RADAR BACKSCATTER AND SURFACE-ROUGHNESS MEASUREMENTS FOR STATIONARY BREAKING WAVES

In this study the surface features and the radar backscatter associate d with breaking waves generated by a uniform flow past a stationary su bmerged hydrofoil were examined. The level of energy dissipation due t o breaking was varied by changing the foil angle of attack. Time serie s of surface elevation profiles were obtained for the breaking crest r egion and the following waves. Radar backscatter (X-band) was also mea sured for an incidence angle of 45 degrees with the radar looking both upwave and downwave for HH and VV polarizations. These measurements w ere compared to model predictions of radar backscatter using the surfa ce elevation data as inputs to the model. The breaking crest region ex hibited the largest surface disturbances, as measured by the temporal variance of the surface elevation. The maximum in the variance was ass ociated with large low-frequency disturbances in the 'toe' region. Dow nstream-moving waves appear just ahead of the crest and, due primarily to interaction with the spatially varying current set up by the stati onary wave, decrease in amplitude by an order of magnitude as they pro pagate downstream. These surface disturbances remain at a low level th ereafter. A maximum radar cross-section per unit area of about 0.5 was observed near the breaking crest, for both HH and VV polarization in the upwave look direction. The maximum value for the upwave look direc tion was about twice as large as for the downwave look direction. Down stream of the breaking crest, the radar cross-section decreased rapidl y and then leveled off, and an increasing difference between the VV an d HH backscatter was observed as the overall backscatter level decreas ed. Near the second crest, there was a small increase in the height va riance and in the radar cross-section. The surface-elevation measureme nts were used as inputs for a Bragg-scattering model and the expected radar backscatter was calculated. The variations in the observed radar cross-section downstream of the breaking crest are satisfactorily exp lained by the Bragg model when surface-tilt effects are taken into acc ount. However, the backscatter from the breaking crest itself is not a ccurately predicted since, in this region, the small-scale surface rou ghness exceed the limits of validity for the Bragg model.