Formulation of the Sea Surface Friction Velocity in Terms of the Mean Wind and Bulk Stability


Over 5000 aircraft eddy-covariancemeasurements from four different aircraft in nine different experiments are used to develop a simple model for the friction velocity over the sea. Unlike the widely used Coupled Ocean–Atmosphere Response Experiment (COARE) bulk flux scheme, the simple model (i) does not use Monin–Obukhov similarity theory (MOST) and therefore does not require an estimate of the Obukhov length, (ii) does not require a correction to the wind speed for height or stability, (iii) does not require an estimate of the aerodynamic roughness length, and (iv) does not require iteration. In comparing the model estimates developed in this work and those of the COARE algorithm, comparable fitting metrics for the two modeling schemes are found. That is, usingMonin–Obukhov similarity theory and the Charnock relationship did not significantly improve the predictions. It is not clear how general the simplemodel proposed here is, but the samemodel with the same coefficients based on the combined dataset does a reasonable job of describing the datasets both individually and collectively. In addition, the simple model was generally able to predict the observed friction velocities for three independent datasets that were not used in tuning themodel coefficients. Motivation for the simple model comes from the fact that physical interpretation of MOST can be ambiguous because of circular dependencies and self-correlation. Additional motivation comes from the large un-certainty associated with estimating the Obukhov length and, especially, the aerodynamic roughness length. 1

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