A New Drag Relation for Aerodynamically Rough Flow over the Ocean

            From almost 7000 near-surface eddy-covariance flux measurements over the sea, we deduce a new air-sea drag relation for aerodynamically rough flow:

                                                      u_*   =   0.0583 U_N10  –  0.243 .

Here, u_* is the measured friction velocity (in m s^–1), and U_N10 is the neutral-stability wind speed at a reference height of 10 m (also in m s^–1).  This relation is fitted to U_N10 values between 9 and 24 m s^–1.  A drag relation formulated as u_* versus U_N10 has several advantages over one formulated in terms of C_DN10 = (u_*/U_N10)² .  First, the multiplicative coefficient on U_N10 has smaller experimental uncertainty than do determinations of C_DN10.  Second, scatter plots of u_* versus U_N10 are not ill-posed when U_N10 is small, as plots of C_DN10 are; u_*–U_N10 plots presented here thus suggest aerodynamically smooth scaling for small U_N10.  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 C_DN10 function that naturally rolls off at high wind speed and asymptotes to a constant value of 3.40 x 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.  We suggest that this roll off in C_DN10 results simply from known processes that influence wind-wave coupling.

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