Resolving a forest-strip induced uplift region using the shaved-grid-cell method with large eddy simulations

Several wind tunnel observations and simulation studies of backward and forward facing steps have described distinct, barrier-induced uplift and downdraft regions. In solid obstacles, the front of the obstacle induces uplift while the downwind end of the barrier induces downdraft. In forest-like semi-porous forward facing steps the uplift zone is advected downwind from the barrier's front edge, at a distance of about 2-8 times the canopy height. A wind break or a narrow strip of forest resembles a narrow obstacle, rather than a forward facing step. Observations we conducted with lidar indicate that the back of the obstacle, and not the front, is associated with uplift.

The RAMS based Forest Large Eddy Simulation (RAFLES) is a large eddy simulation model of flows inside and above forested canopies. RAFLES is spatially explicit, uses the finite volume method to solve the Navier-Stokes equations and accounts for vegetation drag effects on the flow and on the flux exchange between the canopy and the canopy air. The model uses satellite images of forests combined with empirical biological structural data to derive explicit or simplified, 3D heterogeneous simulation domains that include leaf density and tree-stem and branch volumes. For a better representation of the vegetation structure in the numerical grid within the canopy sub-domain, the model uses a modified version of the shaved-grid coordinate system. The hard volume of vegetation elements within each numerical grid cell is represented as a sub-grid-scale process that shrinks the open apertures between grid cells and reduces the open cell volume. Vegetation effects are also attributed by a drag force that acts on the flow through the semi-porous canopy and is proportional to the local leaf density.

We used RAFLES to simulate the effects of forest strips of varying widths and foliage coverage on the mean vertical velocity and on the turbulent kinetic energy under neutrally buoyant conditions. We explicitly tested the effects of the hard volume of the canopy and the effects of the leaf drag by simulating a large number of virtual canopies with various stem diameters and leaf coverage ranging from very sparse to un-naturally dense. The results show that flow through semi-porous forest barriers leads to an uplift at the downwind end of the barrier. The implications of these findings are discussed for dispersion over a narrow forest patch.