Turbulence in Sparse, Organized Vegetative Canopies: A Large-Eddy Simulation Study

A large-eddy simulation study was performed to characterize turbulence in sparse, row-oriented canopies. This was accomplished by simulating a set of heterogeneous row-oriented canopies with varying row vegetation density and spacing. To determine the effects of heterogeneity, results were compared to horizontally homogeneous canopies with an equivalent 'effective' leaf area index. By using a proper effective leaf area index, plane-averaged mean velocities and bulk scaling parameters contained only small errors when heterogeneity was ignored. However, many cases had significantly larger second- and third-order velocity moments in the presence of heterogeneity. Some heterogeneous canopies also contained dispersive fluxes in the lower canopy that were over 20 % as large as the turbulent flux. Impacts of heterogeneity were most pronounced in the cases of large row leaf area density and widely spaced rows. Despite the substantial amount of open space in the sparse canopies, vertical velocity skewness and quadrant-hole analysis indicated that the flow behaved predominantly as a canopy layer even though integral length scales at the canopy top no longer followed mixing-layer scaling. This was supported by the fact that similar composite-averaged coherent structures could be readily identified in both the heterogeneous and homogeneous canopies. Heterogeneity had an effect on coherent structures, in that structure detection events were most likely to occur just upwind of the vegetation rows. In simulations with large row spacing, these structures also penetrated deeper into the canopy when compared to the equivalent homogeneous canopy.