On Variations of Radar Backscatter Due to Short Wind Waves in a Long-Wave Field

An effect of suppression of Ka-band radar backscatter from short wind gravity-capillary waves (GCWs) after the passage of a train of intense long meter-scale waves (LWs), the radar trace, was investigated in a laboratory experiment. It is obtained that radar backscatter intensity in the area of a nonbreaking LW train can increase by an order of magnitude or more, Doppler spectra of the radar signal become wider, and the polarization ratio is close to 1, all of which is typical of the case of micro GCW breaking. The backscatter suppression in the wake reaches one or two orders of magnitude, which is weaker than in the wake that we previously found for breaking LW with overturning crests. The radar trace in the last case was explained by the turbulence excited by the strong LW breaking, which resulted in suppression of short wind GCW. The theoretical interpretation of the radar trace for nonbreaking LW is based on the effect of enhancement of wind drift near the crests of LW, leading to micro breaking of GCW. Small mm–cm-scale structures formed on the GCW profile determine the radar backscatter enhancement in the LW train. The existence of the radar trace is determined by the relaxation of GCW to their undisturbed level in the absence of LWs.