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

A large number of chemical species measured in the planetary boundary layer (PBL) are emitted and/or taken up by vegetation. Until now, numerical PBL studies of scalar transport have largely considered the atmosphere interacting with a surface that is described by a specified surface roughness and surface scalar flux. The presence of a plant canopy at the surface complicates the processes by which scalars are transported to and from the source/sink region which, for the forested situation, is distributed both vertically and horizontally in space.

The top-down and bottom-up gradient functions proposed by Wyngaard and Brost (1984) attempt to generalize the the flux-gradient and flux-variance relationships. These functions are utilized by numerous researchers who are searching for ways to estimate surface or entrainment fluxes of scalar species when their measurements or numerical simulations are limited in vertical resolution. A large percentage of the scalar species and atmospheric regimes investigated by these researchers are either emitted by or interacting with a vegetated surface. The vertical distribution of the scalar source and the complications imposed by elevated drag elements on the mixing process are likely to affect the proposed non-dimensional relationship between a vertical scalar gradient and/or scalar variance and the scalar source.

Through numerical simulation, we explore the canopy-imposed modification of the top-down/bottom-up gradient and variance functions proposed by Wyngaard and Brost (1984).