Observed Drag Coefficient Asymmetry in a Tropical Cyclone

Abstract

The behavior of drag coefficient ((Formula presented.)) 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 (Formula presented.) is closely related to the location relative to the tropical cyclone (TC) center. The (Formula presented.) presents an enhancement when the typhoon is away from the observational site. The spatial distribution of (Formula presented.) on the periphery of a TC is asymmetric, and the (Formula presented.) 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 (Formula presented.) can be explained by the differences in wave properties between the two quadrants. (Formula presented.) is smaller in cross-swell conditions than that in the along-wind wave conditions. Observations also confirm that (Formula presented.) tends to level off and even attenuate with the increase of wind speed, and the critical wind speed for (Formula presented.) 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 (Formula presented.) 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.