Microwave Scattering from Short Gravity Waves
The fundamental mechanisms of microwave backscattering from short gravity waves have been investigated in the laboratory using a CW coherent dualpolarized focused radar and scanning laser slope gauge which provides an almost instantaneous profile of the water surface while scattering is taking place. The surface is also monitored independently for specular reflection by an optical sensor. It was found that microwave backscattering occurs in discrete bursts which are highly correlated with “gentle” breaking of the waves. These backscattering bursts are either completely nonspecular or are partly nonspecular and partly specular. The specular contribution is found to be more important than generally expected, even at moderate to high incidence angle. Its source seems to be the specular facets in the turbulent wake and the capillary waves generated during breaking. Completely nonspecular backscattering bursts have been analyzed by using the method of moments to compute numerically the backscattering complex amplitudes from the measured profiles and then comparing the computed results with the measured results. Using numerical modeling, it can be shown that for a wave in the process of breaking, its small-radius crest is the predominant scattering source in a manner akin to wedge diffraction as described by the geometric theory of diffraction (GTD). The parasitic capillary waves generated during wave breaking also scatter, but their contribution is in general smaller than that of the crest. The relationship and differences between GTD and small perturbation theory (SPT) in the description of wedge diffraction are established. One important implication is that the breakdown of the composite model in the description of scattering from short gravity waves may be traced to the nonlinearity of water waves. Preliminary implications of the result for microwave backscattering from the ocean surface are examined.
KeywordsSpecular Reflection Capillary Wave Turbulent Wake Polarization Ratio Large Incidence Angle
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