Comparison between measured tracer fluxes and footprint model predictions over a homogeneous canopy of intermediate roughness

Fast response tracer flux measurements are compared against flux footprint predictions from both a Lagrangian stochastic simulation and an analytical solution to the equation of diffusion over a canopy of intermediate roughness. A turbulent tracer flux experiment was conducted over a peach orchard for a range of mildly unstable to very unstable conditions. For this purpose, a line source was used to release sulfur hexafluoride at treetop and fast response tracer flux instrumentation placed in the roughness sub-layer was mounted on four towers perpendicular to the line source to measure the vertical tracer flux. There was excellent agreement between Lagrangian simulated fluxes and their experimental counterparts. The analytical solution to the advection–diffusion equation used also shows a good agreement with the tracer fluxes particularly far from the tracer source. These results suggest that for canopies of intermediate roughness, the flux footprint predictions from both models presented work very well, despite their simplifying assumptions.