Wind Energy Forecasting Using a Three-Dimensional Planetary Boundary Layer Parameterization

Accurate numerical weather prediction (NWP) of atmospheric flows for wind energy applications is challenging in many geographical regions, oftentimes due to complex topography or horizontal variability in, for example, land characteristics. In NWP models, turbulent stresses and fluxes are handled typically using a one-dimensional (vertical) treatment based on the assumption of horizontal homogeneity. While this approach is convenient and appropriate when the horizontal grid spacing is relatively coarse (on the order of 10 km), it is not valid as the grid spacing approaches the “terra incognita” or “grey zone” (approximately 100 m to 1 km) because horizontal gradients in turbulent statistics become non-negligible. Consequently, three-dimensional (3D) effects are important, as the most energetic turbulent eddies are neither fully parameterized (traditional mesoscale simulations) nor fully resolved (traditional large eddy simulations).

To address the issue of assuming horizontal homogeneity in high-resolution mesoscale simulations – which are needed to improve wind forecasting in complex terrain – we have developed a 3D planetary boundary layer (PBL) parameterization for the Weather Research and Forecasting (WRF) model. Based on the algebraic model developed by Mellor and Yamada (1982), the new PBL scheme accounts for the 3D effects of turbulence by calculating explicitly the momentum, heat, and moisture flux divergence in addition to the turbulent kinetic energy (TKE). Recently, we implemented a prognostic form of the TKE equation into the 3D PBL parameterization. In the present study, we conduct numerical simulations to explore the potential benefit of using a prognostic equation, compared to using a diagnostic equation, for TKE in the context of wind energy forecasting.