The damping of short gravity-capillary waves by experimental sea slicks measured by an airborne multi-frequency microwave scatterometer

Abstract

As a part of the joint US-German SAXON-FPN experiment radar signatures of five experimental monomolecular sea slicks of different visco-elastic properties were measured by an airborne five-frequency (L-, S-, C-, X- and Ku-band) microwave scatterometer. The ratio of the backscattered radar power from slick-free and slick-covered sea surfaces were measured at different microwave frequencies and incidence angles such that the Bragg wavelength ranged from 2 to 40 cm. At wind speeds up to 6 m/s this ratio exhibits the wavenumber dependence that is consistent with the Marangoni damping theory, which describes the damping of ocean surface waves by visco-elastic surface films in the centimeter to decimeter wavelength region. At wind speeds between 9 and 11 m/s the ratio decreases in the wavenumber range between k=60 m/sup -1/ and k=300 m/sup -1/. No statistically significant dependence of the damping ratio on radar look direction and polarization of the radar was measured at wind speeds between 1.5 m/s and 6 m/s.<>