Wave tank studies of radar Doppler shifts in the presence of surfactant films on the water surface

Doppler shifts of Ka-band radar signals backscattered by short wind waves in the presence of organic films have been studied in wave tank experiment at low wind speeds and at different surfactant concentrations. It is shown that phase velocities of mm-scale wind ripples, retrieved from radar Doppler shifts, do not obey the linear dispersion relationship and are determined by the velocities of free gravity-capillary waves and of bound waves - nonlinear harmonics of decimetre-scale dominant waves. A relative intensity of the bound waves as a function of surfactant concentration is estimated. Damping of free and bound waves due to surfactant films has been analysed theoretically and a qualitative explanation of changing of radar Doppler shifts in the presence of films is given.