Turbulent Fluxes and Coherent Structures in Marine Boundary Layers: Investigations by Large-Eddy Simulation

Abstract

The surface planetary boundary layer (PBL) is a dynamically important region in oceanic and atmospheric flows. This is because globally significant fluxes of momentum, energy, and materials occur across the air-sea boundary. The boundary fluxes cause large normal (vertical) gradients in the adjacent velocity and buoyancy fields. The consequent shear and convective instabilities give rise to vigorous turbulence that carries the boundary fluxes some distance into the fluid interior and thereby reshapes the mean vertical profiles. The vertical extent of the PBL is limited by the efficiency of turbulent transport away from its generation sites against its dissipation by cascade to small scales where molecular diffusion acts and its suppression by stable density stratification in the interior. The Coriolis force due to the Earth’s rotation is significant in PBL dynamics whenever the Rossby number, V * /fh (where V* is a turbulent velocity scale, f is the Coriolis frequency, and h is the PBL vertical extent), is not large; this typically is true for oceanic and atmospheric shear layers, but it is only atypically true for purely convective layers (it occurs in especially deep oceanic convection). We seek a fuller understanding of these processes, in order to better interpret PBL observations and develop more skillful models.