Radar backscatter from stationary breaking waves

Abstract

Radar backscatter measurements from stationary breaking waves were used to examine how the surface roughness generated by wave breaking affects radar backscatter at moderate incidence angles. Stationary breaking waves were generated by submerging a stationary hydrofoil in a uniform flow. X band radar backscatter measurements were made at numerous streamwise positions along the stationary breaking waves from an incidence angle of 45° for horizontal transmit and receive polarization (HH) and vertical transmit and receive polarization (VV) looking both upwave and downwave. The radar returns increased substantially, and the HH‐to‐VV polarization ratio approached unity near the breaking crests. This radar signature is consistent with those observed in the field. Detailed optical measurements of the breaking surfaces revealed that the observed radar returns near the breaking crests were the result of increased incoherent backscatter from the small‐scale surface roughness generated by the breaking waves, although surface tilt effects also modified the radar return. Scattering models based upon the small perturbation solution performed well in the wake of the stationary breaking crest, but they significantly underestimated the HH‐to‐VV polarization ratio near the breaking crest. More advanced scattering solutions such as the integral equation method produced more accurate results in regions containing the largest surface roughness. These findings suggest that incoherent backscatter from surface disturbances produced by deep water breaking waves may be the source of the high radar returns and small polarization ratios observed from the ocean at moderate incidence angles.