Typhoon air‐sea drag coefficient in coastal regions

Abstract The air‐sea drag during typhoon landfalls is investigated for a 10 m wind speed as high as U 10 ≈ 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.