Laboratory study of polarized microwave scattering at grazing incidence

Laboratory measurements of Ku-band scattering at grazing incidence, as well as the fine space-time structure of the short wind waves in the Scripps Institution of Oceanography and Delft Hydraulics Laboratory wind-wave channels are presented. This study was stimulated by the need to verify the processes which significantly contribute to scattering at grazing incidence. A dual polarized (VV, HH) coherent pulsed Ku-band scatterometer with good temporal resolution (3 ns) was used to obtain simultaneous Doppler spectra and the absolute cross-section of scattered signals for grazing angles from 6-25 degrees and winds in the range 2-12 m/s. A scanning-laser slope meter measuring capillary waves from measurements of the scattered signal showed that the Doppler spectra has a bimodal structure. While the low-frequency part of the spectrum corresponds to Bragg scattering from free capillary waves, the high-frequency part is clearly associated with scattering from bound capillary waves on the crests of the longer wind waves. This type of scattering is dominant for the upwind direction (especially for HH) and becomes larger as the wind and fetch increase