Momentum flux budget across the air-water interface under strongly forced wind conditions

Kianoosh Yousefi, University of Texas at Dallas, Mechanical Engineering, Richardson, United States, Fabrice Veron, University of Delaware, School of Marine Science and Policy, Newark, DE, United States and Marc P Buckley, Helmholtz-Zentrum Hereon, Institute of Coastal Ocean Dynamics, Geesthacht, Germany
Abstract:
In recent years, the exchange of momentum between the atmosphere and the ocean has been the subject of several investigations. Although the role of surface waves on the air-sea momentum flux is now well established, detailed quantitative measurements of the momentum fluxes over surface wind waves remain scarce. The accurate predictions of momentum fluxes between the air and the ocean demand a thorough understanding of the turbulent boundary layer above ocean surface waves, particularly in high winds and extreme conditions.

In the current study, we present a detailed investigation of the various terms of the mean momentum equation, derived explicitly in an orthogonal wave-following coordinate system, for the airflow in the wave boundary layer over wind-generated surface waves. To this end, quantitative airside velocity measurements were obtained in the laboratory over wind-driven surface waves through a combined particle image velocimetry (PIV) and laser-induced fluorescence (LIF) system for several wind-wave conditions with 10-m wind speeds ranging from 0.89 to 16.59 m~s {}^{-1} . The mean, turbulent, and wave-induced velocity fields were then extracted from the instantaneous velocity fields using a linear triple decomposition technique.

Our data showed that, as a general trend, the mean wave-induced stress decreases to a negative minimum from a near-zero value far from the surface and then increases rapidly to a positive value (which supports a substantial portion of the total stress) near the interface where the turbulent stress is reduced. Far away from the surface, however, the turbulent stress supports nearly all the air-sea stress, and the wave stress is zero. Closer to the surface, the wave-induced and turbulent stresses are both approaching zero, and therefore, the stress is supported by the viscosity within the viscous sublayer. The profiles of the viscous stress rapidly fall to zero farther away from the surface in the bulk of the flow. For the low wind speed of 2.3 m~s {}^{-1} , the viscous stress at the surface contributes more than 80\% to the total stress. In winds higher than approximately 15 m~s {}^{-1} , however, viscosity ceases to play a significant role in the overall air-sea momentum flux.

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