Footprints and fetches for fluxes over forest canopies with varying structure and density

A stochastic trajectory model was used to estimate scalar flux footprints in neutral stability for canopies of varying leaf area distributions and leaf area indices. An analytical second-order closure model was used to predict mean wind speed, second moments and the dissipation rate of turbulent kinetic energy within a forest canopy. The influence of source vertical profile on the flux footprint was examined. The fetch is longer for surface sources than for sources at higher levels in the canopy. In order to measure all the flux components, and thus the total flux, with a desired accuracy, sources were located at the forest floor in the footprint function estimation. The footprint functions were calculated for five observation levels above the canopy top. It was found that at low observation heights both canopy density and canopy structure affect the fetch. The higher above the canopy top the flux is measured, the more pronounced is the effect of the canopy structure. The forest fetch for flux measurements is strongly dependent on the required accuracy: The 90% flux fetch is greater by a factor of two or more compared to the 75% fetch. The upwind distance contributing 75% of flux is as large as 45 times the difference between canopy height and the observation height above the canopy top, being even larger for low observation levels.