Cross-Polarization Radar Backscattering From the Ocean Surface and Its Dependence on Wind Velocity

Recent wind retrieval algorithms using crosspolarization (cross-pol) radar sea return (VH) assume that VH is independent on the azimuth angle and mainly varies with the wind speed. Incidence angle dependence is either absent or is only in wind speeds less than 21 m/s. However, azimuth and incidence angle variations are expected since theory and data comparisons show the dominance of surface effects in the scattering mechanisms; thus, both co-polarization and cross-pol cross sections reflect the directional distribution of the ocean surface roughness and wave breaking. Here, the VH dependence on wind velocity is analyzed with special focus on the variations with the incidence and azimuth angles. The results show that, for the typical incidence angle range of radar images used for hurricane wind retrieval (20°-50°), the magnitude of these angular variations is equivalent to a difference of about 10 m/s in the retrieved wind speed for low-to-strong winds (~21 m/s) as well. It is prudent to incorporate the incidence angle dependence and the azimuth angle dependence in the wind retrieval algorithm and in the signal simulation for the design of next-generation scatterometers. The dependence on the wind speed is also examined. It reconfirms that the VH sensitivity increases toward high winds, but signal saturation may occur.