On the Reduction of the Radar Backscatter by Oceanic Surface Films: Scatterometer Measurements and Their Theoretical Interpretation

During the two SIR-C/X-SAR missions in 1994, surface film experiments were performed in the North Sea with a 5-frequency/multipolarization scatterometer flown on a helicopter, in order to investigate the reduction of the radar backscatter in the presence of quasibiogenic and anthropogenic sea surface films, particularly, at different wind speeds. Under all wind conditions encountered in this study, the measured damping ratio (i.e., the ratio of the radar backscatter from a slick-free and a slick-covered water surface) increases with increasing Bragg wavenumber. It is shown that not only Marangoni damping theory, but also wind-induced effects, primarily the energy input by the wind into the wave spectrum, also have to be taken into account. The reductions measured at low to moderate wind speeds (3.5–4 m/s and 5 m/s) are qualitatively explained by means of a comparison of the different source terms of the action balance equation. For the case of high wind speed (12 m/s) a theoretical model for the damping ratios is developed. Using this model, the experimental data can well be reproduced, and the absence of the Marangoni damping maximum at intermediate Bragg wavenumbers (approximately 100 rad/m) can be interpreted. Furthermore, the model can explain the similarities between the radar backscatter reductions measured over quasibiogenic and anthropogenic surface films under high wind conditions.