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Ring wave frequency spectra: measurements and model

Ring waves are generated when rain drops impact water surfaces. There is evidence that ring waves can be an important factor contributing to radar scatterometer returns. Because little is known about ring-wave spectra, modeling of rain effects on scatterometer returns from the sea surface is hampered. Consequently the authors are conducting experiments at the Rain-Sea Interaction Facility at Wallops Island to investigate ring-wave spectra. For this study, the simulated rain is 2.8 mm diameter drops at terminal velocity and the rainfall rate ranges from 5 to 200 mm hr/sup -l/. The authors measured elevation time-series using a capacitance probe and used them to compute ring-wave frequency spectra. The spectra all have a similar shape; so for ease of comparison, each spectrum was normalized with respect to its variance. The mean normalized spectrum for all rain rates reveals that the roll-offs are asymmetric, i.e., the low frequency side decays faster than the high frequency tail. The authors model these spectral features by using a logarithmic Gaussian relationship.