PHYSICAL PROCESSES OF MICROWAVE BACKSCATTERING FROM LABORATORY WIND-WAVE SURFACES

Physical processes of microwave backscattering from wind wave surfaces are investigated in a wind wave tunnel by using a X-band (9.6 GHz) mi crowave scatterometer. Detailed analysis of time series of the backsca ttered intensity and Doppler spectrum shows that the physical processe s of microwave backscattering are closely associated with the processe s of the wind wave field. At slant incidence (45-degrees) the backscat tered intensity is in phase with the wave profile, and the Doppler vel ocity also follows the phase of the individual waves with high Doppler velocity observed at the crests of the individual waves. This velocit y is equal to the propagating speed of the crests. It is concluded tha t the fine structures of wind wave surfaces, which are trapped near th e crests and propagating with the crests, are the main contributor to microwave backscattering. It is also pointed out that the effect of wa ve breaking with bubble entrainment on the microwave backscattering is not significant under the condition of the present experiment. At nor mal incidence the backscattered intensity has a different phase relati onship with the wave profile. The intensity has two peaks, one at the crest and the other at the trough of the individual waves. The peak at the trough is stronger than that at the crest. This result is consist ent with specular point scattering and also with the asymmetrical feat ures of young laboratory wind waves. At intermediate incident angles, backscattering processes are rather complicated because both specular point scattering and Bragg resonance scattering contribute to the rada r backscattering.