Doppler radar backscatter from ring waves
The effects of rain falling on the sea surface have been investigated using a 3 GHz Doppler radar. In the absence of wind, the field measurements of radar backscatter show an enhancement of surface roughness due to a 4 mm hr 1 rainfall, which is equivalent to a light wind of several m s 1, but without the azimuthal variation characteristic of wind generated roughness. The radar Doppler spectra are dominated by dual peaks corresponding to scatterers both approaching and receding from the radar at the phase velocity of the resonant Bragg wave. This indicates that the dominant backscatter mechanism from this rain-roughened sea was resonant Bragg scatter from ring-waves expanding outward from the sites of droplet impacts. A secondary feature of the Doppler spectra which may be backscatter from stalks, is characterized by intermittent, weaker returns at velocities that are approximately stationary relative to the water surface. The Doppler spectral widths indicate that ring-wave lifetimes exceed 300ms.