Improved subfilter turbulence modeling for large eddy simulation using WRF

A new suite of models for representing subfilter-scale (SFS) turbulence stresses has been implemented into the Weather Research and Forecasting (WRF) model for improved large eddy simulation (LES), particularly in the presence of complex terrain. Our subfilter models are physically based, permit backscatter of energy, and do not assume a local balance between turbulence production and dissipation as many SFS models do. As such, our models are well suited to simulating flow in urban environments, where eddies tend to be smaller and turbulence production and dissipation are frequently not in local balance. Our approach is based upon reconstruction of the resolvable subfilter-scale (RSFS) stresses combined with a subgrid-scale (SGS) eddy viscosity model. Velocity scales near the grid cutoff that are attenuated by discretization effects are reconstructed to compute the RSFS stresses. The RSFS stresses are then added to the SGS component to account for all of the subfilter stresses. The SGS model can be chosen from either of WRF's two native models (Smagorinsky or TKE-based) or from among our new implementations which include both a nonlinear and a dynamic eddy-viscosity model and a near-wall canopy stress model. The greatest improvements are achieved with our combined RSFS and our new SGS models. Results from idealized test cases illustrate the improvements of our models over WRF's standard LES closures.

This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. UCRL-ABS-230878