Passive scalar flux footprint analysis over horizontally inhomogeneous plant canopy using large-eddy simulation

The large-eddy simulation (LES) method has been used to simulate passive scalar such as carbon dioxide (CO2) flux and its footprint or source weight function over a horizontally inhomogeneous canopy, in which half of the physical domain is patched soil or clearcut upwind. A step change of aerodynamic roughness length (z01/z02=20) across a clearcut–canopy interface, which simulates the forest edge, is examined. In order to simulate the transport and dispersion of CO2 within and above the canopy, a constant crosswind line source (tracer) is placed at the upwind boundary. This study investigates the influence of the clearcut interface on the scalar flux and its footprint at a downwind measurement tower. The study reports the contribution of all sources to a point flux measurement. The footprint function demonstrates a large platform effect, which is due to the clearcut–forest interface. The numerical results demonstrate an influence region from 3 to 5 tree heights upwind of the interface to more than 15 tree heights downwind the interface. It is observed that the rate of adjustment for a passive scalar is much slower than that of vectors, such as momentum. Thus, a much larger fetch is required than the one demanded in otherwise homogeneous conditions for the tower flux measurements to be robust and representative of the underlying forest and not of the clearcut. The results from this study provide useful information on the examination of the measurement uncertainties associated with the net ecosystem–atmosphere exchange (NEE) of CO2 under neutral conditions. The numerical results are also compared with a set of experimental data and analytical methods such as the Lagrangian stochastic (LS) simulation. The study shows that LES as an effective tool can be used to simulate atmospheric flows over heterogeneous canopies.