Cross-Polarization GMF for High Wind Speed and Surface Stress Retrieval

Abstract

This paper describes the construction of geophysical model function (GMF) for wind speed and surface stress retrieval at high winds from cross-polarized radar backscatter at the water surface. The starting point is the laboratory experiment designed for the study of X-band backscattering from water surface. In particular, it was shown that cross-polarized normalized radar cross section (NRCS) keeps sensitivity to wind friction velocity at high winds alternatively to co-polarized radar return. Basing on the analysis of the Doppler spectra and simultaneous measurements of parameters of surface waves, we suggest a hypothesis that at high winds the cross-polarized backscattered signal is formed mainly due to scattering from wave breakers. The hypothesis is supported by the experimentally detected proportionality between the power of the scattered signal at cross polarization and the area swept by wave breakers. These results obtained in the laboratory environment are applied for constructing the similar dependencies for field conditions. Using the phenomenological statistical physics approach, a parameterization of active whitecap coverage fraction dependence on wind friction velocity is suggested. With the use of this parameterization, the empirical function expressing the cross-polarized NRCS by u* is derived. Using the surface drag parameterization applicable at strong winds, this dependence is verified on the base of available data sets containing collocated satellite measurements of cross-polarized C-band NRCS and ground measurements of wind speed. GMFs for u* and U10 retrieval are suggested. Analysis shows that taking into account the angular dependence of cross-polarized radar backscattered power improves the accuracy of wind speed and wind friction velocity retrieval.