The influence of a forest canopy on top‐down and bottom‐up diffusion in the planetary boundary layer

Results from two nested‐grid large‐eddy simulations comparing cases with and without a plant canopy are presented. Through comparisons of numerically generated mean and turbulence statistics, the influence of a plant canopy with a leaf area index of two is shown to modify the air flow compared with an identical case without plants. Investigations of instantaneous fields and spatial spectra show that a plant canopy alters the spatial structure of turbulence and acts to shift the dominant scale to a scale on the order of the canopy height. Distributed drag and scalar sources, representing the presence of a scalar emitting deciduous forest, have little influence on top‐down diffusion, but enhanced mixing and increased turbulence intensities result in a dramatic modification to bottom‐up scalar diffusion up to ∼4.5 times the height of the canopy. Use of previously proposed bottom‐up gradient function with observations of scalar gradients under unstable stability conditions at 50 m over a 25 m tall forest (leaf area index of two) are shown to lead to an underestimate of the scalar emission flux by a factor of four. New top‐down and bottom‐up functions are proposed to include these canopy‐induced effects for this particular canopy. Copyright © 2003 Royal Meteorological Society