Radar backscatter from mechanically generated transient breaking waves - Part I: Angle of incidence dependence and high resolution surface morphology

This paper describes the results of an experimental investigation of the microwave backscatter from several laboratory generated transient breaking waves. The breaking waves were generated mechanically in a 35 m x 0.7 m x 1.14 m deep wave tank, utilizing chirped wave packets spanning the frequency range 0.8-2.0 Hz, Backscatter measurements were taken by a X/K-band (10.525 GHz, 24.125 GHz) continuous wave Doppler radar at 30 degrees, 45 degrees, and 60 degrees angles of incidence, and at azimuth angles of 0 degrees and 180 degrees relative to the direction of wave propagation. Surface profiles were measured with a high-speed video camera and laser sheet technique. Specular facets were detected by imaging the surface from the perspective of the radar. The maximum radar backscatter occurred in the upwave direction prior to wave breaking, was nearly polarization independent and corresponded to the detection of specular facets on the steepened wave face. This peak radar backscatter was predicted through a finite conductivity corrected physical optics technique over the measured surface wave profiles. Post break backscatter was predicted using a roughness corrected physical optics technique and the small perturbation method, which was found to predict the returns for vertical (VV) polarization, but to under predict the horizontal (HH) returns.