The Effects of Vegetation Density on Coherent Turbulent Structures within the Canopy Sublayer: A Large-Eddy Simulation Study

Terrestrial ecosystems are characterized by a wide range of canopy vegetation density, which is known to affect turbulent transport processes across the canopy-atmosphere interface. In the presence of a dense and horizontally homogeneous canopy, the canopy sublayer has been described as resembling a plane mixing layer. At the other extreme, where the canopy is essentially absent, the canopy sublayer is typically assumed to be similar to a turbulent boundary layer over a rough surface. However, it remains unclear how the canopy turbulence changes from boundary-layer-like to mixing-layer-like as the vegetation density increases. We use large-eddy simulation to study five different vegetation densities varying from an extremely sparse canopy to an extremely dense canopy. This investigation draws on the study of flow statistics as well as large-scale coherent turbulent structures within the canopy sublayer. The coherent structures are identified through the use of proper orthogonal decomposition. The results of skewness of velocity components and characteristic length scales suggest that, as the vegetation density increases, the canopy turbulence gradually undergoes a transition from resembling a rough-wall boundary layer to being similar to a mixing layer. As demonstrated by others, we found that the coherent structures within the canopy sublayer consist of a strong sweep/ejection motion framed by a counter-rotating vortex pair with elliptical cross-sections. As the canopy becomes denser, these important structures are shown here to be more elevated. Vegetation density does not appear to have a significant effect on the percentage of the total turbulent kinetic energy that is represented by the coherent structures.