Wave Tank Study of Steep Gravity-Capillary Waves and Their Role in Ka-Band Radar Backscatter

Nonlinearity of gravity-capillary waves (GCWs) plays an important role in the wind wave dynamics and in microwave scattering from the sea surface. This study is aimed to investigate in wave tank experiment characteristics of nonlinear features of mechanically generated centimeter-decimeter-wavelength GCW--parasitic capillary ripples (PCRs) and bulge/toe structures, and their contribution in microwave radar return. An optical method of visualization of GCW profile with high spatial resolution was developed to study GCW profile and Ka-band Doppler scatterometer was used to measure backscattering at vertical (VV) and horizontal (HH) polarizations. Dependencies of PCR slopes and bulge curvature on steepness of dm-scale GCW have been obtained and indicated sharp growth in the vicinity of the limiting steepness of order 0.3. PCR slopes and bulge curvature both decreased in the presence of film, but PCR appeared to be stronger suppressed by film than the bulges. The velocities of microwave scatterers retrieved from radar Doppler shifts were found to be consistent with the phase velocities of 4-Hz GCW, but smaller for GCW of lower frequencies. Both the phase velocities and the scatterer's velocities slowly increased with GCW steepness. Polarization ratio (PR) values decrease with GCW steepness and in the presence of film, thus indicating the enhanced relative contribution of nonpolarized scattering for very steep GCW and when film is applied on the water surface. We consider that the aforementioned results can be explained under an assumption that PCR contributes mostly to Bragg scattering component, while bulge/toe structures contribute to nonpolarized scattering.