Footprint considerations in BOREAS

Comparisons of observations of concentration or flux from platforms at various heights, such as tower and aircraft, must take into account differences in the location and extent of upwind surface source or sink areas which affect the individual observations, with their physical and biological characteristics. Such "footprint" estimates are based on solutions of the diffusion/advection equation which have not previously been evaluated over a boreal ecosystem. In order to adjust an analytical footprint model within the surface layer above forest canopies typical for the Boreal Ecosystem-Atmosphere Study (BOREAS) sites, 29 tracer gas release experiments were carried out between August 30 and September 9, 1994, at three tower sites in the northern study area (NSA). Sulphur hexafluoride (SF6) was released from point sources at various upwind distances from the towers under various meteorological, terrain, and release conditions. Wind, temperature, and stability parameters, during each release period, were used as input into calculations of vertical concentration profiles sampled at the towers, based on a three-dimensional diffusion model. Predictions of concentration profiles, or back calculation of source strength from observed profiles, were in good agreement with observed concentrations or actual release rates, respectively. The diffusion model was then used to compute footprint estimates for stable to unstable conditions, for tower and aircraft-based observation platforms. They show spatially constrained footprints in the surface layer, due to effective vertical coupling, so observations from towers and low-flying aircraft must be expected to be very site specific, and scaling up to larger areas will have to be done with careful consideration of surface mosaics. Our study also included footprint estimates made for airborne observations above the surface layer, based on large-eddy simulations over "BOREAS-like" terrain, for boundary layer structures comparable to those observed in BOREAS. They document the progressive decoupling of airborne observations from the surface features at these heights.