A subgridscale model for largeeddy simulation of planetary boundarylayer flows
Abstract
A longstanding problem in largeeddy simulations (LES) of the planetary boundary layer (PBL) is that the mean wind and temperature profiles differ from the MoninObukhov similarity forms in the surface layer. This shortcoming of LES has been attributed to poor grid resolution and inadequate subgridscale (SGS) modeling. We study this deficiency in PBL LES solutions calculated over a range of shear and buoyancy forcing conditions. The discrepancy from similarity forms becomes larger with increasing shear and smaller buoyancy forcing, and persists even with substantial horizontal grid refinement. With strong buoyancy forcing, however, the error is negligible. In order to achieve better agreement between LES and similarity forms in the surface layer, a twopart SGS eddyviscosity model is proposed. The model preserves the usual SGS turbulent kinetic energy formulation for the SGS eddy viscosity, but it explicitly includes a contribution from the mean flow and a reduction of the contributions from the turbulent fluctuations near the surface. Solutions with the new model yield increased fluctuation amplitudes near the surface and better correspondence with similarity forms out to a distance of 0.1 0.2 times the PBL depth, i.e., a typical surfacelayer depth. These results are also found to be independent of grid anisotropy. The new model is simple to implement and computationally inexpensive.
 Publication:

BoundaryLayer Meteorology
 Pub Date:
 November 1994
 DOI:
 10.1007/BF00713741
 Bibcode:
 1994BoLMe..71..247S