The stable boundary layer (SBL) in the atmosphere is of considerable interest because it is often the 'worst case' scenario for air pollution studies and health effect assessments associated with the accidental release of toxic material. Traditional modeling approaches used in such studies do not simulate the non-steady character of the velocity field, and hence often overpredict concentrations while underpredicting spatial coverage of potentially harmful concentrations of airborne material. Large-eddy simulation (LES) can reproduce the non-steady character of the SBL. The challenge for LES is to be able to resolve the rather small energy-containing eddies of the SBL while still maintaining an adequate domain size. This requires that the subgrid-scale (SGS) parameterization of turbulence incorporate an adequate representation of turbulent energy transfer. Recent studies have shown that both upscale and downscale energy transfer can occur simultaneously, but that overall the net transfer is downscale. Including the upscale transfer of turbulent energy (energy backscatter) is particularly important near the ground and under stably-stratified conditions. The subgrid-scale (SGS) turbulence model used in our LES approach is a dynamic, mixed model that allows the upscale transfer (backscatter) of energy and has made possible the simulation of episodes of enhanced turbulence in the SBL. The turbulent episode is associated with the breakdown of large-scale wave-like activity in the upper part of the SBL. Such episodes of enhanced turbulence have been observed in the SBL (i.e. Coulter, 1990).

Large-eddy simulations of the SBL have been conducted for a range of meteorological forcing conditions in terms geostrophic wind speed and surface cooling. The simulations illustrate the key role of mechanical turbulence supported by the geostrophic forcing, and the lesser competing effects of turbulence damping by buoyancy that develops in response to the surface cooling. With sufficient geostrophic forcing, the SBL has continuous turbulence with periods of enhanced turbulence. As the surface cooling is increased and/or the geostrophic forcing decreased, the periods of enhanced turbulence become less prevalent and the overall intensity of continuous turbulence is reduced. The SBL exhibits a two-regime structure in the vertical direction, with vorticity dominating the lower part and larger-scale wave-like motions dominating the upper part and continuing above the SBL. Episodes of enhanced turbulence are seen mainly in the upper part of the SBL, where a mix of waves and turbulence exist and turbulent fluxes are often countergradient due to overturning. In contrast, turbulence in the near-surface region remains rather constant and turbulent fluxes are downgradient.