Radar signature of the sea surface perturbed by rain

The attenuation and reflectivity of drops falling through the atmosphere affects the radar signature of the sea surface. Another, much less documented, effect is the perturbation of the surface roughness due to the impacting drops. It is the subject of the present paper. As for the surface description, the present authors refer to the rain-only experiments performed by Bliven et al. (1997) at the NASA/WallopslRain-Sea Interaction Facility. These experiments consisted of wire-probe measurements of a water surface perturbed by rain, which was generated by a simulator with 2.8 mm drops placed at 13 m height; this height enabled the drops to fall practically at terminal velocity. The observed frequency spectrum S/sub f/ has been modelled by a log-Gaussian shape and then transformed to an isotropic surface spectrum with the help of the dispersion relationship of surface waves w(K). The present authors decided to introduce two minor improvements in this model. First, for very small rain rates, the surface roughness tends toward the superposition of independent ring-waves. Hence, they decided to linearize the peak value of the spectrum S/sub f,peak/ between O and 5 mm hr/sup -1/. Secondly, they introduced an exponential law to represent the high frequency tail (f>12 Hz) of the rain spectrum. As for the wind-only spectrum the used model is the fully-developed limit of the model proposed by Lemaire et al. In a first approach, they assume that the rain spectrum simply adds to the wind-only spectrum. The related nonlinearities are described.