Mechanisms of low‐grazing‐angle scattering from spilling breaker water waves

Abstract

The low‐grazing‐angle electromagnetic scattering from spilling breaker water waves has been examined using a numerical approach. A moment‐method‐based electromagnetic technique was used to find the instantaneous scattering from the crests of breakers generated mechanically in a wave tank in the absence of wind. A high‐speed imaging system carried on an instrument carriage traveling at the phase velocity of the breakers provided continuous measurements of the temporal evolution of the crest shape, in turn allowing a continuous calculation of the microwave backscatter from the crest. As the wave steepens and a bulge forms on the forward face of the crest, horizontal‐to‐vertical polarization backscattering ratios (HH/VV) as high as 0 dB are observed. Greatly reduced ratios are observed after breaking when no steep features remain on the wave. A time‐dependent Fourier analysis identifies “fast” scattering whose Doppler shifts are nearly equal to or greater than the phase velocity of the breaker, and “slow” scattering whose Doppler shifts are consistent with the orbital motion of the breaker. Comparison of the Doppler shifts with the measured wave profiles shows that the fast scattering identified is associated with very steep surface features that give specular (or nearly specular) reflection points on the surface, but the magnitude and HH/VV ratio of the response to a specific steep feature depends upon its electromagnetic size, thus giving a frequency dependence. The slow scattering identified (in the absence of wind) is correlated with the “turbulent scar” that remains after breaking.