The effects of canopy leaf area index on airflow across forest edges

Large-eddy simulations (LES) were performed to explore the effect of a homogeneous variation in the forest leaf area index (LAI) on the mean characteristic of the turbulent flow across forest edges. The LES here predicted first-order and second-order mean velocity statistics within the canopy that agree with reported Reynolds-Averaged Navier-Stokes (RANS) model results, field and laboratory experiments. The LAI was varied between 2 and 8. By increasing the forest LAI, the mean flow properties were found to be altered in two fundamental ways: at the forest-to-clearing transition a recirculation was observed at higher LAI. Another recirculation zone sets up downstream of the clearing-to-forest transition as the flow enters the tall forest canopy. The genesis of this within-forest recirculation zone was investigated and a simplified analytical model was also proposed to explain the initial location of the negative mean velocity. A simplified scaling argument that decomposes the mean velocity at the forest-to-clearing edge into a superposition of exit flow and recirculating flow with their relative importance determined by LAI was investigated.