3.2 A New Drag Relation for Aerodynamically Rough Flow over the Ocean

Monday, 9 July 2012: 3:45 PM
Essex North (Westin Copley Place)
Edgar L. Andreas, NorthWest Research Associates, Inc., Lebanon, NH; and L. Mahrt and D. Vickers
Manuscript (1.9 MB)

            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 UN10  –  0.243 .

Here, u* is the measured friction velocity (in m s–1), and UN10 is the neutral-stability wind speed at a reference height of 10 m (also in m s–1).  This relation is fitted to UN10 values between 9 and 24 m s–1.  A drag relation formulated as u* versus UN10 has several advantages over one formulated in terms of CDN10 = (u*/UN10)2 .  First, the multiplicative coefficient on UN10 has smaller experimental uncertainty than do determinations of CDN10.  Second, scatter plots of u* versus UN10 are not ill-posed when UN10 is small, as plots of CDN10 are; u*–UN10 plots presented here thus 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 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 CDN10 results simply from known processes that influence wind-wave coupling.


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