Spectral Contrasts of Short Wind Waves in Artificial Slicks from the Sea Surface Photographs

Abstract

Purpose. The aim of the work is to evaluate the contrasts between the two-dimensional spectra of the short wind waves on a clean sea surface and on the surface covered by a thin film of vegetable oil. The contrast angular dependence, which is still not understood, is of particular interest. The study is intended to widen the base of empirical notions of wave suppression on the surfactant films in field conditions. Its results may be useful both for theoretical modeling the short wind wave spectra, and for developing the methods for remote monitoring of the ocean. Methods and Results. The contrasts were assessed by analyzing the sea surface photographs taken from the Platform of the Black Sea hydrophisical subsatellite polygon (Katsiveli) during the specialized experiments aimed at obtaining artificial slicks using vegetable oil spills. The applied in the study simple method for estimating the contrasts is based on the assumptions of a linear relationship between the brightness and the sea surface slope, and of the invariability of the brightness – slope transfer function at transition from a clean sea surface to a slick. In contrast to the previously applied methods, this approach makes it possible to obtain the contrasts varying both in wavenumber and direction. Obtaining the estimates of the shortest wave characteristics usually constitutes the utmost technical difficulty. In the work, the spectral contrasts are evaluated for the wind waves whose lengths are from ~ 20 to ~ 1 cm. Conclusions. At moderate wind speeds (6–8 m/s), the obtained contrasts increase monotonically with the wavenumber up to the values ~ 10. Under calm conditions (wind speed 0.5 m/s), the spectral contrast maximum (~ 30–50) is observed at the wavenumber peak ~ 100 rad/m that is qualitatively confirmed by the estimates from a string wave gauge. These results are consistent with the previous measurements performed by the other authors. The two-dimensional contrast distributions are anisotropic with the maximum in the direction perpendicular to the wind one. At moderate winds, the anisotropy increases with growth of a wavenumber.