Experimental study of the microwave radar backscatter from transient deep-water breaking waves.

Abstract

This thesis describes two comprehensive laboratory experiments to investigate the radar backscatter from several types of mechanically generated, transient, deep-water breaking waves. The major objective of these experiments was to identify and model the scattering mechanisms dominating the instantaneous radar return from breaking waves. This was achieved by taking radar backscatter measurements at incidence angles ranging from 30 to 85 degrees, and at azimuth angles ranging from 0 to 180 degrees. Radar measurements were complemented by wave characterization measurements using a laser sheet and high-speed imaging system. The surface profiles extracted from the image data were used in several numerical scattering models to predict the theoretical radar cross section (RCS). Specular scattering from the forward wave face, as it steepened through specular angles in relation to the radar, was found to produce the largest instantaneous radar backscatter observed. The largest RCS values were polarization independent and were observed in the upwave direction, just prior to wave breaking. Doppler spectra corresponding to the peak returns were narrow banded and possessed mean Doppler shifts given by the phase velocity of the waves. After wave breaking, radar returns were obtained at all azimuth angles. The backscatter during this period was dominated by the post-break small scale roughness and appeared to be strongly affected by surface tilt effects. The post-break Doppler spectra were broad banded and split into several groups of peaks. The broadness of these spectra were attributed to the distribution of the velocities of the post-break small scale roughness. For upwave look directions at angles of incidence smaller than the maximum observed surface slope, the radar backscatter transitioned from a specular scattering dominated pre-break region, predicted by physical optics analysis, to a roughness dominated post-break region. The post-break vertically polarized radar returns were consistent with calculations using the small perturbation method. The radar returns for angles of incidence larger than the maximum surface slope were dominated by the backscatter from the post-break small-scale roughness.