Radar backscattering measurements of artificial rain impinging on a water surface at different wind speeds

At the wind-wave tank of the University of Hamburg the authors carried out laboratory measurements with artificial rain and wind. The radar backscatter from the rain- and wind-roughened water surface was measured with a coherent 9.8 GHz (X-band) scatterometer at different polarizations (VV, HH, and HV) and incidence angles (28°, 40°, and 55°). At VV-polarization the radar Doppler spectra show two Doppler-shifted maxima at all incidence angles. At HH-polarization a third maximum is visible at zero Doppler shift. With increasing incidence angle the two Doppler-shifted maxima are weakened, whereas the third (non Doppler-shifted) maximum increases. The authors explain this as being an effect of the different backscatter mechanisms caused by the different splash products, like Bragg scattering at ring waves and scattering at the static splash products like stalks. They measured that the relative radar cross sections at HV-polarization increase with increasing high rain rate (50 mm/h and 100 mm/h), which they attribute to enhanced edge and multiple scattering