The Effects of Canopy Leaf Area Index on Airflow Across Forest Edges: Large-eddy Simulation and Analytical Results

Abstract

The structure of turbulent flows along a transition between tall-forested canopies and forest clearings continues to be an active research topic in canopy turbulence. The difficulties in describing the turbulent flow along these transitions stem from the fact that the vertical structure of the canopy and its leaf area distribution cannot be ignored or represented by an effective roughness length. Large-eddy simulation (LES) runs were performed to explore the effect of a homogeneous variation in the forest leaf area index (LAI) on the turbulent flow across forest edges. A nested grid numerical method was used to ensure the development of a deep boundary layer above the forest while maintaining a sufficiently high resolution in the region close to the ground. It was demonstrated that 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. In the simulations reported here, the LAI was varied between 2 and 8 spanning a broad range of observed LAI in terrestrial ecosystems. By increasing the forest LAI, the mean flow properties both within the forest and in the clearing near the forest edge were altered in two fundamental ways: near the forest edge and into the clearing, the flow statistical properties resembled the so-called back-facing step (BFS) flow with a mean recirculation zone near the edge. Another recirculation zone sets up downstream of the clearing as the flow enters the tall forest canopy. The genesis of this within-forest recirculation zone can be primarily described using the interplay between the mean pressure gradients (forcing the flow) and the drag force (opposing the flow). Using the LES results, a simplified analytical model was also proposed to explain the location of the recirculation zone inside the canopy and its dependence on the forest LAI. Furthermore, a simplified scaling argument that decomposes the mean velocity at the outflow edge into a superposition of 'exit flow' and BFS-like flow with their relative importance determined by LAI was explored.

Characterisation of drag and wake properties of canopy patches immersed in turbulent boundary layers

“…a reverse ow region) were observed to disappear in some PIV snapshots. Similar intermittent features of the recirculation region are reported by Cassiani et al (2008) who conducted large eddy simulations to simulate turbulent ows across forest edges. Anyway, since the focus of the present paper is on the mean ow and drag properties of porous patches these instantaneous features of the ow arenot discussed further and we leave them to be addressed in further work.…”

Predicting the dry deposition of aerosol‐sized particles using layer‐resolved canopy and pipe flow analogy models: Role of turbophoresis

“…This may have induced secondary recirculation zones between their roughness elements and the open ground that enhanced particle deposition. We should note that the existence of such secondary recirculation zones between more extensive canopies and open gaps has already been documented in recent forest edge experiments [Detto et al, 2008] and detailed large eddy simulation runs [Cassiani et al, 2008]. Both studies confirm that these recirculation zones are Comparison between measured (diamond) and modeled particle flux ratio (subcanopy to above-canopy) as a function of the particle Reynolds number (Re p ) derived by using above-canopy u * for all the runs.…”

The Parallelized Large-Eddy Simulation Model (PALM) version 4.0 for atmospheric and oceanic flows: model formulation, recent developments, and future perspectives

“…This rapid breakdown of turbulence is also known as the spectral shortcut (e.g., Shaw and Patton, 2003). This type of canopy model has been successfully applied by various authors to study turbulent flows inside and above homogeneous as well as heterogeneous canopies such as forest edges (Cassiani et al, 2008;Finnigan et al, 2009;Dupont and Brunet, 2009, among others).…”