NUMERICAL-ANALYSIS OF THE SEA-STATE BIAS FOR SATELLITE ALTIMETRY

Theoretical understanding of the dependence of sea state bias (SSB) on wind wave conditions has been achieved only for the case of a unidire ctional wind-driven sea [Jackson, 1979; Rodriguez et al:, 1992; Glazma n and Srokosz, 1991]. Recent analysis of Geosat and TOPEX altimeter da ta showed that additional factors, such as swell, ocean currents, and complex directional properties of realistic wave fields, may influence SSB behavior. Here we investigate effects of two-dimensional multimod al wave spectra using a numerical model of radar reflection from a ran dom, non-Gaussian surface. A recently proposed ocean wave spectrum is employed to describe sea surface statistics. The following findings ap pear to be of particular interest: (1) Sea swell has an appreciable ef fect in reducing the SSB coefficient compared with the pure wind sea c ase but has less effect on the actual SSB owing to the corresponding i ncrease in significant wave height. (2) Hidden multimodal structure (t he two-dimensional wavenumber spectrum contains separate peaks, for sw ell and wind seas, while the frequency spectrum looks unimodal) result s in an appreciable change of SSB. (3) For unimodal, purely wind-drive n seas, the influence of the angular spectral width is relatively unim portant; that is, a unidirectional sea provides a good qualitative mod el for SSB if the swell is absent. (4) The pseudo wave age is generall y much better for parametrizing the SSB coefficient than the actual wa ve age (which is ill-defined for a multimodal sea) or wind speed. (5) SSB can be as high as 5% of the significant wave height, which is sign ificantly greater than predicted by present empirical model functions tuned on global data sets. (6) Parameterization of SSB in terms of win d speed is likely to lead to errors due to the dependence on the (in p ractice, unknown) fetch.