There have been much effort to understand the processes of momentum, heat and mass exchange between forest canopies and the free atmosphere and advances have been achieved through both numerical simulations and field observations. A comprehensive review of turbulent flow within plant canopies was recently published by Finnigan(2000). While early theories suggested that canopy turbulence involves a superposition of energetic small scale eddies from plant wakes with surface layer turbulence, more recent studies have shown that canopy flows are dominated by transient, large coherent structures on the scale of the entire canopy. This implies that the traditional “steady-state” Reynolds-average approach to model canopy turbulence and diffusion may be inadequate and more advanced modeling approach such as Large-Eddy Simulation (LES) is required.

A computational fluid dynamics model, FEM3MP, was used for the present investigation. FEM3MP is a model developed primarily for simulating airflow and the dispersion of hazardous pollutants released near buildings in the urban environment (Chan and Stevens, 2000). It employs the finite element methodology for accurate representation of variable terrain and complex building shapes, together with a linearized, fully implicit projection method for efficient time integration. In addition to treating aerosols, UV radiation decay, surface energy budget, and canopy effects, the model also has an LES and a non-linear eddy viscosity turbulence submodel.

The LES approach was used in this study to simulate the turbulent flow and diffusion above and within forest canopies. The particular field studies being simulated herein include an experiment conducted in a South Carolina coastal forest (Shinn, 1969) and a recent air tracer experiment performed in a tropical forest (Shinn and Pletcher, 2001). Our numerical predictions for the velocity profiles, turbulence kinetic energy, and concentration of the tracer gas are generally very consistent with the measured data. In this paper, we will describe briefly the salient features of our model, present a comparison of our model predictions against the field data, and discuss the performance of our model.

REFERENCES

Chan, S. and D. Stevens, 2000, An Evaluation of Two Advanced Turbulence Models for Simulating the Flow and Dispersion Around Buildings, Proc. of the Millennium NATO/CCMS Int. Tech. Meeting on Air Pollution Modeling and its Application, Boulder, CO, pp. 355-362.

Fennigan, J., 2000, Turbulence in Plant Canopies, Annu. Rev. Fluid Mech., 32:519-571.

Shinn, J.H., 1969, Analysis of Wind Data From a South Carolina Coastal Forest, U.S. Army Electronics Command, Research and Development Tech. Report ECOM-6036.

Shinn, J.H. and R.J. Pletcher, 2001, An Air Tracer Dispersion Experiment in a Forest, LLNL report (in press).

This work was performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.