Large‑Eddy Simulation Analysis of Momentum Balance and Turbulent Adjustment across a Forest‑to‑Clearing Roughness Change

Abrupt transitions from forest to clearing create complex shear layers, pressure gradients, and coherent vortices that challenge conventional atmospheric‑flow models. Here we apply large‑eddy simulation (LES) with a Localised Dynamic Kinetic sub‑grid model to reproduce a canonical forest–clearing configuration previously studied in a wind tunnel. The vegetation is represented through vertically distributed drag and leaf‑area density terms that absorb momentum and generate canopy turbulence. Model validation against particle‑image‑velocimetry and hot‑wire data demonstrates good agreement for mean velocity, friction velocity, and turbulence intensities within the canopy, while highlighting a systematic over‑prediction of the Reynolds stress \(\overline{u'w'}\) above the canopy. Momentum‑budget diagnostics reveal that advection, pressure gradients, and Reynolds‑stress divergences peak within five canopy heights of the edge, and that their combined effect sustains elevated turbulent stresses up to \(x/h \approx 20\). Beyond this fetch the flow relaxes: turbulent stresses decay, the internal boundary layer thickens, and the turbulent length scale \(L_s\) grows, indicating adjustment to the smoother clearing. Despite local discrepancies, the global momentum balance closes satisfactorily downstream of \(x/h \approx 10\), confirming that the dominant transport mechanisms are resolved. The results underscore the need to represent canopy heterogeneity and roughness discontinuities in mesoscale and climate models, and they establish LES as a robust framework for developing realistic parameterisations of fragmented landscapes with direct implications for pollutant dispersion, micro‑climate regulation, and land‑use planning.