Numerical Simulation and Modeling of Air-Sea Coupling at Small Scales

We use direct simulation of air and water turbulent flows to study atmosphere-ocean coupling dynamics at low wind speeds at small spatial scales. The fluid motions are solved by simulation of the Navier-Stoke equations subject to air-water coupled free-surface kinematic and dynamic boundary conditions, and the transport of passive scalars is simulated with a convection-diffusion equation. It is found that the flow feature at the air-water interface is mainly controlled by the underlying water motions. The flow structure near the waterside is characterized by both coherent structures approaching from the bulk flow underneath and the wind stress disturbance, which makes the aqueous boundary layer qualitatively distinct from the boundary layers near a solid wall or a shear-free free surface. On the airside, a micro low-level jet structure is identified which is found to be induced by waterside upwelling and streamwise vortices. Based on the extensive simulation data obtained, we are able to quantify flow statistics in terms of turbulent kinetic energy budget and wind momentum and energy input to the wave field, which are useful for the comparison and cross-validation with field measurements.