Influence of unstable thermal stratification on forest edge flows: Insights from large-eddy simulation

Forest edges are widespread transition zones that strongly influence forest microclimates and land–atmosphere exchanges. They modify the turbulent transfer of momentum, heat, and trace gases, with potential feedbacks on regional climate. Yet, their flow dynamics under non-neutral thermal stratification remain poorly understood. Using large-eddy simulations that couple canopy-scale biophysical processes with atmospheric boundary-layer (ABL) dynamics, we examine how thermal stratification, from neutral to free convection, modifies the micrometeorology at the transition between a crop and a dense forest over a flat terrain. Under windy but unstable conditions, the internal boundary layer (IBL) developing at the transition keeps the main turbulent edge-flow features identified under neutral conditions. Scalars adjust to the forest more slowly than momentum, due to their accumulation within the stable lower forest layer and their partial upward entrainment by intermittent intrusions of ABL-scale motions. Under free convection, the contrast in buoyancy forces between the crop and the forest generates thermally-driven, ABL-scale circulations, producing a weak near-surface breeze towards the forest and a persistent rising motion above the forest center. Although too weak to form a classical shear-driven IBL, this breeze still induces a sub-canopy jet similar to that in windy conditions. At the top of the forest canopy, thermally-driven turbulence is less effective at coupling with the forest interior than shear-driven turbulence under windy conditions, strengthening the stable layer in the understorey where scalar accumulate towards the forest center, where the breeze converges. Overall, the micrometeorological fields never fully adjust to the forest. These findings, albeit specific to the chosen forest and soil moisture conditions, offers valuable insights for environmental and ecological applications.