Observed Drag Coefficient Asymmetry in a Tropical Cyclone

The behavior of drag coefficient (CD ${C}_{\mathrm{D}}$) 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 CD ${C}_{\mathrm{D}}$ is closely related to the location relative to the tropical cyclone (TC) center. The CD ${C}_{\mathrm{D}}$ presents an enhancement when the typhoon is away from the observational site. The spatial distribution of CD ${C}_{\mathrm{D}}$ on the periphery of a TC is asymmetric, and the CD ${C}_{\mathrm{D}}$ 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 CD ${C}_{\mathrm{D}}$ can be explained by the differences in wave properties between the two quadrants. CD ${C}_{\mathrm{D}}$ is smaller in cross-swell conditions than that in the along-wind wave conditions. Observations also confirm that CD ${C}_{\mathrm{D}}$ tends to level off and even attenuate with the increase of wind speed, and the critical wind speed for CD ${C}_{\mathrm{D}}$ saturation over the coastal region (similar to 20 m/s) is much lower than that over the open ocean (similar to 30 m/s). The observational spatial distribution of CD ${C}_{\mathrm{D}}$ 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.