Grid convergence in a large-eddy simulation of the stable boundary layer

The effects of stable stratification in the atmosphere and more specifically the stable atmospheric boundary layer present many challenges for numerical models at all scales. In order to improve the simulation of stable atmospheric conditions, we develop and validate a subgrid scale turbulence closure suitable for the large-eddy simulation of stratified turbulent flows. The SGS closure is based on the stretched-vortex SGS model, a physics-based LES-SGS closure. The novel buoyancy correction accounts for unstably and stably stratified conditions, it is consistent with several aspects of stratified turbulence dynamics, takes into account the anisotropy of motions and mixing as stratification increases and contains no flow-adjustable parameters. The buoyancy correction reverts to the standard formulation of the SGS closure for neutrally stable flows, thus allowing the seamless simulation of conditions with variable stability. Using the new SGS closure we perform LES of a stable boundary layer corresponding to the conditions of the GABLS field campaign. The mean profiles of wind, potential temperature and buoyancy flux exhibit grid resolution independence even at resolutions that can be considered coarse. A criterion for convergence for stably stratified conditions based on the ratio of subgrid scale to total turbulence kinetic energy is discussed.