The effect of steep sea-waves on polarization ratio at low grazing angles

A backscattering numerical model based on a two-scale representation of surface roughness is developed for one-dimensional (1D) sea surfaces at low grazing angles. The effect of the large-scale roughness component is accounted for by a numerical solution of the integral equation for surface field obtained in the forward-scattering approximation. The presence of the small-scale roughness responsible for backscattering is treated by the small-perturbation theory. The numerical simulations accomplished support the viewpoint that the significant difference between experimentally observed and numerically calculated values of polarization ratio for low grazing angles is most likely due to inadequate modeling of surface roughness. It is demonstrated that adding a few relatively minor steep-wave-like features to the surface with the standard Pierson-Moskowitz spectrum will change the average polarization ratio dramatically, bringing its theoretical values from about -20 dB to experimentally observed values of a few negative dB. Half of this increase is due to steepening of the front faces of the undulating waves. However, the other 10 dB of increase is due to diffraction effects, which enhance the scattering coefficient for the HH-polarization on the front faces of the steep waves.