Recent years have seen the development of robust and efficient numerical techniques for exact calculations of rough surface scattering. We discuss how such methods, typically formulated for time-independent surfaces, can be extended to calculate scattering from time-evolving ocean-like surfaces. Estimates are provided for the choice of parameters in such time-varying simulations. The method of ordered multiple interactions (MOMI) is used to calculate time-varying scattering from surfaces generated according to linear and nonlinear (Creamer) models for incidence angles ranging from normal to low grazing. We discuss the runtime considerations and demonstrate that combining the MOMI with a fast multipole method (FMM)-type acceleration technique makes large-scale time-varying Monte Carlo simulations possible. The average Doppler spectra of backscattered signals obtained from such simulations are compared for different incident angles, polarizations, and surface models. In particular, the simulations show a broadening of the Doppler spectra for nonlinear surfaces, especially at low grazing angles (LGA) and a separation of the vertical and horizontal polarization spectra at LGA for nonlinear surfaces. This spectral separation at LGA is not observed when the linear surfaces are used.Index Terms-Doppler spectrum, electromagnetic scattering by rough surfaces, low grazing angles, magnetic field integral equation, numerical methods, Pierson-Moskowitz spectrum.

[1] Depolarized (de-pol) radar backscatter is now produced by many spaceborne satellites. Analysis of RADARSAT-2 (R2) quad-polarization (quad-pol) data with collocated in situ ocean wind measurements reveals that the de-pol radar backscatter does not saturate in high winds. This is a significant development for radar wind sensing, because wind retrieval with copolarized (co-pol) backscatter suffers from problems of incidence-and-azimuth-angle-dependent signal saturation and dampening in high winds. We present a study comparing satellite quad-pol measurements with the composite surface Bragg (CB) theory of radar backscattering from the ocean surface. The co-pol data are in good agreement with the CB theory. De-pol data are more sensitive to wind speed compared to theoretical prediction, thus retrieval of high winds is more accurate using the de-pol return. The cubic wind speed dependence of de-pol returns in high winds reflects the significant breaking wave contributions. The relationship can be used to obtain wave-breaking properties from space.

Abstract-ResultsInstabilities in the West model due to formation of steep wave features limit the study to L-band backscattering for wind speeds less than 2 m/s, so that the surfaces considered are only slightly rough on an electromagnetic scale. The small slope approximation for electromagnetic scattering is shown to provide reasonable predictions in this limit. Statistics of the resulting surface profiles and backscattered fields are compared for the three models and are found to be similar in most respects. Backscattered field Doppler spectra, however, show differences, with the West model apparently capturing more nonlinear interactions in the surface evolution.Index Terms-Doppler spectrum, rough surface scattering, sea scattering.

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