A Three Dimensional PBL Parameterization to Improve Wind Simulations over Complex Terrrain

Numerical weather prediction (NWP) models are being run at sub-kilometer horizontal grid spacings. However, NWP models use planetary boundary layer (PBL) parameterizations that assume statistically homogeneous turbulent motions in the horizontal. Under this simplification, PBL schemes only account for one-dimensional (1D) mixing resulting from vertical gradients in turbulent fluxes. Although a convenient assumption for grid spacings of several kilometers, assuming horizontally homogeneous turbulent motions is no longer valid at sub-kilometer grid spacings. This limits the benefit of fine grid spacings over complex terrain regions. Hence, we propose a three-dimensional (3D) PBL parameterization to account for both vertical and horizontal gradients in turbulent fluxes.

The 3D PBL parameterization is based on the turbulence model developed by Mellor and Yamada (1982). Our implementation in the Weather Research and Forecasting (WRF) model proceeds in steps from a pure algebraic model that diagnoses the turbulent kinetic energy (TKE) equation to a higher level model that predicts the TKE. During each step, we compare results from the 3D PBL parameterization with both state-of-the-art 1D PBL and large eddy simulations (LES). These include both idealized simulations of the convective boundary layer and simulations over the Columbia River Gorge wherein the Wind Forecast Improvement Project 2 (WFIP2) field campaign is taking place. Over the WFIP2 region, WRF is configured with the innermost nest at 111 m of horizontal resolution using both 1D and 3D parameterizations whereas the WRF-LES is run with ~50 m grid spacing. The evaluation focuses on the performance of the 3D PBL scheme to reproduce the wind over complex terrain at resolutions where the assumption of horizontal homogeneity is no longer valid.