The Structure of Canopy Turbulence and their Implications to Scalar Dispersion

The canonical form of atmospheric flows near the land surface, in the absence of a canopy, resembles a rough-wall boundary layer and has been extensively studied for more than half a century. However, in the presence of an extensive and dense canopy, the flow within and just above the foliage resembles a perturbed mixing layer – a hypothesis proposed about 2 decades ago and appears supported by several field experiments. However, 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 is shown to be dominated by vortices connected with von Kármán 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 better represented by 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 α. The relative enhancement of turbulent diffusivity over its surface-layer value near the canopy top depends on the magnitude of α. In the uppermost zone, the flow follows the classical surface-layer similarity theory. Another important attribute of turbulent flows inside canopies is wake production and short-circuiting of the energy cascade. How these processes affect passive scalar concentration variability in general and their spectral properties in particular remains largely unexplored. Progress on this problem is frustrated by the shortage of high resolution spatial concentration measurements, and by the lack of simplified analytical models that connect spectral modulations in the turbulent kinetic energy (TKE) cascade to scalar spectra. Here, we report the first planar two-dimensional scalar concentration spectra cc φ inside tall dense canopies derived from flow visualization experiments. We found that in the spectral region experiencing wake production, the cc φ exhibits directional scaling power laws. In the longitudinal direction (x), or the direction experiencing the largest drag force, the ( ) cc x k φ was steeper than and followed an approximate at wavenumbers larger than the injection scale of wake energy, where 5/3