A new drag relation for aerodynamically rough flow over the ocean

Abstract

From almost 7000 near-surface eddy-covariance flux measurements over the sea, the authors deduce a new air-sea drag relation for aerodynamically rough flow: u * = 0.0583U N10-0.243. Here u * is the measured friction velocity, and U N10 is the neutral-stability wind speed at a reference height of 10 m. This relation is fitted to U N10 values between 9 and 24 ms -1.Adrag relation formulated as u * versus UN10 has several advantages over one formulated in terms of CDN10 5(u */U N10) 2. First, the multiplicative coefficient on U N10 has smaller experimental uncertainty than do determinations of CDN10. Second, scatterplots of u * versus UN10 are not ill posed when UN10 is small, as plots of CDN10 are; u *-U N10 plots presented here suggest aerodynamically smooth scaling for small UN10. Third, this relation depends only weakly on Monin-Obukhov similarity theory and, consequently, reduces the confounding effects of artificial correlation. Finally, with its negative intercept, the linear relation produces a CDN10 function that naturally rolls off at high wind speed and asymptotically approaches a constant value of 3.40 3 10 -3. Hurricane modelers and the air-sea interaction community have been trying to rationalize such behavior in the drag coefficient for at least 15 years. This paper suggests that this rolloff in C DN10 results simply from known processes that influence wind-wave coupling. © 2012 American Meteorological Society.