The effect of vegetation density on canopy sub-layer turbulence

The canonical form of atmospheric flows near the land surface, in the absence of a canopy, resembles a rough-wall boundary layer. However, in the presence of an extensive and dense canopy, the flow within and just above the foliage behaves as a perturbed mixing layer. To date, no analogous formulation exists for intermediate canopy densities. Using detailed laser Doppler velocity measurements conducted in an open channel over a wide range of canopy densities, a phenomenological model that describes the structure of turbulence within the canopy sublayer (CSL) is developed. The model decomposes the space within the CSL into three distinct zones: the deep zone in which the flow field is shown to be dominated by vortices connected with von Karman vortex streets, but periodically interrupted by strong sweep events whose features are influenced by canopy density. The second zone, which is near the canopy top, is a superposition of attached eddies and Kelvin-Helmholtz waves produced by inflectional instability in the mean longitudinal velocity profile. Here, the relative importance of the mixing layer and attached eddies are shown to vary with canopy density through a coefficient a. We show that the relative enhancement of turbulent diffusivity over its surface-layer value near the canopy top depends on the magnitude of a. In the uppermost zone, the flow follows the classical surface-layer similarity theory. Finally, we demonstrate that the combination of this newly proposed length scale and first-order closure models can accurately reproduce measured mean velocity and Reynolds stresses for a wide range of roughness densities. With recent advancement in remote sensing of canopy morphology, this model offers a promising physically based approach to connect the land surface and the atmosphere without resorting to empirical momentum roughness lengths.