Typhoon air-sea drag coefficient in coastal regions

The air-sea drag during typhoon landfalls is investigated for a 10 m wind speed as high as U-10 approximate to 42 m s(-1), based on multilevel wind measurements from a coastal tower located in the South China Sea. The drag coefficient (C-D) plotted against the typhoon wind speed is similar to that of open ocean conditions; however, the C-D curve shifts toward a regime of lower winds, and C-D increases by a factor of approximately 0.5 relative to the open ocean. Our results indicate that the critical wind speed at which C-D peaks is approximately 24 m s(-1), which is 5-15 m s(-1) lower than that from deep water. Shoaling effects are invoked to explain the findings. Based on our results, the proposed C-D formulation, which depends on both water depth and wind speed, is applied to a typhoon forecast model. The forecasts of typhoon track and surface wind speed are improved. Therefore, a water-depth-dependence formulation of C-D may be particularly pertinent for parameterizing air-sea momentum exchanges over shallow water.