Scattering From Nonlinear Gravity Waves: The "Choppy Wave" Model

To progress in the understanding of the impact of non-linear wave profiles in scattering from sea surfaces, a non-linear model for infinite depth gravity waves is considered. This model, termed the “Choppy Wave Model” (CWM), is based on horizontal deformation of a linear, reference random surface. It is numerically efficient and enjoys explicit second-order statistics for height and slope, which makes it well adapted to a large family of scattering models. We incorporate the CWM into a Kirchhoff or Small-Slope Approximation and derive statistical expressions for the corresponding incoherent cross section. We insist on the importance of “undressing” the wavenumber spectrum to generate a non-linear surface with a prescribed spectrum. Interestingly, the inclusion of non-linearities is found to be practically compensated by the spectral undressing process, an effect which might be specific to the CWM and needs to be investigated in the framework of fully non-linear models. Accordingly, the difference between the respective NRCS is rather small. The most noticeable changes are faster azimuthal variations and a slight increase of the radar returns at nadir. A statistical analysis of sea clutter in the framework of a TwoScale Model is also performed at large but nongrazing incidence. It shows a pronounced polarization dependence of the distribution of large backscattered amplitudes, the tail being much larger in horizontal polarization and for small resolution cell. Surface non-linearities are shown to increase the tail of the amplitude distribution, as expected. Less obviously, their relative impact is found lesser in horizontal polarization. This raises the question of the actual contribution of non-linearities in radar sea spikes at nongrazing angles.