Laboratory measurements of artificial rain impinging on a wind-roughened water surface

Laboratory measurements in the wind-wave tank of the University of Hamburg with artificial rain have been carried out by using a coherent X-band scatterometer, a resistance-type wire gauge and a two-dimensional laser slope gauge. The authors used raindrops with a diameter of 2.9 mm, rain rates between 160 mm/h and 300 mm/h, and wind speeds between 2 m/s and 12 m/s. They show that, by comparing the along-wind and the crosswind slope spectra, a wind dependent orientation of the entire wave field (consisting of wind- and rain-generated waves) can be found. From the X-band Doppler spectra measured at low wind speeds they infer that the backscattered radar power (BRP) at co-polarization is caused by the rain-generated ring waves, whereas at cross-polarization it is associated with the cavities and stalks generated by the impinging rain drops. Moreover, they measured a strong increase of the relative BRP, σ_rel, at co-polarizations (VV, HH) in the presence of rain at wind speeds below approximately 8 m/s. However, at cross-polarization (HV) σ_rel was enhanced more strongly in the presence of rain than at co-polarization for all wind speeds (i.e. up to 12 m/s) and rain rates