Wave tank study of phase velocities and damping of gravity-capillary wind waves in the presence of surface films

Abstract

The wave number-frequency spectra of gravity-capillary waves were measured using two optical spectrum analysers and an artificial gradient illuminator. It was found that phase velocities of centimetre-millimetre (cm-mm) scale waves on clean water do not obey the dispersion relation of free surface waves, but they increase with fetch, while frequencies of dominant decimetre (dm)-scale wind waves decrease with fetch. This observation implies that the wind wave spectrum contains nonlinear cm-mm-scale harmonics bound to the dominant waves and propagating with the phase velocities of the dominant waves. The relation between bound and free waves can be estimated from measurements of phase velocity. Wind ripple damping was found to be maximum at wavelengths around 5-7 mm. The latter effect agrees with results of our field experiments using artificial slicks and can be explained qualitatively by a nonlinear cascade damping mechanism, when the damping of dm-scale dominant waves leads to strong damping of their cm-mm-scale nonlinear harmonics including damping of parasitic capillary ripples. © 2006 Springer-Verlag Berlin Heidelberg.