Air-sea momentum flux at high wind speeds

The mean wind profile and the Charnock coefficient, or drag coefficient, over mature, growing, and complex seas are investigated. We develop a model of the wave boundary layer, which consists of the lowest part of the atmospheric boundary layer that is influenced by surface waves and wave breaking, based on the conservation of momentum and energy. Energy conservation is cast as a bulk constraint, integrated across the depth of the wave boundary layer, and the turbulence closure is achieved by parameterizing the dissipation rate of turbulent kinetic energy. Over mature seas, we identify the conditions necessary for the Charnock coefficient to be a true constant, an assumption routinely made in existing bulk parameterizations. The drag coefficient is also estimated over growing and complex seas, such as under hurricane wind forcing, by combining the numerical wave model. At higher wind forcing the effect of breaking waves on the drag coefficient is investigated in detail.