Consequences of the Effect of Surface Gravity Waves on the Mean Air Flow
The effect of wind-generated gravity waves on the air flow is discussed using quasi-linear theory of wind-wave generation. In this theory both the effects of the waves and the effect of air turbulence on the mean wind profile is taken into account, but effects of turbulence on the wave-induced air motion are disregarded. In addition, effects of wave breaking on the air-sea momentum transfer are not considered. Nevertheless, this relatively simple model of the momentum transport from air to water is shown to produce realistic results.
The main result of this theory is that the momentum transfer at the air-sea interface is sea-state dependent, explaining the scatter in plots of the experimentally observed drag coefficient as a function of wind speed. As a matter of fact, a very good agreement between observed and modeled stress is found, much better than when the usual Charnock relation for the roughness length is used.
A proper description of the physics of the momentum transfer can therefore only be given by considering a coupled ocean-wave, atmosphere model. Results of the coupling of a simple surface layer model with a third-generation wave model will be discussed. Finally, consequences for coupled climate models of ocean and atmosphere are briefly pointed out.
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