Eddy structure near the plant canopy interface

We have elucidated the vortical structure of eddies that characterize turbulence at the interface between vegetation canopies and the lower atmosphere. Early analyses of field observations of coherent structures at forest sites describe a combination of an ejection of air from the forest followed by a downward sweep of air from aloft. Large-eddy simulation of lower boundary-layer turbulence, with resolution of the layers of a canopy, has been used to examine the three-dimensional vortical nature of organized structures. Two procedures have been employed to identify structures in such simulations. The first of these methods involves composite averaging of events detected by selecting regions of the flow field in which canopy-top static pressure exceeds a certain threshold. Positive pressure perturbations identify the streamwise velocity convergence at the interface between ejection and sweep. The second procedure is non-prejudicial and involves Lumley's Empirical Orthogonal Function (EOF) decomposition to extract the characteristic eddy. Both techniques clearly reveal counter-rotating vortices when v,w velocity vectors are displayed over y,z cross-sections. Applying the lambda2 method of vortex identification, the composite average procedure exposes distinct horseshoe-shaped vortices associated with both the ejection and the sweep. The former is a head-up vortex with an updraft between the two legs of the horseshoe, while the latter is a head-down vortex with a downdraft between the two legs. The EOF technique emphasizes the downdraft component of the structure and minimizes the preceding ejection. Nevertheless, the head-down vortex associated with the sweep exhibits a very similar pattern in terms of the streamwise slope and the lateral separation of the legs of the vortex.