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 mi crowave backscatter from several laboratory generated transient breaking wa ves. The breaking waves were generated mechanically in a 35 m x 0.7 m x 1.1 4 m deep wave tank, utilizing chirped wave packets spanning the frequency r ange 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, a nd 60 degrees angles of incidence, and at azimuth angles of 0 degrees and 1 80 degrees relative to the direction of wave propagation. Surface profiles were measured with a high-speed video camera and laser sheet technique. Spe cular facets were detected by imaging the surface from the perspective of t he radar. The maximum radar backscatter occurred in the upwave direction pr ior to wave breaking, was nearly polarization independent and corresponded to the detection of specular facets on the steepened wave face. This peak r adar backscatter was predicted through a finite conductivity corrected phys ical optics technique over the measured surface wave profiles. Post break b ackscatter was predicted using a roughness corrected physical optics techni que and the small perturbation method, which was found to predict the retur ns for vertical (VV) polarization, but to under predict the horizontal (HH) returns.