Physics-based parameterization of air-sea momentum flux at high wind speeds and its impact on hurricane predictions

A new bulk parameterization of the air-sea momentum flux at high wind speeds is proposed. It is based on coupled wave-wind (CWW) model simulations of ten tropical cyclones in the western Atlantic Ocean during 1998-2003. In the CWW model, the surface wave directional frequency spectrum near the spectral peak is calculated using the WAVEWATCH III [Tolman, 2002] model and the high frequency part of the spectrum is parameterized using the theoretical model of Hara and Belcher [2002]. The complete wave spectrum is then introduced to the wave boundary layer model of Hara and Belcher [2004] to estimate the surface roughness length for different wind and wave conditions. The new parameterization describes that the roughness length increases linearly with wind speed and the neutral drag coefficient tends to level off at high wind speeds. The proposed parameterization is tested for real hurricane predictions during the 2003-2004 seasons using the operational GFDL/URI coupled hurricane-ocean prediction model. The impact of the new formula is mainly found in improved prediction of the 10-m maximum wind speed, especially for strong hurricanes.