Parameterization of microwave emission due to foam to improve the accuracy of satellite-based retrieval algorithms

WindSat, the first polarimetric microwave radiometer on orbit, has as its primary objective the demonstration of robust retrieval of the sea surface wind vector from measured brightness temperatures. The ocean surface wind vector is one of the key environmental data records for the NPOESS Conical Microwave Imager/Sounder (CMIS) instruments, first planned for launch in 2009. To date, aircraft and satellite measurements, as well as modeling results, indicate that brightness temperature variations with wind direction are small, on the order of 1-3 K peak-to-peak. Therefore, quantitative understanding of the dependence of the ocean surface emissivity on properties such as surface roughness and wave breaking is critical for wind vector retrieval. Despite the importance of this, some basic physical properties such as the azimuthal angle dependence of the microwave emission from foam have not been well characterized to date. Recent measurements from the R/P FLIP indicated that the increase in ocean surface emission due to breaking waves may depend on both the incidence and azimuthal angles. The need to quantity this dependence motivated systematic measurement of the emissivity of reproducible breaking waves at varying incidence and azimuthal angles. Results from these recent field measurements provide the first parameterization with wind speed of the change in brightness temperatures due to breaking waves.