The reduction of the radar backscatter from the ocean in the presence of biogenic and anthropogenic surface films measured by an airborne scatterometer

During the two SIR-C/X-SAR missions in 1994, surface film experiments were performed in the German Bight of the North Sea in order to investigate the reduction of the radar backscatter in the presence of biogenic and anthropogenic sea surface films. For this purpose, backscatter measurements were carried out with a 5-frequency/multi-polarisation scatterometer flown on a helicopter. Monomolecular slicks consisting of the same substance (oleyl alcohol) were deployed under different environmental conditions, particularly, at different wind speeds. It was found that, under all wind conditions, the measured damping ratios (i.e., the ratios of the radar backscatter from a slick-free and a slick-covered water surface) increase with increasing Bragg wavenumber. This cannot be explained by Marangoni damping theory (which describes the damping of small sinusoidal water waves by monomolecular slicks). Instead, wind-induced effects, primarily the energy input by the wind into the wave spectrum, have to be taken into account. For the case of high wind speed (12 m/s, where the energy input by the wind is larger than the viscous dissipation), a theoretical model for the damping ratios is developed. The results of this model agree well with the experimental data, in particular, the absence of the Marangoni damping maximum at intermediate Bragg wavenumbers (approx. 100 rad/m) can be explained. Furthermore, the similarities between the radar backscatter reductions measured over biogenic and anthropogenic surface films under high wind conditions are reproduced.