Abstract

As wind farms grow in number and size worldwide, it is important that their potential impacts on the environment are studied and understood. The Fitch parameterization implemented in the Weather Research and Forecasting (WRF) model since version 3.3 is a widely used tool today to study such impacts. We identified two important issues related to the way the added turbulent kinetic energy (TKE) generated by a wind farm is treated in the WRF model with the Fitch parameterization. The first issue is a simple bug in the WRF code and the second issue is the excessive value of a coefficient that relates TKE to the turbine electro-mechanical losses, called CTKE. These two issues directly affect the way a wind farm wake evolves and they impact properties like near-surface temperature and wind speed at the wind farm as well as behind it in the wake. We provide a bug fix and a revised value of CTKE that is one quarter of the original value. This 0.25 correction factor is empirical; future studies should examine its dependency on parameters such as atmospheric stability, grid resolution, and wind farm layout. We present the results obtained with the Fitch parameterization in the WRF model for a single turbine with and without the bug fix and the corrected CTKE and compare them against high-fidelity Large-Eddy Simulations (LES). These two issues have not been discovered before because they interact with one another in such a way that their combined effect is a somewhat realistic vertical TKE profile at the wind farm and a realistic wind speed deficit in the wake. All WRF simulations that used the Fitch wind farm parameterization are affected and their conclusions may need to be revisited.

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