Characteristics of turbulent slope flows in stably-stratified environments: numerical results from Large-Eddy Simulation

Over the past couple of decades, Large Eddy Simulation (LES) has become a successful and popular tool to numerically investigate the structure and characteristics of the atmospheric boundary layer (ABL). However, its applicability to stably-stratified conditions is still an area of ongoing research.

In this study, LES is used to develop, maintain, and analyze turbulent slope flows within a stably-stratified environment. The background thermal stratification of the environment is taken to be constant (potential temperature increases linearly with height) and the Coriolis force is set to zero. Initially, the velocity field is taken to be at rest. The slope flows are then induced in the model by applying a surface buoyancy forcing across a sloped, planar surface. The resulting velocity field is characterized by an extremely shallow near-surface jet and weak return flow aloft. This experiment is repeated for a range of slope angles and various subgrid-scale (SGS) models that are commonly used for LES of ABL studies. Once the slope flows are in a quasi-steady state, an analysis of the turbulence kinetic energy (TKE) budget is presented for each case. In this analysis, the buoyancy production of TKE is found to have significant components in the along-slope and slope-normal directions due to the sloping nature of the surface, which violates the assumptions made for Monin-Obukhov similarity theory. Additionally, large magnitudes of the TKE production terms, which are commonly used in SGS models, are observed near the sloping surface. Both of these features have implications on the type of SGS model that would be considered the most suitable for LES of these high-shear, stably-stratified conditions.