Momentum flux budget analysis of wind-driven air-water interfaces

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Accurate knowledge of the air-sea momentum flux plays a critical role in ocean-atmosphere modeling. In this study we investigate the momentum flux budget at an air-water interface in the presence of wind-driven gravity-capillary waves. At the air-sea interface the total momentum flux (wind stress) partitions into viscous and wave-induced stress; the latter transfers momentum into surface waves. We estimate the wave growth rate (momentum transfer rate into waves) to close the momentum flux budget, using previous laboratory measurements of total and viscous stress, as well as surface wave spectra. The wave-induced stress is computed via a parameterized wave growth rate, which is proportional to the turbulent stress divided by the wave-phase speed squared. The constant of proportionality is then determined under two different assumptions. The first assumption (nonsheltering assumption) is that the turbulent stress is equal to the total wind stress. In the second assumption (sheltering assumption) the wave growth rate is determined by the local turbulent stress, which is a reduced turbulent stress due to the presence of longer waves. Both assumptions yield simple closed form expressions for the stress-partitioning ratio (ratio between the total stress and the viscous stress). With the sheltering assumption the growth rate agrees with previous theoretical and empirical estimates. Without sheltering, the growth rate is significantly lower than previous estimates. Therefore our results indicate that the growth rate of surface waves is determined by the local, reduced turbulent stress rather than the total wind stress. Copyright 2005 by the American Geophysical Union.

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Journal of Geophysical Research: Oceans