The damping of ocean waves by surface films: A new look at an old problem

Abstract

A new theory is presented explaining why not only short surface ripples, but also longer ocean surface waves are damped by oil films floating on the sea surface. The wave attenuation by viscoelastic surface films is attributed to the Marangoni effect, which causes a strong resonance‐type wave damping in the short‐gravity‐wave region, and to nonlinear wave‐wave interaction, by means of which wave energy is transferred from the longer waves to the energy sink in the Marangoni resonance region. A viscoelastic surface film changes the free surface boundary condition in the tangential direction and thus strongly modifies the flow pattern in the boundary layer. As a consequence, wave energy is dissipated by enhanced viscous damping in the short‐gravity‐wave region due to large velocity gradients induced in the viscous boundary layer. Estimates of the influence of surface films on nonlinear transfer rates are given. Data on wave damping obtained in laboratory and field experiments by previous investigators are discussed in the light of the proposed theory. It is found that this theory is capable of explaining the observed strong wave damping by viscoelastic surface films. The theory predicts that in the equilibrium range of the spectrum, but outside the Marangoni resonance region, wave damping increases with wave number k as k3/2 and increases quadratically with wind speed (up to the limit where the film is “washed down”). The higher the elasticity of the surface film, the stronger is the wave damping.

SAR raw signal simulation of oil slicks in ocean environments

“…Oil influence on the ocean spectrum is a widely studied issue in literature, with great attention to measurements techniques [8], [31]- [39], but no sound theoretical model for the spectrum of water covered by oil is available. On the other hand, a theoretical model of ocean spectra modification in presence of surfactant is highly desirable for SAR simulation purposes.…”

“…An oceanographic model that predicts sea surface modifications caused by an oil layer [8] is chosen and used, and the effect of the oil layer over the electromagnetic properties of the surface is analyzed in detail. The simulator generates the corresponding SAR raw signal by using 0196-2892/02$17.00 © 2002 IEEE an appropriate SAR model that accounts for the ocean dynamics and for the electromagnetic wave-ocean surface interaction.…”

First evidence for the detection of natural surface films by the QuikSCAT scatterometer

“…[14] It is well known that imaging radars can be used for detecting small to medium-scale slicks which have dimensions from some tens of meters to tens of kilometers [Alpers and Hühnerfuss, 1989], but till now it has not been realized that spaceborne scatterometers can be used for detecting large-scale slicks which have dimensions of several hundred kilometers and thus have the potential to carry out global measurements of slick coverage. In this paper we have presented evidence that indeed scatterometers can be used for this task.…”

“…They strongly damp shortscale surface waves (short gravity and capillary waves). Thus they become detectable by active microwave remote sensors because a reduction of the amplitude of short-scale waves gives rise to a reduction of the Normalized Radar Cross Section (NRCS) [Alpers and Hühnerfuss, 1989]. Although the detection of slicks by space-borne Synthetic Aperture Radars (SARs) is well reported [Alpers and Hühnerfuss, 1989], their narrow swath (typically 100 -500 km) and their long revisit time (typically 24 -35 days) inhibit the mapping of slick coverage on a global scale within a reasonable timeframe.…”

“…Thus they become detectable by active microwave remote sensors because a reduction of the amplitude of short-scale waves gives rise to a reduction of the Normalized Radar Cross Section (NRCS) [Alpers and Hühnerfuss, 1989]. Although the detection of slicks by space-borne Synthetic Aperture Radars (SARs) is well reported [Alpers and Hühnerfuss, 1989], their narrow swath (typically 100 -500 km) and their long revisit time (typically 24 -35 days) inhibit the mapping of slick coverage on a global scale within a reasonable timeframe. Since the QuikSCAT scatterometer has a 1,800 km swath covering 93% of the global ocean daily [Liu et al, 1998], it would be highly desirable to use QuikSCAT data in performing such a task.…”