Wave shadowing and modulation of microwave backscatter from the ocean

Abstract

Shadowing and modulation of microwave backscatter by ocean waves are studied using coherent X‐band radars. Two types of shadowing are investigated: geometric shadowing (complete blockage of incident rays) and partial shadowing (polarization‐dependent diffraction combined with weak scatterers). We point out that the frequency of occurrence of zero signal‐to‐noise ratio samples cannot depend on the incident power level or the polarization if geometric shadowing occurs but can if partial shadowing exists. We then compare this behavior with observations, and show that the data do not support the hypothesis that geometric shadowing plays a significant role in low‐grazing‐angle microwave scattering from the ocean surface. Furthermore, our data indicate that partial shadowing only depends significantly on polarization for the steep waves found near shorelines. We also study the modulation of microwave backscatter by ocean waves using these data by looking at the phase differences between received power and scatterer velocity. These phase differences appear to be rather well explained by standard composite surface theory at VV polarization, having values that are positive looking up wave and negative looking down wave. For HH polarization, however, breaking effects come into play and overshadow composite surface effects of free waves. They cause the phase difference to be near zero for up wave looks and near 180° for down‐wave looks. A simple model that involves both breaking and freely propagating waves but does not include any shadowing effects is shown to account for observed phase differences at both polarizations to within about 10°.