuracy comparable to traditionally estimated winds with less “noise” in the directional estimates, particularly at low wind speeds. In addition, wind fields generated using the model‐based procedure can be used to detect and correct ambiguity removal errors in ambiguity‐removed point‐wise wind fields. A separate procedure, based only on the wind field model, can also be used as a data quality check to detect errors in point‐wise ambiguity removal.