Laboratory study of polarized microwave scattering from steep waves at grazing incidence

Laboratory measurements of the fine space-time structure of short gravity-capillary waves, as well as Ku-band scattering at grazing incidence in mechanical and wind-generated waves are presented. This study was stimulated by the need to verify the impact of nonlinear features of the surface that contribute to scattering at grazing incidence. A scanning laser slope gauge (SLSG) was used for measuring 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 3 ns temporal resolution was used to obtain Doppler spectra of the scattered signals. The simultaneous time-series for wave slope and the scattered signal, both the local Doppler frequency and amplitude, were measured along a long wave profile. Due to their different phase velocities, two-dimensional space-time filtering could be used to separate the free and bound capillary waves. Both types of waves were found to co- and counter-propagate relative to the direction of the dominant wave propagation. Measurements of the Doppler frequency and amplitude of a scattered signal are consistent with this classification of the wave field and reveal Bragg-scattering for both types of waves. Wave breaking at the crests of the long wave packets is a possible source of the observed scatterers