Multipath EM scattering from breaking ocean waves at grazing incidence

Abstract

The electromagnetic scattering from a breaking water wave with near grazing illumination has been examined using both experimental measurement and numerical calculation. The scattering from a metal model of a breaking wave of 1.2 m wavelength was characterized at grazing angles ranging from 0° to 25° and at frequencies ranging from 6 GHz to 12 GHz. Strong multipath interference is observed in the vertical polarization (VV) backscatter and much weaker interference is seen in the horizontal polarization (HH) backscatter. Scattering from the wave profile both with and without the breaking plume shows that the plume is the dominant scattering feature. Numerical calculation of the scattering from modified surface profiles shows that the interference is due to multipath scattering from the plume and front face of the wave. HH interference is substantially smaller than that of VV because the scatter from the plume in the multipath direction (toward the front face) is greatly reduced owing to the conducting surface boundary conditions. The relative levels of the plume and expected distributed-surface scattering are consistent with the wave-tank measurement of horizontally polarized sea spikes from breaking waves of this scale.