Three-Dimensional Planetary Boundary Layer Parameterization for High-Resolution Mesoscale Simulations of Flows Over Complex Terrain

In numerical weather prediction (NWP) models turbulent stresses and fluxes are commonly parameterized using one-dimensional parameterizations based on the assumption of horizontal homogeneity. If horizontal grid cell sizes are relatively large (e.g., greater than 10 km) this assumption is justified. As the grid-cell size of mesoscale simulations decreases the assumption of horizontal homogeneity is violated. Recently Honnert and Masson (2014) and Honnert et al. (2016) used large-eddy simulations (LES) of convective boundary dry and cloud-topped boundary layers over flat terrain to determine the scale at which three-dimensional turbulence effects cannot be neglected due to the presence of large convective structures. Three-dimensional effects at mesoscale grid-cell size are particularly pronounced in flows over complex terrain, characterized by either complex topography or surface heterogeneities (e.g., land-sea interface). To study turbulence effects in complex terrain at different scales we therefore carried out LES based on the Wind Forecast Improvement 2 (WFIP2) field study from 2016 and 2017 in Columbia River Gorge area. Field study observations and validated LES results are then used to assess the new three-dimensional planetary boundary layer (3D-PBL) parameterization implemented in the Weather Research and Forecasting model based on the work of Mellor and Yamada (1982). We focus on selected cases when physical phenomena of significance for forecasting in complex terrain such as mountain waves, topographic wakes, and gap flows were observed. We use filtered LES fields to compute turbulence quantities at different scales and provide an estimate of a scale below which 3D PBL parameterization should be used.