Simultaneous radar and video observations of the sea surface in field conditions

Results of joint radar and video observations from the Black Sea Research platform are presented. Ka-band dual-copolarized (VV and HH) radar was used for measurements of backscattering cross-section and Doppler shifts. The radar footprint was observed by video camera, which was strictly synchronized with the radar. The total coverage of the radar footprint by bright features are computed using measured radar angular antenna pattern. Large foaming breaking waves resolved by the radar footprint are analyzed individually. Active phase of breaking wave process is accompanied by short intensive spike in radar backscattering. During the spike, the polarization ratio (HH/VV) approaches 1, indicating specular mechanism in backscattering from disturbed water mass in breaking crest. However, consequent passage of foam through the radar footprint leads to depression in radar signal, which lasts few times longer than the spike and may reduce an overall contribution of large foaming events to the mean cross-section. In contrast to the cross-section, wave breaking events do not produce intensive spikes in Doppler velocity, suggesting that speed of the scatterer associated with breaker is less than phase speed of breaking wave. Micro-scale breakers do not generate foam, but produce short-living solar glints which are associated with anomalous slope about 40-50°. Comparison of the glint coverage with instantaneous radar cross-section reveals surprisingly high correlation for both VV and HH polarization, suggesting that micro-scale breaking events are important in Ka-band radar backscattering. Combination of long wave properties retrieved from the Doppler velocity with video observation of the breakers allows to estimate modulation transfer function (MTF) of the wave breaking. The magnitude of such MTF decreases with wind speed and increases with wave ageing. The maximum of the large-scale wave breaking is located on the long wave crest, while the peak of micro-scale breaking is shifted on the long wave front face.