Chimneys of the Amazon: Effects of Gentle Topography on Gas Fluxes Emitted within Forests

One of the main assumptions used in interpreting tower-based eddy-covariance (EC) measurements above canopies as emissions or uptake from large areas surrounding the tower is the constant turbulent flux with height. This assumption is plausible for stationary, planar homogeneous flow in the absence of subsidence. However, such ideal conditions are a nicety rarely encountered in forested ecosystems, where undulations in topography are routinely prevalent. Several studies with regular and gentle topography (e.g., two dimensional ridges and isolated three-dimensional hills) have shown that mild topographical features lead to spatial variations in mean flow and turbulence. These changes lead to modifications in the turbulent exchange of gases between the canopy and the boundary layer above even if when emissions or uptake from the canopy remains spatially uniform. The magnitude and spatial distribution of these modifications have not been quantified and frame the scope of the talk. A large eddy simulation (LES) with neutral stratification and realistic topography is employed to explore these effects over a small region of the Amazon forest. Particle trajectories are tracked using an offline Lagrangian model driven by the LES flow field. The results show that even for a uniform release from a horizontally homogeneous canopy, the presence of gentle topography produces appreciably heterogeneity in scalar fluxes. Most of the transport across the canopy top occurs through “chimneys” located downwind of topographic features. Sensitivity of the results to source and “measurement height” are also reported. Significant spatial heterogeneity persists through the canopy roughness sublayer, where most eddy-covariance measurements are performed above tall forests.