Observed Drag Coefficient Asymmetry in a Tropical Cyclone

The behavior of drag coefficient (C_D) in two different motion-relative quadrants of Typhoon Mujigae (2015) is investigated through the flux observations conducted on a fixed platform over the coastal region in the northern South China Sea. Observations reveal that the variation of C_D is closely related to the location relative to the tropical cyclone (TC) center. The C_D  presents an enhancement when the typhoon is away from the observational site. The spatial distribution of C_D on the periphery of a TC is asymmetric, and the C_D in the right rear quadrant is much larger than that in the right front quadrant for the same wind speed range. This asymmetric distribution of C_D can be explained by the differences in wave properties between the two quadrants. C_D is smaller in cross-swell conditions than that in the along-wind wave conditions. Observations also confirm that C_D tends to level off and even attenuate with the increase of wind speed, and the critical wind speed for C_D saturation over the coastal region (~20 m/s) is much lower than that over the open ocean (~30 m/s). The observational spatial distribution of C_D in TC quadrants not only improves our understanding on the air-sea momentum flux but also provides a potential solution for the long-standing scientific bottleneck on TC intensity forecasting.