The necessity for a new parameterization of an empirical model for wind/ocean scatterometry

This study reveals that the power‐law form of the Seasat A scatterometer system (SASS) empirical backscatter‐to‐wind model function (SASS 1), when combined with the sum‐of‐squares (SOS) wind retrieval algorithm, does not uniformly meet the instrument performance specification requirements. Analysis indicates that the horizontally polarized ( H pol ) and vertically polarized ( V pol ) components of the benchmark SASS 1/SOS wind retrieval system relating signal strength (backscatter) to wind speed yield self‐consistent results only for a small mid‐range of speeds at larger incidence angles and for a somewhat larger range of speeds at smaller incidence angles. An approach is presented that differs from previous calibration studies: here the internal V pol versus H pol consistency of the model is examined by the use of a set of pairwise collocated SASS‐produced winds, where one member of a wind pair ( U VV ) derives from only V pol backscatter measurements and the other ( U HH ) from only H pol measurements. This data set was created by extracting from the Seasat mission Geophysical Data Record (GDR) data base all pairs of SASS winds of the form ( U VV , U HH ) such that U VV and U HH are observations separated by no more than 5 min and 50 km. The set contains 377,289 such pairs. Comparisons of SASS 1/SOS derived wind data with wind data taken off the coast of Scotland during JASIN and with wind data from NDBO buoys off the U.S. Atlantic, Pacific, and Gulf coasts further underscores the shortcomings of the SASS 1/SOS wind retrieval system. These in situ wind comparisons to SASS indicate geographical differences in the retrieved scatterometer winds that are potentially attributable to environmental differences such as sea surface temperature. Taken together, the in situ comparisons to SASS, and the SASS H pol to V pol wind intercomparison indicate that the SASS 1/SOS wind retrieval system appears deficient in retrieving some winds, particularly from backscatter measurements made at higher incidence angles. We also find that H pol backscatter measurements show greater sensitivity than V pol backscatter measurements to wind speed variations above 10 m s −1 , while the reverse holds for wind speeds less than 10 m s −1 . It should be emphasized that it is the geophysical algorithms that lead to the errors cited here, not the instrumentation or the principles of scatterometry. In displaying these sources of error we have been obliged to outline the algorithmic methodology in a manner of presentation not previously attempted either in the published literature or in the voluminous code documentations. We hope that this material will be of value in itself to users of scatterometer data who seek to enhance their understanding of how such data are derived.