Impact of Forest Density on Atmospheric Boundary-Layer Flow and Turbulence for Wind Energy Applications

Abstract Nowadays, it is common to build wind farms in forested areas as wind-energy production is growing rapidly. However, surrounding forests change the local wind conditions drastically, and therefore, there is a need to understand the interaction between local wind conditions and forest properties. The present work aims to investigate the impact of various forest densities on the flow characteristics in the lower part of the Atmospheric Boundary-Layer (ABL). Here, Large-Eddy Simulations (LES) of wind flow over horizontally homogeneous forest canopies were carried out for four different forest densities. The selected forest cases range from extremely dense forests with a Leaf Area Index (LAI) of 10.5 to extremely sparse forests with an LAI of 0.44. The LES results reveal a strong impact of forest density on wind shear and turbulence at rotor-relevant heights. At hub height, the wind-shear exponent varies by up to 16% across the four forest densities, while turbulent kinetic energy (TKE) shows a non-monotonic dependence on LAI. Intermediate forests (LAI $$\approx $$ 2.2 $$-$$ 4.4) produce up to 24% higher hub-height TKE than both sparse (LAI = 0.44) and dense (LAI = 10.5) canopies, leading to stronger turbulence intensities and Reynolds shear stresses. At hub height, horizontal integral-length scale varies by as much as 63%, and the gust factor by about 8% across the forest cases. These variations directly affect turbine performance: higher turbulence levels and gust factors increase fatigue loads and extreme stresses, but also promote faster wake recovery and improved farm efficiency. In contrast, extremely sparse or dense canopies suppress hub-height turbulence, resulting in slower wake mixing and less favourable conditions for large wind-farm layouts. The findings emphasize that optimal wind-farm siting in forested landscapes critically depends on canopy density, with intermediate LAI values presenting both challenges and opportunities for turbine operation.