Effects of Vegetation and Topography on the Boundary Layer Structure above the Amazon Forest

The presence of tall and dense vegetation greatly enhances the effects of topography on the flow field. As a consequence, even very gentle topography, when covered by forests, causes flow separation and the development of recirculation zones in the lee of hills and ridges. Despite significant theoretical advances for flows over simple gentle topography covered with a canopy and the increasing number of modeling studies, a framework to analyze tower data in these conditions is still not available. We use data from two field campaigns in central Amazonia, the GoAmazon at the K43 tower and an intensive campaign at the ATTO tall tower, to study the vertical structure of turbulence above the forest. Both towers are situated in regions of gentle topography covered by primary dense forest. We focus on the TKE budget and use the reduced TKE phase space to show that the patterns in observations can only be explained by the effects of topography. In particular, we observe regions above the canopy in which the local rate of TKE dissipation is larger than the local rate of production, which is inconsistent with TKE budgets in the canopy roughness sublayer or the convective boundary layer (over flat topography). By using large eddy simulation of flow over forest-covered ridges, we identify the cause for such striking feature in the observed TKE budget. Thus, in the Amazon forest, two "roughness sublayers" overlap and interact with each other, one caused by the presence of the vegetation, and the other cause by the topography. The latter is much deeper, and merges directly into the mixed layer portion of the CBL, preventing the existence of an inertial sublayer that follows Monin-Obukhov scaling. This work also illustrates the power of the reduced TKE phase space as a tool to investigate turbulence in disturbed surface layers.