A laboratory study of strong modulation of radar signals due to long waves on the water surface covered with a surfactant film

摘要

Modulation of radar backscatter due to long wind waves in the presence of surfactant films is studied for th first time under wave-tank conditions. It is found that the modulation coefficient can increase significantly with increasing surfactant concentration. We show that the existing theoretical models of modulation of the surface waves underestimate the observed modulation levels. It is demonstrated that the experimental radar Doppler shifts cannot be explained by only the scattering due to free gravity-capillary waves (GCW) and are also determined by coupled waves, which are nonlinear harmonics of the longer wind wave components. Estimates of the relative intensity of coupled waves in the wind-wave spectrum at the Bragg wavelength are presented. It is shown that the contribution of coupled waves is considerable and increases with increasing surfactant concentration, thus confirming the hypothesis for a "cascade" mechanism of strong backscatter modulation in the long-wave field.