Low-grazing-angle (LGA) sea-spike backscattering from plunging breaker crests

The low-grazing-angle (LGA) microwave backscattering from a series of wave profiles representing the time evolution of a plunging breaker water wave is numerically calculated and compared with modeled predictions. The crest regions of the waves are isolated to remove large-scale multiple back-reflection paths that give interference. The horizontally polarized backscatter (HH) significantly exceeds that at vertical polarization (VV) during the breaking, despite the lack of multipath. Existing scattering models with some heuristic corrections are applied to the profiles to identify the dominant scattering mechanisms. The large HH-to-VV ratio is predicted from single scattering using an extended geometrical optics (EGO) approach. The initial rise in the backscatter as the wave steepens is predicted as diffraction from inflection points in the surface curvature at the crest using a modification of the geometrical theory of diffraction. The calculations show that the LGA backscattering from breaking wave crests is very complicated even in the absence of multipath and simple optically based models that treat the crest cross sections as equal at the two polarizations will be inaccurate.