Investigation of shadowing in rough surface scattering at small grazing angles

Electromagnetic scattering from rough surfaces such as the ocean is affected by surface shadowing when the angle of incidence is large (small grazing angle). Traditional rough-surface scattering theories such as the small-perturbation method, Kirchhoff approximation, and two-scale model do not directly include shadowing effects. Attempts have been made to include shadowing in these models by incorporating a "shadowing function". The shadowing function is usually determined from a geometrical optics approximation that ignores diffraction into the shadowed area. Using a Fresnel description of the diffraction, Wetzel (1990) showed that significant energy can be diffracted into the shadowed region under conditions common on the sea surface, thereby potentially contributing to the backscatter. The validity of the shadowing approximation is investigated using a hybrid moment method/geometrical theory of diffraction technique. A small-scale random roughness is superimposed on a large-scale surface approximating an ocean wave. The backscatter from the resulting two-scale surface is compared with that predicted by the two-scale scattering model extended using shadowing. The backscatter is numerically calculated both from a surface that is small-scale rough across its entire profile and from a surface that has small-scale roughness only in areas that are directly illuminated (smooth in the shadowed regions).