Short gravity-capillary waves modulation due to long surface and internal wave: laboratory and field experiment

Abstract

Modulation of short wind gravity-capillary waves (SGCW) due to long surface and internal waves in the presence of surface films of different surfactant concentrations has been studied in wave tank and field experiments using Ka-band radar. Wave tank experiments were carried out in the oval wind wave tank of the Institute of Applied Physics RAS at different wind velocities (2 m/s to 5 m/s) and at two fetches. It was obtained that the Modulation Transfer Function (MTF) magnitude in film slicks was several times larger than in non-slick areas, the phase of MTF was also changed in the presence of films. Similar conclusions were made from field observations of radar backscatter modulation due to long surface and internal waves, here the effect of enhanced surface waves modulation in slick zones was revealed, too. Different physical mechanisms were taken into account to explain experiment, namely, geometrical effects, modulation of surfactant concentration, transformation of the wind velocity over the long wave profile. Theoretical analysis has demonstrated that the effect of enhanced modulation could not be explained using only the mechanisms mentioned above. MTF is assumed to be determined by the effect of bound components of short wind-wave spectrum.