An investigation of coupled atmospheric and oceanic boundary layers using large-eddy simulation

The marine atmospheric boundary layer (ABL) and oceanic boundary layer (OBL) are a two-way coupled system. At the ocean surface, the ABL and OBL share surface fluxes of momentum and buoyancy that incorporate variations in sea surface temperature (SST) and currents. To investigate the interactions, a coupled ABL–OBL large-eddy simulation (LES) code is developed and exercised over a range of atmospheric stability. At each time step, the coupling algorithm passes oceanic currents and SST to the atmospheric LES, which in turn computes surface momentum, temperature, and humidity fluxes driving the oceanic LES. Equations for each medium are time advanced using the same time step but utilize different grid resolutions: the horizontal grid resolution in the ocean is approximately four times finer, e.g., (Δ x o , Δ x a ) = (1.22, 4.88) m. Interpolation and anterpolation (its adjoint) routines connect the atmosphere and ocean surface layers. In the simplest setup of a statistically horizontally homogeneous flow, the largest scale ABL turbulent shear-convective rolls leave an imprint on the OBL currents in the upper layers. This result is shown by comparing simulations that use coupling rules that are applied either instantaneously at every x – y grid point or averaged across an x – y plane. The spanwise scale of the ABL turbulence is ∼1000 m, while the depth of the OBL is ∼20 m. In these homogeneous, fully coupled cases, the large-scale spatially intermittent turbulent structures in the ABL modulate SST, currents, and the connecting momentum and buoyancy fluxes, but the mean profiles in each medium are only slightly different.