The structure of stably stratified atmospheric boundary layers: A large‐eddy simulation study

Dry stably‐stratified planetary boundary layers have been studied by large‐eddy simulation. Results from simulations using two different subgrid‐scale models are compared. A recently suggested improved subgrid‐scale model version is shown to give improvement in the near‐surface region comparable to what have previously been obtained with inclusion of random subgrid‐scale stresses. A comparison of two flows with or without wave motions aloft shows that effects of these waves are negligible within the boundary layer. This is attributed to the small amplitudes of these waves, being forced by the turbulent eddies in the planetary boundary layer. an analysis of second moments and their budgets shows that a quasi‐steady regime has been reached for turbulence statistics, whereas mean fields are, as expected, still evolving. Accordingly profiles of vertical fluxes deviate from the forms assumed within local scaling theories. A picture of eddy structure in the turbulent layer is obtained through a combination of quadrant analysis of vertical fluxes and distributions of vorticity vector orientations. It is found that 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 transition from a typical shear‐flow structure near the surface to what might be expected for an internal‐wave field above the boundary layer.