Drag coefficients derived from ocean current and temperature profiles at high wind speeds

Two bottom-up methods based on the turbulence closure and bulk model were utilised to estimate drag coefficients at high wind speeds based on ocean current and temperature profiles observed by two subsurface buoys during Typhoon Megi in the South China Sea. A numerical experiment was conducted using the turbulence closure model to test the impact of missing measurements in the upper mixed layer on the wind stress estimate and reconstruction of the upper ocean current. The results were sufficiently robust after several time steps. The wind stresses derived from the two methods were consistent with each other. Wind stress increased quickly with increasing wind force, and then was constant. A parametric typhoon wind model was applied to obtain the wind field and estimate the corresponding drag coefficient. The results showed that the drag coefficient increased to a maximum at a critical wind speed of about 30 m s⁻¹, and then levelled off with increasing wind speed. The uncertainty of the maximum drag coefficient was about ±0.5 due to neglect of nonlinear terms, the uncertainty of the model wind speeds and the drag coefficient. The critical wind speed for the maximum drag coefficient varied by at least ±5 m s⁻¹. The uncertainty of the drag coefficient parameterisation was about 24% due to wind speed. These results indicate that a bottom-up method can be used to estimate drag coefficients at the forced stage during the passage of typhoons or hurricanes.