Abstract: Large-eddy simulations of flow over a “horizontally” uniform model forest are used to investigate the effects of gentle topography on the turbulent kinetic energy (TKE) budget within the canopy roughness sublayer. Despite significant differences between simulations using idealized sinusoidal topography and real topography of the Amazon forest, results indicate that the effects of topography are located predominantly in the upper canopy and above, and are mostly caused by mean advection of TKE. The “horizontally” averaged TKE budget from idealized and real gentle topographies are almost identical to that for flat terrain, including a clear inertial layer above the roughness sublayer in which shear production is balanced by local dissipation. At topography crests, where observational towers are usually located, mean vertical advection of TKE can be as important as horizontal advection. We propose the use on an approximate TKE balance equation to estimate mean advection from single tower measurements, and introduce a new advection index that can be used as a proxy to quantify the importance of the topography on the TKE budget.

Abstract: The interpretation of tower‐based eddy‐covariance (EC) turbulent flux measurements above forests hinges on three key assumptions: (1) steadiness in the flow statistics, (2) planar homogeneity of scalar sources or sinks, and (3) planar homogeneity in the flow statistics. Large eddy simulations (LESs) were used to control the first two so as to explore the break‐down of the third for idealized and real gentle topography such as those encountered in Amazonia. The LES runs were conducted using uniformly distributed sources inside homogeneous forests covering complex terrain to link the spatial patterns of scalar turbulent fluxes to topographic features. Results showed strong modulation of the fluxes by flow features induced by topography, including large area with negative fluxes compensating “chimney” regions with fluxes almost an order of magnitude larger than the landscape flux. Significant spatial heterogeneity persisted up to at least two canopy heights, where most eddy‐covariance measurements are performed above tall forests. A heterogeneity index was introduced to characterize and contrast different scenarios, and a topography categorization was shown to have predictive capabilities in identifying regions of negative and enhanced fluxes.

The presence of coarse mineral dust in the atmosphere has been substantiated in several recent measurement campaigns, which include observations of particles up to and above 100 μm in diameter. Yet, atmospheric dust models either do not include particles larger than 20 μm or severely underestimate their concentrations. One possibility for the underestimated concentrations is that models do not represent enhancements of particle transport due to subgrid-scale topography. Here, large-eddy simulations are used in combination with Lagrangian particle tracking to assess the impact of gentle two-dimensional topography with 50 and 100 m elevation on the vertical transport of coarse dust in neutrally stratified conditions. The presence of topography significantly increases the likelihood that 5 and 20 μm particles reach several hundred meters in altitude. Further, topography increases this likelihood by orders of magnitude for larger 60 μm particles. Three mechanisms are observed to contribute to the increased vertical transport: a strong upward mean flow region on the uphill side of the topography, ejection of particles downwind of the topography crest, and enhanced vertical dispersion in the wake of the crest. The compounding effects of these mechanisms provide a pathway for coarse dust emitted from the surface to reach elevations where they can be further transported into the free atmosphere by large-scale motions such as convective plumes. While these findings are motivated by mineral dust observations, they are generally applicable to other heavy aerosols such as pollen.

Abstract: Observational data from two field campaigns in the Amazon forest were used to study the vertical structure of turbulence above the forest. The analysis was performed using the reduced turbulent kinetic energy (TKE) budget and its associated two-dimensional phase space. Results revealed the existence of two regions within the roughness sublayer in which the TKE budget cannot be explained by the canonical flat-terrain TKE budgets in the canopy roughness sublayer or in the lower portion of the convective ABL. Data analysis also suggested that deviations from horizontal homogeneity have a large contribution to the TKE budget. Results from LES of a model canopy over idealized topography presented similar features, leading to the conclusion that flow distortions caused by topography are responsible for the observed features in the TKE budget. These results support the conclusion that the boundary layer above the Amazon forest is strongly impacted by the gentle topography underneath.