Scaling of Drag Coefficients Under Five Tropical Cyclones

Abstract

The drag coefficient, often used to parameterize the surface wind stress τ, beneath tropical cyclones (TCs) is a critical but poorly known factor controlling TC intensity. Here, τ is estimated using current measurements taken by 12 Electromagnetic Autonomous Profiling Explorer floats beneath the forward half of five TCs. Combining estimates of τ and aircraft measurements of winds U10, the downwind drag coefficient (Formula presented.) and the angle ϕ clockwise orientation from U10 to τ are computed. At |U10| = 25–40 m/s, (Formula presented.) and ϕ vary over (0.8–3.1) × 10−3 and −15–40°, respectively. A new nondimensional parameter “effective wind duration,” a function of |U10|, storm translation speed, and positions in TCs, predicts (Formula presented.) to within 25%. The largest (Formula presented.) and smallest ϕ 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.