Observed drag coefficients in high winds in the near offshore of the South China Sea

This paper investigates the relationships between friction velocity, 10m drag coefficient, and 10m wind speed using data collected at two offshore observation towers (one over the sea and the other on an island) from seven typhoon episodes in the South China Sea from 2008 to 2014. The two towers were placed in areas with different water depths along a shore-normal line. The depth of water at the tower over the sea averages about 15m, and the depth of water near the island is about 10m. The observed maximum 10min average wind speed at a height of 10m is about 32m s . Momentum fluxes derived from three methods (eddy covariance, inertial dissipation, and flux profile) are compared. The momentum fluxes derived from the flux profile method are larger (smaller) over the sea (on the island) than those from the other two methods. The relationship between the 10m drag coefficient and the 10m wind speed is examined by use of the data obtained by the eddy covariance method. The drag coefficient first decreases with increasing 10m wind speed when the wind speeds are 5–10m s , then increases and reaches a peak value of 0.002 around a wind speed of 18m s . The drag coefficient decreases with increasing 10m wind speed when 10m wind speeds are 18–27m s . A comparison of the measurements from the two towers shows that the 10m drag coefficient from the tower in 10m water depth is about 40% larger than that from the tower in 15m water depth when the 10m wind speed is less than 10m s . Above this, the difference in the 10m drag coefficients of the two towers disappears.