Validation of Multi-Scale Simulations of the Flow over Big Southern Butte Using the Weather Research and Forecasting Model

Advances in high performance computational resources and frameworks now make possible the use of Numerical Weather Predication (NWP) models for high-resolution simulations of atmospheric flows. In order to develop best practices, standards, and procedures for multi-scale simulations, we need to carry out extensive validation of NWP models across unprecedented range of scales from hundreds of kilometers to tens of meters. However, turbulence parameterizations in planetary boundary layer (PBL) schemes used in NWP models are based on the assumption that turbulent motions are statistically homogeneous in horizontal planes over a grid cell. While this assumption can be justified as approximately valid when horizontal grid cell sizes are greater than several kilometers, the assumption breaks down in complex terrain as grid cell sizes are reduced. We therefore propose developing a fully three-dimensional (3D) PBL scheme that accounts for horizontal divergence of turbulent stresses and horizontal gradients of turbulent fluxes. The proposed three-dimensional PBL scheme is based on the work of Mellor and Yamada (1982) utilizing algebraic turbulence stress and flux parameterization.

The development of the three-dimensional PBL scheme proceeds in steps from a reduced algebraic model including a diagnostic equation for turbulent kinetic energy (TKE) to a full algebraic model with a prognostic equation for TKE and potential temperature variance. At each step of development we compare the performance of the 3D PBL scheme in comparison to existing one-dimensional PBL parameterizations implemented in the Weather Research and Forecasting (WRF) model. For the purpose of validation of multi-scale simulations using the WRF model we use data from a field study in Idaho carried out in 2010 and centered on the Big Southern Butte. We carry out both mesoscale simulations and large-eddy simulations (LES). Nested mesoscale simulations are carried out using the innermost nest with grid cell size of 111 m with both 1D and 3d PBL schemes while nested WRF-LES are carried with grid cell size of ~50m. The results of this analysis are used to assess the performance of the new 3D PBL schemes in complex terrain where the assumption of horizontal homogeneity is violated.