Field studies on the action of rain on the radar backscattering from wind-roughened water surfaces

In order to gain further insight into the radar backscattering from rain-and wind-roughened water surfaces, we performed field experiments at the mouth of the river Elbe in the German Bight. The backscattered radar power from the water surface was measured at different wind speeds, wind directions, and rain rates, as well as at different radar frequencies, polarizations, and incidence angles. Our results show that the dominant scattering mechanism at VV-polarization is Bragg scattering which shows a strong dependence on the Bragg wavelength. The enhancement and reduction of the surface roughness by splash products and sub-surface phenomena, respectively, result in a transition wavenumber between enhancement of the radar backscattering (due to additional surface roughness) and its reduction (due to wave damping) of about 100 rad/m. We hypothesize that this transition wavenumber depends on the drop-size distribution of the rain. Taking into consideration the different dependencies on rain rate of the radar backscatter at different frequencies and polarizations, we suggest a method to estimate rain rates by calculating the ratio of the radar cross-sections at L band, VV-polarization and at C band, HV-polarization. Provided an availability of synthetic aperture radar (SAR) data at the respective frequency/polarization combinations, this method will allow for investigating the nature of small-scale (convective) rain events over the ocean by analyzing multi-frequency/multi-polarization SAR images.