Satellite scatterometer studies of ocean surface stress and drag coefficients using a direct model

Abstract

This study evaluates the accuracy of estimates of sea surface friction velocity derived using a new model function that operates directly on the NASA scatterometer normalized radar cross‐section (NRCS) measurements. These NRCS data are first collocated with numerous National Data Busy Center buoys (within ±0.5° latitude and longitude) and are then processed using the new Weissman et al. [1994] Frontal Air‐Sea Interaction Experiment model to produce 25‐km by 25‐km spatial averages of the friction velocity for several contiguous areas within this region. One method of validation of these estimates involves comparisons with friction velocity estimates derived from data collected by the buoys using bulk methods. This was conducted at numerous locations; the Atlantic and Pacific coastlines and within the Gulf of Mexico. It is found that there is excellent agreement between these two different measurements. In addition, this approach enabled the derivation of estimates of the surface drag coefficient, with the use of the buoy winds, neutral stability winds at 10‐m elevation, U10N, and the scatterometer surface friction velocity u*. This permitted the study of the dependence of this drag coefficient CD on wind speed in these different regions on spatial scales, which had not previously been possible. Comparisons are made to previously published models of CD that only depend on wind speed, with generally good agreement at winds over 6 m−1 s, but with significant regional differences at winds less than 6 m−1 s.