Presenter: Xiao-Hui Zhou (University of Rhode Island)

Description:
The drag coefficient under tropical cyclones and its dependence on sea states are investigated by combining upper ocean current observations (using EM-APEX floats deployed under five tropical cyclones) and a coupled ocean-wave (Modular Ocean Model 6 - WAVEWATCH III) model. The estimated drag coefficient averaged over all storms is around 2~3*10^-3 for wind speeds 25-55 m/s. The drag coefficient is significantly reduced by misaligned swell when the misalignment angle between the dominant wave direction and the wind direction exceeds about 45^o. Since the misaligned swell is more common in the far front and in the left front quadrant of the storm, the drag coefficient also tends to be lower in these areas and shows a distinct spatial distribution. The normalized roughness length (Charnock coefficient) decreases with increasing wave age in lower wind bins (25-30m/s). However, this trend disappears at higher wind speeds.