Large-eddy simulation of tracer gas transport and dispersion within and above a successively thinned loblolly pine canopy

Transport of mass, momentum, and scalars into and out of a forest canopy are dominated by large-scale turbulent structures. These structures exist at time scales on the order of seconds and have length scales of several meters. Bursts and sweeps are examples of large scale structures that move packets of air into (sweep) and out of (burst) a forest (Gao et al. 1989). One numerical method used to capture large-scale structures in boundary layer flow is large-eddy simulation (LES). LES has been used to simulate the flow within and above canopies by Shaw and Schumann (1992) and others since. LES captures the full physics of atmospheric turbulence by explicitly solving the governing equations for large eddies while modeling small eddies with a sub-grid scale model.

We have used LES to simulate the flow and dispersion of a tracer gas within and above a loblolly pine canopy. Each simulation was evaluated with observations from a 2004 tracer gas experiment in a loblolly pine canopy (Thistle et al. 2005). Both the tracer experiments and numerical simulations were repeated in each of four stages of thinning where the experimental plot was thinned from dense boles and thick understory (basal area greater than 13 m2) to a final basal area equal to 6.5 m2.

A porous media model based on estimated leaf area density (LAD) profiles was used to simulate the effects of the pine canopy on momentum and turbulent kinetic energy. Heating of the air by the canopy was included as a heat source term to the energy equation based on measured heat fluxes above the canopy. A transport equation for sub-grid turbulent kinetic energy was used as a sub-grid scale model. The results, evaluated in terms of mean profiles of wind speed, potential temperature, and turbulence statistics, show that instantaneous bursting and sweeping occur in the canopy and have an influence on tracer gas dispersion.

References:

Gao, W., Shaw, R. H., and Paw U, K. T., 1989. “Observation of organized structure in turbulent flow within and above a forest canopy”. Boundary-Layer Meteorology. 47, 349-377.

Shaw, R. H., and Schumann, U., 1992. “Large-eddy simulation of turbulent flow above and within a forest”. Boundary-Layer Meteorology. 61, 41-64.

Thistle, H. W., Peterson, H. G., Allwine, G., Edburg, S., Lamb, B. K., Strom, B., 2005. “Pheromone movement in four stand thinning scenarios: high frequency plume observations”. Presented at the 2005 ASAE annual international meeting. Tampa Florida, July 2005.