Sea State Dependence of Wind Stress and Drag Coefficient under High (Hurricane) Winds

Resolving the exchange of momentum between wind and the ocean is essential for accurately predicting tropical cyclone intensity and track as well as ocean responses. The bulk parameterization for calculating wind stress from wind speed does not account for sea state dependence of the wind stress and assumes that the angle between the wind speed and wind stress is negligible. It is therefore desirable to develop a robust method to estimate the wind stress vector for given surface wave field and wind speed vector, such that it can be implemented in coupled atmosphere/wave/ocean models for tropical cyclones. There are multiple theories for explicitly solving for the sea state dependent wind stress. Major differences among these theories stem from the parameterization of wave form drag (or wave growth rate), the assumption of the mean wind profile, and the parameterization of the spectral tail. In this study we investigate the sea state dependence of the momentum flux and its sensitivity to different approaches by performing three experiments. The first set of experiments is fetch dependent simulations under steady uniform wind speeds. The second set of experiments uses idealized tropical cyclone wind fields. The final set of experiments is case studies using wind fields from real tropical cyclones. NOAA's WAVEWATCH III wave model is used to predict the wave height spectrum in each experiment. Our results suggest that sea state dependence of the drag coefficient is generally weak and the predicted misalignment between the wind direction and the stress direction varies for different theories.