THE STRUCTURE OF STABLY STRATIFIED ATMOSPHERIC BOUNDARY-LAYERS - A LARGE-EDDY SIMULATION STUDY

Dry stably-stratified planetary boundary layers have been studied by l arge-eddy simulation. Results from simulations using two different sub grid-scale models are compared. A recently suggested improved subgrid- scale model version is shown to give improvement in the near-surface r egion comparable to what have previously been obtained with inclusion of random subgrid-scale stresses. A comparison of two flows with or wi thout wave motions aloft shows that effects of these waves are negligi ble within the boundary layer. This is attributed to the small amplitu des of these waves, being forced by the turbulent eddies in the planet ary boundary layer. An analysis of second moments and their budgets sh ows that a quasi-steady regime has been reached for turbulence statist ics, whereas mean fields are, as expected, still evolving. Accordingly profiles of vertical fluxes deviate from the forms assumed within loc al scaling theories.A picture of eddy structure in the turbulent layer is obtained through a combination of quadrant analysis of vertical fl uxes and distributions of vorticity vector orientations. It is found t hat bursts dominate the fluxes. A few intense events make up for most of the flux. This effect of intermittency increases towards the top of the boundary layer. Distributions of vorticity vectors indicate a tra nsition from a typical shear-now structure near the surface to what mi ght be expected for an internal-wave field above the boundary layer.