Radar backscatter and surface roughness measurements for stationary breaking waves

Abstract

In this study the surface features and the radar backscatter associated with breaking waves generated by a uniform flow past a stationary submerged hydrofoil were examined. The level of energy dissipation due to breaking was varied by changing the foil angle of attack. Time series of surface elevation profiles were obtained for the breaking crest region and the following waves. Radar backscatter (X-band) was also measured for an incidence angle of 45 degrees with the radar looking both upwave and downwave for HH and VV polarizations. These measurements were compared to model predictions of radar backscatter using the surface elevation data as inputs to the model. The breaking crest region exhibited the largest surface disturbances, as measured by the temporal variance of the surface elevation. The maximum in the variance was associated with large low-frequency disturbances in the `toe' region. Downstream-moving waves appear just ahead of the crest and, due primarily to interaction with the spatially varying current set up by the stationary wave, decrease in amplitude by an order of magnitude as they propagate downstream. These surface disturbances remain at a low level thereafter. 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 direction was about twice as large as for the downwave look direction. Downstream of the breaking crest, the radar cross-section decreased rapidly and then leveled off, and an increasing difference between the VV and HH backscatter was observed as the overall backscatter level decreased. Near the second crest, there was a small increase in the height variance and in the radar cross-section. The surface-elevation measurements 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 explained by the Bragg model when surface-tilt effects are taken into account. However, the backscatter from the breaking crest itself is not accurately predicted since, in this region, the small-scale surface roughness exceed the limits of validity for the Bragg model.