Flux Contribution of Large-Scale Coherent Structures within Canopy Sub-layer

The traditional gradient-diffusion theory is problematic for estimating the turbulent transport of scalar quantities (e.g., carbon dioxide and water vapor) across the canopy-atmosphere interface due to the predominant role of large-scale coherent structures in the transport process within the canopy sub-layer (CSL). An accurate estimation of the fraction of the flux transport that the coherent structures are responsible for often suffers from the lack of a quantitative and objective identification method. In this talk, large-eddy simulation (LES) is performed to simulate canopy turbulence, and the large-scale coherent structures are identified through the use of the proper orthogonal decomposition (POD). It is shown that the results are in a good agreement with prior analysis conducted for data collected in numerical, laboratory and field experiments. The three-dimensional geometric features of the coherent structures are shown to be described by a strong sweep motion framed by a vortex pair with elliptical cross-sections inclined at a tile angle of approximately 45 degree in the spanwise direction. Furthermore, above the treetop the vortex pair curves upward in the streamwise direction. We also examine the role of these coherent structures in transporting scalar quantities. Different methods of calculating the inner product of the state vector in the POD are tested to estimate the flux contribution of the coherent structures.