Turbulent Flow across a clearing-forest transition: A Large Eddy Simulation Study

Abstract Forest fragmentation leads to changes in scalar concentration and momentum fluxes. These changes create superficial heterogeneity in forests, affecting mass, energy, and meteorological flows across various spatial scales. The abrupt alteration of canopy roughness from clearing-to-forest wind flow triggers instabilities downstream of the forest border, revealing the significant contribution of advective flows. Current methodologies investigate canopy flows with idealized foliage distribution, using the same Leaf Area Index as the experimental data. In these experiments, leaf density is typically higher near the top of the canopy. To simulate clearing-to-forest wind flow, we employ Large Eddy Simulation (LES) with momentum drag dependent on foliage density. The maximum value of this drag occurs at the middle of the forest height (0.5h). Our results demonstrate a well-adjusted turbulent profile downstream of the forest. Near the border, increased skewness at the top and in the middle of the canopy indicates a large variability in the wind field. At the top of the canopy, we find maximum vertical momentum fluxes, and at z=0.5h, positive values indicate downward fluxes. Within the canopy, the dominant structures are sweeps, representing gust events responsible for momentum flux transport from the wind to the forest. Despite the profiles being well-resolved in the clearing and deep into the forest, close to the border, high levels of turbulence create instabilities, increasing uncertainty in LES simulations. To address this, it is necessary to refine the clearing fetch dimension and inflow boundary conditions.