NUMERICAL-SIMULATION OF LOW-GRAZING-ANGLE OCEAN MICROWAVE BACKSCATTERAND ITS RELATION TO SEA SPIKES

This paper presents the results of numerically simulating microwave ba ckscatter from a deep-water breaking wave profile. Enhanced microwave backscatter from the crests of breaking waves has been hypothesized as the source of bright short-lived microwave radar echoes that are obse rved at low-grazing angles (LGA's), The characteristics of these ''sea spikes'' are distinctly different from the Bragg-scatter echoes that dominate measurements made at moderate grazing angles, Of particular i nterest is the high contrast that sea spikes present against ocean bac kground backscatter when observed with horizontally polarized transmit /receive configurations [horizontal (HH) versus vertical (VV)]. This H H/VV contrast disparity has been attributed to polarization-selective cancellation of the direct reflection from the wave crest by the surfa ce reflection, This hypothesis is reinforced first by showing evidence that VV polarization is suppressed in the intensity range that would normally be populated by the brightest scatterers. Histograms of unave raged Doppler-centroid measurements show further that the depleted VV backscatter population is responding to scatterers that are moving muc h more slowly than the HH scatterers. The Doppler-centroid histograms provide a sharper delination between the two scattering populations th an do the unconditionally averaged Doppler spectra that are more commo nly reported, Finally, our numerical simulations show evidence of an i nterference mechanism that selectively suppresses VV backscatter. In o ur simulations, the polarization selectivity comes from the phase depe ndence of the backscatter from the wave crest, A Brewster phenomenon a t the surface reflection point is not necessary.