Recent Development of Drag Coefficient, Foam, and Surface Roughness for High Wind EM Emission and Scattering Computation

Ocean surface roughness and whitecaps are driven by the ocean surface wind stress, thus their values calculated from the wind speed input depend on the applied drag coefficient formula. Roughness and whitecaps are two critical elements of the ocean surface response in microwave remote sensing. Recent analysis of the microwave radiometer measurements in tropical cyclones have yielded the result that when expressed as a wind speed power function, the exponent in high winds (greater than about 35 m/s) is about -1 for the drag coefficient, 0.5 for the wind friction velocity, and 1.25 for the whitecap coverage. The updated information on the drag coefficient, whitecap coverage, and surface roughness is incorporated in the microwave emission and scattering models. The calculated brightness temperature and the co- and cross-polarized scattering radar cross sections in tropical cyclone wind conditions for a range of microwave frequencies and incidence angles are reported.