Assessing the effect of topography on the Atmospheric Flow over the Amazon Forest by means of Large Eddy Simulation and tower measurements

Contrary to our intuition forest are rarely situated on uniform and flat terrain in which the horizontally homogeneity of turbulence holds. The orography of the Amazon Tall Tower Observatory (ATTO, https://www.attoproject.org/) makes no exception and this complicates the interpretation of its measurements. The spatial representativeness of anemometric tower data and the underlying hypothesis of horizontal homogeneity of the atmospheric flows have long being questioned in the literature, as it has been long known that forested canopies enhance the effects of even mild topography. Using the Parallelized Large Eddy Simulation Model (PALM) with the highly vertically resolved measurements of the two ATTO towers (19 levels from 5 m to 316 m) allowed the assessment of the influence of topography on the atmospheric flow within and above the roughness sublayer in the Amazon Forest. To totally ascribe the flow variability to spatial orography gradients, a horizontally homogeneous leaf area density was considered. Simulations considering the real topography of the ATTO site and different wind directions are compared with measured profiles and with simulations ran over flat topography. The impact of the orography on turbulence is presented. Focus is also given to the ability of the PALM model to increase the spatial representativeness of the tower data.