Polarized microwave scattering by surface water waves and turbulence

Measurements of the fine space-time structure of short gravity-capillary waves in the presence of turbulence, as well as Ku-band scattering at grazing and moderate incidence from mechanically-generated waves in a wave channel are presented. This study was stimulated by the need to investigate the influence of nonlinear features of the scattering surface, which contribute to scattering at grazing incidence. Mechanically generated steep periodic (3 to 6 Hz) waves were used as a source of bound parasitic capillary waves. An oscillating (2 to 7 Hz) grid was used as a source of subsurface turbulence. A scanning laser slope gauge (SLSG) and an imaging slope gauge were used to measure short waves of length 0.2-20 cm, and frequencies up to 150 Hz. A dual polarized (VV, HH) coherent pulsed Ku-band scatterometer with 3ns temporal resolution was used to obtain Doppler spectra of the scattered signals for grazing angles from 12 to 30 degrees. Simultaneous time series of wave slopes and microwave scattering and their frequency spectra permitted a comparison of these parameters both with and without turbulence. It was found that the energy of bound short waves increased substantially (by 10-40%) on interaction with the turbulence. Measurements of the Doppler frequency shift and amplitude of the scattered signal are consistent with this change in the wave field, and reveal Bragg scattering by the bound waves.