The nature of microwave backscattering from water waves

Our recent laboratory study shows that Bragg scattering by itself is not an adequate description for microwave backscattering from water waves. It may account for part of the scattering, but reflection from specular facets and wedge-like diffractive scattering from small radius crests of waves can predominate. Our first experiment was performed on wave paddle-generated short gravity waves. Using a scanning laser slope gauge to measure the surface and the moments method to compute the scattering, we found that the small radius crests of such waves can be the more dominant source of scattering and that the description of such scattering is closer to wedge diffraction than Bragg scattering. Bragg scattering does describe the scattering from the parasitic capillaries. We also found that specular reflection is more important than generally expected. Our second experiment was performed on wind waves. We found that at low wind, the Doppler spectrum is narrow peaked but it gradually evolves to become a doubly peaked spectrum at high wind. Analysis shows that low wind scattering is indeed Bragg scattering. At high wind, the lower frequency peak again is due to Bragg scattering from rough patches, whereas the higher frequency peak is due to scattering from waves not unlike the paddle-generated short gravity waves.