Wind stress over the open ocean

Abstract

An automatic inertial dissipation system was used during three cruises of the RRS Discovery in the Southern Ocean to obtain a large data set of open-ocean wind stress estimates. The wind speed varied from near-calm to 26 m/s, and the sea-air temperature differences ranged from -8 to +4°C. It is shown that, under unstable atmospheric conditions, the assumption of a balance between local production and dissipation of turbulent kinetic energy is false, and that the sign and magnitude of the imbalance, !D, depends critically on both stability, z / L , and wind speed, U10N: !D = z L 0.5 " U 10N 6.5 # $ % & ' ( z / L < 0 Application of this empirical term increased the wind stress values obtained under unstable conditions, and brought them into agreement with the data obtained under neutral conditions. The flow of air around the RRS Discovery was simulated in three dimensions using a computational fluid dynamics model. The vertical displacement and the acceleration of the air flow reaching the anemometer site were quantified. The results were used to correct the measured drag coefficient, CD10N , and wind speed estimates. The resulting mean wind stress to wind speed relationship: 1000 CD10N = 0.53 + 0.064U10N 6 !U10N ! 26 m/ s confirmed those obtained by Smith (1980) and Large and Pond (1981). Wave measurement suggested that the sea state was not, on average, fully developed for wind speeds above 12 m/s. However, contrary to findings from other studies, no persistent anomalies in the drag coefficient were detected despite the range of conditions and sea states encountered. It is shown that the wave-age dependent wind stress formulae, derived by previous authors from data obtained over shallow water, do not apply to open ocean conditions.

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