Examination of the Spatio-temporal Variability of the Terms of the Turbulent Kinetic Energy Budget over a Complex Terrain in the Convective Boundary Layer: A Tool for Parameterization Development

An accurate representation of the spatial variability of the Turbulent Kinetic Energy (TKE) in large-scale models, such as in the Weather Research and Forecasting (WRF) model is a key part of simulations to assess the wind energy resource and for short-term forecast. Current large-scale models parameterize turbulence only in the vertical direction, whereas horizontally homogeneity is assumed within the mesoscale model grid cell, which can span kilometers.  This assumption may hold for cases with flat terrain in which a coarser grid spacing, but could be more problematic in regions of complex terrain. This work considers the spatial variability in terms of quantitative and qualitative results particularly for areas with both simple and complex terrain. In this study, a three dimensional flow field was generated using real-case WRF-LES (large-eddy simulation) for a region of complex terrain centered on the location of the Columbia Basin Wind Energy Study (CBWES – also within Wind Forecast Improvement Project 2 (WFIP2) study region) and over a location with flat terrain (Scaled Wind Farm Technologies (SWiFT) -  Texas, Panhandle). We calculated every term of the TKE budget in a number of vertical and horizontal planes and vertical profiles within both domains. The relative contribution to the TKE tendency due to both horizontal and vertical components of the budget terms was computed, and the WRF-LES results from the two were compared. The simulations showed a great deal of spatial variability in both the CBWES and SWiFT cases. Both horizontal and vertical components of budget terms exhibited a comparable contribution to the TKE. The variability within the complex terrain domain was found to be greater than in the flat terrain case.  Along with this result, the quantitative analysis of each term suggests that the effects of horizontal variability of the TKE should be included in the parameterizations applied in large-scale models in order to better represent the impact of boundary-layer turbulence.