Surface Layer Drag Coefficient at Different Radius Ranges in Tropical Cyclones

Abstract

Using dropsonde data and a flux–profile method, this study investigates the drag coefficient (Cd)–wind speed relationship within different radius ranges. The results show a systematic decrease of friction velocity u* from the range of R/RMW > 1.05 to that of R/RMW < 0.95 (R is the radial location of a dropsonde profile, and RMW is the radius of maximum wind), and the reduction is 5~25% for different wind speeds. Further, within the ranges of either R/RMW > 1.05 or R/RMW < 1.05, a clear feature of “roll-off” at about 35 m s−1 can be obtained. However, the roll feature becomes vague in the ranges of R/RMW < 0.95, R/RMW < 0.85, and R/RMW < 0.75, indicating the TC dynamics within and near RMW play a role in affecting the flux–profile relationship. Even more, Cd of R < 0.75RMW deviates significantly from the Cd of R < 0.85RMW and R < 0.95RMW, while the deviation between R < 0.85RMW and R < 0.95RMW is much smaller. Especially when 10 m winds exceed 40 m s−1, u* of R < 0.75RMW is significantly larger than that of R < 0.85RMW. This phenomenon is also linked to the TC dynamics (e.g., the large radial gradients of winds and the drastic vertical variation of the bulk Richardson number), but the speculation needs to be verified in future study.

Atmospheric Boundary Layer Processes, Characteristics and Parameterization

“…An exceptional finding in this paper was the variation in the turbulent parameters with regard to the distance to the tropical cyclone center: outside three times of the radius of maximum wind (RMW) from the tropical cyclone center, the turbulent kinetic energy and eddy diffusivity values increased with increasing wind speed; however, in the area that was within one to three times the RMW from the tropical cyclone center, these values decreased slowly with increasing wind speed [1]. The other three papers investigated the turbulent fluxes exchanged over the ocean under tropical cyclone conditions through aircraft eddycovariance measurements [2], dropsondes observations [3], and numerical simulations [4]. Gao et al proposed new equations to parameterize the surface drag coefficient over the ocean surface through aircraft eddy-covariance measurements [2]; Ye et al showed from the dropsondes observations that the relationship between the surface drag coefficient and wind speed varied with the distance from the tropical cyclone center [3]; and Ye et al also showed from simulations that the surface flux scheme option, which overestimated the enthalpy exchange coefficient, leads to excessive inflow within the boundary layer and larger eyewall updrafts [4].…”

“…The other three papers investigated the turbulent fluxes exchanged over the ocean under tropical cyclone conditions through aircraft eddycovariance measurements [2], dropsondes observations [3], and numerical simulations [4]. Gao et al proposed new equations to parameterize the surface drag coefficient over the ocean surface through aircraft eddy-covariance measurements [2]; Ye et al showed from the dropsondes observations that the relationship between the surface drag coefficient and wind speed varied with the distance from the tropical cyclone center [3]; and Ye et al also showed from simulations that the surface flux scheme option, which overestimated the enthalpy exchange coefficient, leads to excessive inflow within the boundary layer and larger eyewall updrafts [4].…”