Wave tank optical measurements of phase velocities of wind waves

Wave tank studies of wind waves were carried out using an optical spectrum analyser and an artificial gradient illuminator. The wavenumber-frequency spectrum and phase velocities of gravity-capillary waves were measured over the wavelength range from about 10 cm to 6 mm. It was found that the phase velocities for wavelengths smaller than 2-3 cm remain nearly constant and do not follow the dispersion relation of free surface waves. These phase velocities were found to increase with fetch, in the way that the wave phase velocity of the dominant decimetric wind waves increases with fetch. One thus can conclude that the wind wave spectrum contains nonlinear cm-mm-scale harmonics that are bound to the dominant wind waves, propagating with the phase velocities of the decimetric waves