VIERS-1 SCATTEROMETER MODEL

In this paper a description is given of a physically based theoretical ocean backscatter model (called the VIERS-1 model) for intermediate i ncidence angles, and a comparison of its performance against the CMOD4 empirical model is made. The VIERS-1 scatterometer algorithm is based on a two-scale composite surface model which includes both specular a nd Bragg scattering. Its short wave model is based on the energy balan ce equation and accounts for viscous damping, slicks, dissipation due to whitecapping, and nonlinear three-and four-wave interactions. A num ber of parameters in the model have been determined by means of labora tory data and analyzed European Centre for Medium-Range Weather Foreca sts (ECMWF) winds. Because: of the two-scale approach the wave number up to which Bragg scattering applies should be determined. This is don e by means of laboratory data at X band. In addition, laboratory data of the wave spectrum have been utilized to validate the VIERS-1 short wave spectrum. An inverse of the algorithm is developed to derive wind speed and direction from the observed (ERS-1) backscatter and by comp arison with ECMWF analyzed winds' three parameters for the short wave spectrum, namely, the Phillips parameter, the directional width of the spectrum, and the wave number boundary between gravity waves: and sho rt waves have been obtained. Comparisons between VIERS-1, C band model , version 4 (CMOD4), and ECMWF analyses are made. VIERS-1 performs bet ter in the high wind speed range, and this feature is of importance wh en scatterometer winds are assimilated into an atmospheric model. Howe ver, in terms of backscatter rather than wind speed, CMOD4 shows bette r results. It is suggested that this is caused by the too simple direc tional distribution of the VIERS-1 short wave spectrum.