Friction Residual of the Near-Surface Momentum Budget at the Eyewall: Role of the Radial Momentum Transport

Laboratory (Donelan et al. 2004) and field (Powell et al. 2003) measurements now agree that the drag coefficient (Cd) over the ocean generally increases linearly with wind speed up to hurricane speeds (~30 m/s) and levels off at higher speeds. The Powell et al. (2003) results even show a decreasing cd with wind speed beyond this point, such that there is an approximately 50% difference in cd values at ~50 m/s. The laboratory-based measurements are developed from the eddy-correlation method, while the field measurement are developed using the profile method based on GPS dropwindsonde wind speed profiles. Both methods necessarily require approximately steady, horizontally-homogeneous, and non-divergent conditions, which is fairly well satisfied in the lab and well outside of the hurricane eyewall. Near the large-gradient eyewall, however, the radial advection of tangential momentum may become significant, complicating the inherent assumptions.

From radial profiles of SFMR surface winds obtained in over 25 hurricanes, a near-surface momentum budget is constructed to estimate the residual frictional loss of momentum for the cases where radial transport is both neglected (as the field profile method assumes) and included in the balance. When the advection term is included, the friction residual near the radius of maximum wind is found to be substantially larger in magnitude than when it is neglected. Physically, the advection appears as an additional source (sink) of momentum inside (outside) of Rmax, causing the wind speed profile to exhibit a modified logarithmic slope near the surface. Assuming the high wind speed profiles used by Powell et al. to estimate cd were obtained near the radius of maximum winds, the result here may help to partly explain the differences in Donelan et al. and Powell et al. Cd estimates.