Sensitivity of multiscale large Eddy simulations for wind power calculations in complex terrain

Coupling Large Eddy Simulations (LES) with Numerical Weather Prediction (NWP) models has promising applications for wind energy. Regional climatology, optimal siting of wind turbines as well as short term wind energy forecasts can be improved by considering all the energetic scales of atmospheric motions. However, the complexity of NWP-LES coupled simulations introduces challenges and uncertainties that need to be addressed. This study focuses on understanding the relative importance of different factors and assumptions in NWP-LES calculations for wind energy applications. Using a recent large ensemble of LES simulations driven by NWP boundary conditions over the complex terrain of the Perdigão area, our analysis reveals significant discrepancies in wind power estimates across ensemble members, particularly over hilltops. Depending on the model configuration and the coupling technique, instantaneous predictions can be as sensitive as 800 kW for a 2 MW wind turbine, in terms of ensemble standard deviation. On multi-day time averages, the model sensitivity is in the order of 150 kW. We further analyze the main factors that lead to the observed model sensitivity. Results from a four-way analysis of variance identify topography and land use datasets as the primary drivers of variability, for time averaged estimates. The temporal analysis shows strong inter-daily variability and the importance of turbulence modeling and the coupling techniques for instantaneous predictions. Overall, most of the sensitivity is observed during day-to-night and synoptic transitions. By understanding the relative importance of different factors, future model development and applications can be informed to enhance the accuracy and reliability of wind energy assessments.