An analysis of scatterometer returns from a water surface agitated by artificial rain: evidence that ring-waves are the main feature

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Abstract
Both wind and rain roughen the sea surface, but whereas wind generates waves, rain generates craters, stalks and ring-waves. Average backscat-tered power for scatterometer returns From water surfaces is closely related to small scale features on the water surface, so we use backscattered power from short wind-waves as a basis to evaluate the importance of ring-waves. Experiments were conducted with a 13-5 GHz scatterometer (30° incidence angle, vertical polarization) in a wind-wave tank that is enhanced by a rain simulator. Rain intensities ranged from 3-30 mm h−1 and wind friction velocities were between 10 and 50cms−1. The variance of surface elevation for small scale features ξ2 smi.e., ring-waves and short wind-waves, was computed for each case using data from a capacitance probe. Comparison of the data sets shows that the range of ξ2 sm for the rain cases is comparable to that from light to moderate wind cases—so ring-wave amplitudes are not negligible. Analysis of the radar data provides evidence that ring-waves are the dominant feature contributing to the backscattered power. Thus ring-waves need to be included in scatterometer numerical models that contain rain effects.

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