Ku-band backscatter from the Cowlitz River: Bragg scattering with and without rain

Ku-band backscatter from the Cowlitz River in southwestern Washington State was measured for incidence angles from 0° to 80°. The measurements were made for light-wind conditions with and without rain. In rain-free conditions, Bragg scattering was the dominant scattering mechanism for both horizontal (HH) and vertical (VV) polarizations out to 75°, beyond which the SNR dropped very low at HH. When a light rain was falling on the river, the cross section increased substantially at moderate incidence angles. Doppler spectra taken during rain showed that VV polarized backscatter is primarily from Bragg scattering from ring waves, while HH polarization scatters from both ring waves and stationary splash products, depending on the incidence angle. From the VV polarized measurements, surface wave height spectrum for ring waves is inferred for light rains. Finally, a change in spectral properties was observed when rain changed to hail.