An Investigation of Coupled Atmospheric-Oceanic Boundary Layers Using Large Eddy Simulation (Core Science Keynote)

The marine atmospheric and oceanic boundary layers (ABL and OBL) are a two-way coupled dynamical 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 which drive 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, \eg $(\triangle x_o, \triangle x_a) \,=\, (1.22, 4.88)$\,m. Custom 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. The spanwise scale of the ABL turbulence is $\sim 1000$\,m while the depth of the OBL is $\sim 20$\,m. This surprising result is shown by comparing simulations that use either instantaneous or $x\text-y$ spatial coupling rules. 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. We found that the spectral slopes in the ABL depend on stability, with E(ky) ~ -1.9 in the convective regime and E(ky) ~ -1.1 in the convective roll regime in agreement with recent observations.