Is WRF-LES a Suitable Tool for Realistic Turbulence Analyses in Wind Resource Assessment Applications?

Large eddy simulations (LES) have traditionally been used to study the planetary boundary layer (PBL) fluxes under different ideal conditions and such as a laboratory for testing and improving turbulence parameterizations. With the exponential increment of the computational resources, real cases are about to be "feasible" for a LES approach. Recently, different studies have reported promising results that motivate the research in LES technology for wind resource industry applications.

In this work, the Weather Research and Forecasting LES (WRF-LES) model is assessed to obtain reliable turbulence information that wind and site engineers require. The WRF-LES formulation employed in these studies follows the potential temperature perturbation method proposed by Muñoz-Esparza et al. (2014) which has been adapted to real terrain cases driven by dynamic boundary conditions provided by reanalysis data-sets. The main questions this work wants to answer are: i) Is WRF-LES model a robust system for producing wind speed time-series? ii) Can real WRF-LES simulations generate turbulence? And more important, iii) is the turbulence pattern obtained from WRF-LES simulations realistic?

Answers to these questions are addressed within the frame of a comprehensive validation exercise where one year period WRF turbulence parameterized (WRF-PBL) and WRF-LES runs were carried out at eight different sites over Europe, South America and South Africa. Selected sites represent varying degrees of complexity in terrain characteristics as well as in synoptic and local weather regimes. A validation against wind mast observations (i.e. compliant with industry standards) was conducted.

The results show that WRF-LES outcomes provide realistic 10-min averaged wind condition patterns, remarkably improving WRF-PBL results for wind speed distribution shape and tail. Turbulence intensity was effectively triggered in WRF-LES, improving the previous results obtained by Montornes et. al. (2015a, 2015b). WRF-LES shows a good agreement with respect to the observations with typical biases ranging between 3% and 5% for winds higher than 5 m/s. These results represent a significant step forward to consolidate WRF-LES technology as a reliable modeling solution for wind and site engineers and wind turbine manufacturers demand of realistic turbulence characterization. It also confirms the perturbed method as a robust turbulence's trigger.

Muñoz-Esparza, D., Kosovic, B., Mirocha, J., and van Beeck, J.: Bridging the transition from mesoscale to microscale turbulence in numerical weather prediction models. Boundary-Layer Meteorology, 153(3), 409-440. 2014.

Montornes, A., Kosovic, B., Casso, P., Lizcano, G. Can mesoscale models reach the microscale? EWEA Resource Assessment Workshop. 2015a.

Montornes, A., Casso, P., Lizcano, G., Moreno, P.: Towards next generation of wind resource modeled time-series. 3rd International Conference of Energy and Meteorology. 2015b.