A deterministic, coherent, and dual-polarized laboratory study of microwave backscattering from water waves, Part I: Short gravity waves without wind

Abstract

The fundamental mechanisms of microwave backscattering from mechanically generated short gravity waves with 25-cm wavelength have been investigated in the laboratory using a CW coherent dual-polarized focused radar operating at 9.23 GHz and a laser scanning slope gauge which provides an almost instantaneous profile of the water surface while scattering is taking place. The surface was also monitored independently for specular reflection using an optical sensor. It is 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 partially specular in nature. The specular contribution is found to be more important than generally expected, even at moderate to high incidence angles, and its source seems to be the specular facets in the turbulent wake and the capillary waves generated during breaking. Completely nonspecular backscattering bursts are 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. Their contribution is in general smaller than that of the crest and can be understood in terms of small-perturbation theory. The relationship between GTD and small-perturbation theory in the description of wedge diffraction is established. Implications of these results for microwave backscattering from the ocean surface are examined.