IMPACT OF WAVES ON AIR-SEA EXCHANGE OF SENSIBLE HEAT AND MOMENTUM

The impact of sea waves on sensible heat and momentum fluxes is descri bed. The approach is based on the conservation of heat and momentum in the marine atmospheric surface layer. The experimental fact that the drag coefficient above the sea increases considerably with increasing wind speed, while the exchange coefficient for sensible heat (Stanton number) remains virtually independent of wind speed, is explained by a different balance of the turbulent and the wave-induced parts in the total fluxes of momentum and sensible heat. Organised motions induced by waves support the wave-induced stress which dominates the surface m omentum flux. These organised motions do not contribute to the vertica l flux of heat. The heat flux above waves is determined, in part, by t he influence of waves upon the turbulence diffusivity. The turbulence diffusivity is altered by waves in an indirect way. The wave-induced s tress dominates the surface flux and decays rapidly with height. There fore the turbulent stress above waves is no longer constant with heigh t. That changes the balance of the turbulent kinetic energy and of the dissipation rate and, hence the diffusivity. The dependence of the ex change coefficient for heat on wind speed is usually parameterized in terms of a constant Stanton number. However, an increase of the exchan ge coefficient with wind speed is not ruled out by field measurements and could be parametrized in terms of a constant temperature roughness length. Because of the large scatter, field data do not allow us to e stablish the actual dependence. The exchange coefficient for sensible heat, calculated from the model, is virtually independent of wind spee d in the range of 3-10 ms(-1). For wind speeds above 10 ms(-1) an incr ease of 10% is obtained, which is smaller than that following from the 'constant roughness length' parameterization.