Radar backscatter from mechanically generated transient breaking waves - Part II: Azimuthal and grazing angle dependence

This paper describes the results of experimental investigations into the microwave backscatter from mechanically generated transient breaking waves. The investigations were carried out in a 110 m x 7.6 m X 4 m deep model basin, utilizing chirped wave packets spanning 0.75-1.75 Hz, Backscatter measurements were taken by a K-band continuous wave radar (24.125 GHz) at 40 degrees angle of incidence, and at azimuth angles of 0 degrees, 45 degrees, 90 degrees, 135 degrees and 180 degrees relative to the direction of wave propagation. Grazing measurements were conducted using an X-band (10.525 GHz) FMCW radar at 85 degrees angle of incidence, and azimuth angles of 0 degrees and 180 degrees. Results show that the maximum radar backscatter was obtained in the upwave direction prior to wave breaking and was caused by the specular or near specular presentation of the wave to the radar. After breaking, the backscatter transitioned from a specular or near-specular dominated scattering, primarily seen in the upwave direction, to a small scale roughness dominated scattering, observed at all azimuths, Physical optics solutions were found to correctly predict the backscatter for the specular or near-specular dominated scattering and the small perturbation method was found to accurately model the VV polarization post-break radar backscatter.