Modeling rain effects on microwave backscatter from the ocean

During the Kwajalein Experiment (KWAJEX) in 1999 the effect of rain on Ku band normalized radar cross sections of the sea, /spl sigma//sub 0/, was measured for a wide range rainfall rates and incidence angles. The primary result was that rain significantly increases the cross section at moderate to high incidence angles, obscuring, if not eliminating backscatter from wind-generated waves. To verify our understanding of the effect of rain on sea surface roughness and to extend it to smaller incidence angles and other microwave frequencies we modeled microwave backscatter from a rain-disturbed ocean surface. To do this, we developed a realistic representation of the ocean surface perturbed by wind and rain which accounts for the damping of surface waves by rain-induced subsurface turbulence and for the enhancement of gravity-capillary and short gravity waves by rain-generated ring-waves. The spectral representation of the ocean surface is used as input to a scattering model. The multiscale scattering model used here separates surface waves into three distinct scales (short, intermediate, and long) and evaluates /spl sigma//sub 0/ for each scale. Since rain significantly alters short gravity waves (i.e. the intermediate scale), the explicit calculation in the model of backscatter from these waves makes it ideal for diagnosing the effects of rain. We will show that values of /spl sigma//sub 0/ computed by the model are in quantitative agreement with KWAJEX data. We will also show modeled rain effects on the cross section at low incidence angles for a variety of microwave frequencies.