Depolarization of microwave backscattering from a rough sea surface: Modeling with small-slope approximation

Scatterometric wind retrieval based on HHor VV-pol. radar backscatter experiences saturation at high winds, whereas recent measurements demonstrated that the cross-polarized signal, VHor HV-pol. at high winds shows a strong dependence on wind speed. Explanation of such a behavior of the cross-polarized microwave radar from a theoretical standpoint represents a challenge. A classical composite Bragg (CB) theory significantly underestimates the level of measured cross-polarized radar return for all ranges of winds. Our study based on the small-slope approximation of the second order (SSA2) improves an agreement between the theory and the experiment for intermediate range of winds. The SSA2, as contrasted with the CB, always accounts for Bragg scattering of the second order which involves intermediate waves propagating at zero grazing angles (Wood´s anomaly). Explanation of the cross-polarization scatterometric data at high winds would require going beyond the SSA2. One of the possible mechanisms could be breaking waves playing a role of retro-reflectors.