4.2 Evaluation of a three-dimensional PBL parameterization for simulations of flow over complex terrain

Kosovic, Branko, and Pedro Jimenez Munoz, National Center for Atmospheric Research

Currently, most numerical weather prediction (NWP) models include a one-dimensional parameterizations of planetary boundary layer (PBL). These parameterizations are based on the assumption of horizontal homogeneity. As the grid cell size of mesoscale simulations decreases, the assumption of homogeneity is violated, in particular in complex terrain. For high-resolution mesoscale simulations of flows in complex terrain, we have therefore implemented a three-dimensional (3D) PBL parameterization in the Weather Research and Forecasting (WRF) model. The implementation of the 3D PBL scheme follows the developments outlined by Mellor and Yamada (1974, 1982). To evaluate 3D PBL model performance we use observations from the Wind Forecast Improvement Project 2 (WFIP2). The WFIP2 field study took place in the Columbia River Gorge area during 2016. We focus on selected cases when physical phenomena of significance for wind energy applications such as mountain waves, topographic wakes and gap flows were observed. In addition to observations for assessment of the 3D PBL parameterization we also use a large-eddy simulation (LES). We carried out a nested LES with grid cell sizes of 30 m and 10 m covering a large fraction of the WFIP2 study area. Both LES domains were discretized using 6000 x 3000 x 200 grid cells in zonal, meridional, and vertical direction, respectively. The LES results are used to assess the relative magnitude of horizontal gradients of turbulent stresses and fluxes in comparison to vertical gradients.