Comparison of In-Canopy Flux Footprints between Large-Eddy Simulation and the Lagrangian Simulation

Abstract

Flux footprints for neutral shear-driven canopy flows are evaluated using large-eddy simulation (LES) and a Lagrangian stochastic (LS) model. The Lagrangian stochastic model is driven by flow statistics derived from the large-eddy simulation. LES results suggest that both surface and elevated sources inside the canopy contribute equally to the cumulative flux from an upwind distance of 4 times the canopy height. LES flux footprints are more contracted than those obtained using the Lagrangian stochastic model. This is attributed to an enhanced vertical diffusion and reduced horizontal diffusion. The ejection and sweep contributions to momentum exchange in the Lagrangian stochastic model are weaker than those in the large-eddy simulation. Ejections of low-momentum air dominate at all levels in the canopy modeled by the LES. In contrast, high-momentum sweep events are dominant within the LES canopy and low-momentum ejection events are dominant above the canopy. Dispersion parameters for the first- and second-order statistics of concentration from both LES and LS for three line sources representing the canopy crown, midcanopy, and surface sources are also investigated. Lagrangian model results are sensitive to the choice of the time scale. A time scale based on the dissipation rate agrees well with the LS and LES plume heights of surface source. However, flux footprints from LS are closer to those from the LES, while an intermediate time scale (0.15z/σw) was used inside the canopy.