Air-sea coupled simulations of steady and unsteady oceanic waves.

The air-sea interaction over various wind generated water waves is studied for various wind intensities. The structures of atmospheric turbulent shear flows over steady and unsteady growing water waves of low or moderate slope, sinusoidal and Stokes-like peaked waves are numerically simulated using. Reynolds stress (RSM) closure models that allow for the memory and rapid distortion effects and compared to previous results of Direct Numerical Simulations (DNS). The model includes a wave age dependent surface roughness and an enhanced growth coefficient calculated from the wind induced drag.

It is shown that the critical layer height, where the wave speed is equal to the wind speed, lies within the inner surface layer and non-separated sheltering flow determines the drag and the energy transfer. It is shown that there is a thin layer of down-slope, recirculating and asymmetric streamlines or `cat's-eyes' which all have weak effects. The computations are then extended to the cases when the waves are traveling faster than wind and low rate of growth. In this case the critical shear layer forms outside the inner surface shear layer and the drag and cat's-eyes amplitude all reach a maximum value which are further amplified if the wave slope increases. This result is consistent with experiments and the direct numerical simulations. The computations show that both these mechanisms are further amplified when waves grow and the surface become sharp crested.