Gravity -capillary waves and their influence on air-sea interaction under non-stationary conditions

Wind generated gravity-capillary waves have been measured along with wind parameters in a wind-wave tank, using a scanning laser slope gauge capable of good wavenumber and frequency resolution. The experiments address the generation and subsequent evolution of short wind-waves, their modulation by fluctuating wind forcing and their contribution to the momentum flux from the wind. Measured initial growth rates of waves in the along-wind direction are consistent with previous studies. The directionality of the growth rates is not proportional to any constant power of the cosine of the wind-wave angle. Typical features in the evolution of the wave field include the spreading of the wave energy along the dispersion relation Doppler shifted by the surface current, the emergence of the bound waves (harmonics of the dominant waves), reduction in the Doppler shift due to enhanced subsurface mixing, and downshifting of the dominant wavenumber and the consequent separation of the free and bound waves in frequency. The modulation of the wind speed over a 10–20s period induces modulation with hysteresis on the wave spectral density. The relaxation rates required to explain these hysteresis loops are typically one order of magnitude smaller than the initial growth rates. Hysteresis loops are also seen in the wind stress under modulated wind speed, indicating that a significant fraction of the total stress is supported by waves. Total wind stress is estimated as the sum of the turbulent stress over a smooth surface and the momentum fluxes into all wave components. The momentum flux into each wavenumber is estimated from measured spectral density and a parameterized growth rate. The wave-supported stress is generally more than half of the total stress, and explicit estimation of this component improves the stress estimate significantly, especially when the water contains a variable concentration of surfactants. The best estimate of the wind stress is achieved if wave growth rates are artificially enhanced at wavenumbers higher than 200 rad/m. The hysteresis of estimated wind stress is significantly less than that measured directly