Microwave and acoustic scattering from parasitic capillary waves

We report simultaneous microwave and acoustic Doppler backscattering measurements made in a wind-wave tank. The microwave system operated at 35 GHz (0.857 cm), while the acoustic system transmitted at 190 kHz (0.777 cm). The two systems were mounted to view the surface at the same incidence angle, which was varied. The measurements showed that when both systems looked upwind, horizontal transmit and receive polarization (HH) microwave backscatter from the rough water surface was 2 to 12 dB stronger than acoustic backscatter, depending on incidence angle and wind speed. When the acoustic system looked downwind, however, its backscattering level was consistently about 1 dB lower than that of the upward-looking microwave system. We interpret these results to indicate that both the acoustic and microwave systems were scattering from parasitic capillary waves in addition to freely propagating, wind-generated waves. The tilt of the parasitic capillary waves accounts for the observed differences in microwave and acoustic backscatter. We show that Bragg scattering theory predicts both the intensity and the Doppler shift of the microwave and acoustic signals very well using known properties of parasitic capillary waves. Spectral densities of the parasitic capillary waves derived from this Bragg scattering model are in good agreement with those predicted recently by Fedorov and Melville [1998] and observed by Fedorov et al. [1998].