Scaling of Drag Coefficients under Five Tropical Cyclones

The drag coefficient, often used to parameterize the surface wind stress tau, beneath tropical cyclones (TCs) is a critical but poorly known factor controlling TC intensity. Here, tau is estimated using current measurements taken by 12 Electromagnetic Autonomous Profiling Explorer floats beneath the forward half of five TCs. Combining estimates of tau and aircraft measurements of winds U-10, the downwind drag coefficient (C) over tilde (parallel to). and the angle phi clockwise orientation from U-10 to tau are computed. At vertical bar U-10 vertical bar = 25- 40 m/s, (C) over tilde (parallel to). and phi vary over (0.8- 3.1) x 10(-3) and -15-40 degrees, respectively. A new nondimensional parameter "effective wind duration," a function of vertical bar U-10 vertical bar, storm translation speed, and positions in TCs, predicts (C) over tilde (parallel to). to within 25%. The largest (C) over tilde (parallel to). and smallest. occur at high winds, in the forward right quadrant of fast- moving storms. These dependences are explained by variations in surface wave age and breaking under different wave forcing regimes. Plain Language Summary The forecast of tropical cyclone intensification is critical to the protection of coastlines, involving the complicated tropical cyclone-ocean interaction. The wind of storms can force strong near-inertial current via surface wind stress (often parameterized by a drag coefficient C-d), and then induce the upper ocean cooling due to the shear instability. The transferred momentum and reduced heat supply can both restrict tropical cyclones' development. In other words, the C-d can affect the prediction of momentum and thermal response under storms, and thereby the forecast on storm intensity. This study investigates the spatial variability of downwind drag coefficient C-d under five different tropical cyclones, by integrating the storm-induced ocean momentum because previous results of C-d as a function of wind speed |U-10| are scattered significantly at vertical bar U-10 vertical bar=25-40 m/s. Here, larger C-d in the front-right sector of faster storms than that of slower stoms is found, presumably due to the surface wave effect. A new parameterization of C-d using the surface wave properties under tropical cyclones is proposed, which largely improves the conventional parameterization of C-d(vertical bar U-10 vertical bar). Future studies on the tropical cyclone-wave-ocean interaction and storm intensification forecast will be benefited from this new parameterization.