Low-grazing scattering from breaking water waves using an impedance boundary MM/GTD approach

The radar backscattering from water waves of various degrees of breaking Is numerically examined. A hybrid moment method geometrical theory of diffraction (MM/GTD) technique previously used for small-grazing scattering from perfectly conducting surfaces is reformulated using impedance boundary conditions, allowing the treatment of large (but finite) conductivity scattering media such as sea water. This hybrid MM/GTD approach avoids the artificial edge effects that limit the standard moment method when applied to rough surfaces, allowing the calculation of the scattering at arbitrarily small grazing angles. Sample surfaces are obtained through the edge-detection of video stills of breaking waves generated in a wave tank. The numerical calculations show that the strength of the backscatter is closely associated with the size of the plume on the breaking wave. Strong interference appears in the both horizontal (HH) and vertical (VV) backscatter when the surfaces are treated as perfectly conducting. The VV interference is dramatically reduced when a sea water surface is used, but the HH interference is unaffected. The interference leads to HH/VV ratios of up to 10 dB. The behavior of the scattering is consistent with the multipath theory of sea-spike scattering.