Radar backscattering from breaking wind waves: field observation and modelling

Abstract

In this article, the results of a field study of the Ka-band (37.5 GHz) radar backscattering from breaking wind waves are presented. Radar and a video camera were simultaneously used to measure the radar cross section (RCS) of the whitecap zone and the characteristics of wave-breaking events. A comparison is made between absolute geometrical lengths, areas and orientations of the whitecaps and the RCS of the whitecaps at moderate (45°) and high (70°) incidence angles. A thin vegetable oil film covering the illuminated area was made to minimize background Bragg backscattering from the surface free of breaking-wave features. At high incidence angles, a linear-like dependence between breaking crest length and RCS and no significant azimuthal dependence are found. At moderate angles of incidence, a prominent azimuthal variation of the whitecap's RCS is shown, while a linear-like relationship between the active breaking area and the whitecap's RCS is obtained in presence of the film on the sea surface. The results are compared to the model of quasi-specular reflection from the breaking waves. It is shown that accounting for backscattering from the droplets produced by the breaking crest improves prediction of the whitecap's RCS given by this model at high incidence angles.