Abstract
The air–sea momentum exchanges in the presence of surface waves play an integral role in coupling the atmosphere and the ocean. In the current study, we present a detailed laboratory investigation of the momentum fluxes over wind-generated waves. Experiments were performed in the large wind-wave facility at the Air–Sea Interaction Laboratory of the University of Delaware. Airflow velocity measurements were acquired above wind waves using a combination of particle image velocimetry and laser-induced fluorescence techniques. The momentum budget is examined using a wave-following orthogonal curvilinear coordinate system. In the wave boundary layer, the phase-averaged turbulent stress is intense (weak) and positive downwind (upwind) of the crests. The wave-induced stress is also positive on the windward and leeward sides of wave crests but with asymmetric intensities. These regions of positive wave stress are intertwined with regions of negative wave stress just above wave crests and downwind of wave troughs. Likewise, at the interface, the viscous stress exhibits along-wave phase-locked variations with maxima upwind of the wave crests. As a general trend, the mean profiles of the wave-induced stress decrease to a negative minimum from a near-zero value far from the surface and then increase rapidly to a positive value near the interface where the turbulent stress is reduced. Far away from the surface, however, the turbulent stress is nearly equal to the total stress. Very close to the surface, in the viscous sublayer, the wave and turbulent stresses vanish, and therefore the stress is supported by the viscosity.
Highlights
The momentum flux or wind stress is a fundamental parameter in the study of air–sea interactions
We presented a detailed investigation of the mean, wave-induced and turbulent momentum fluxes in the airflow over wind-generated surface waves
Quantitative airside velocity measurements were obtained in the laboratory over wind-driven surface waves at a fixed fetch and for several forcing conditions corresponding to equivalent 10 m wind speeds ranging from 0.89 to 16.59 m s−1
Summary
The momentum flux or wind stress is a fundamental parameter in the study of air–sea interactions. Air–sea momentum fluxes provide crucial boundary conditions for ocean, atmosphere and surface wave models. Modern parameterizations of the air–sea momentum flux at the ocean surface remain insufficient, especially in strongly forced conditions (Hara & Sullivan 2015). The situation over wavy liquid surfaces, is further complicated by the fact that a component of the stress is carried by the surface waves, which may strongly impact the air–sea momentum flux. The air–sea momentum flux or total stress over the ocean is expressed as the sum of turbulent, wave-induced and viscous stress components We examine turbulent, wave-induced and viscous stresses sequentially before evaluating the total air–water stress balance in the final section
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