Abstract-We present a numerical investigation of horizontally polarized microwave scattering from 1-D sea surfaces at extreme grazing angles. Rigorous electromagnetic calculations are performed with a specific integral formalism dedicated to grazing angles. Sample sea surfaces are simulated using a classical Pierson-Moskowitz elevation spectrum together with weakly nonlinear hydrodynamic models, namely, the Creamer solution, the "choppy wave model," and a recent improved version thereof. For this, the electromagnetic integral formalism is extended to surfaces with irregular sampling. For the different nonlinear surface models and assuming no large-scale current, we evidence a dramatic increase, followed by a saturation of the mean Doppler shift in the last few grazing degrees, with a limiting value depending quasilinearly on the significant wave height. Our numerical investigations confirm that breaking events are not necessary to produce fast scatterers but tend to show that they are necessary to reproduce the elevated level of backscattered power. The results of this study also support the hypothesis that the blow-up of the mean Doppler shift at grazing angle is associated to an electromagnetic sharp edge effect on the large surface crests rather than geometrical shadowing of the troughs.Index Terms-Doppler effect, method of moments, sea surface.

Abstract-We present a boundary integral method for the solution of the rigorous problem of microwave scattering from finite conductivity sea surfaces under grazing illumination. Following the locally perturbated plane approach, the roughness is flattened at the edges of a finite patch, allowing us to use a plane wave as incident field. Both theoretical formulation and numerical implementation are addressed. We present simulations of full-polarization radar cross-sectional diagrams for 2-D ocean-like surfaces in both bistatic and monostatic configurations. The conductivity of the sea water is taken into account with a curved surface-impedance approximation, and results are compared with a perfectly conducting surface model. Simulations are finally confronted to approximate theories and empirical sea backscatter models for low grazing angles at L-band, vertical polarization, and a wind speed of 3 m/s.