Topography induced spatial variability of the Atmospheric flow over the ATTO site

The spatial representativeness of anemometric tower measurements and the underlying hypothesis of horizontal homogeneity of the atmospheric flows have long being questioned in the literature. Further, forest are rarely situated on uniform and flat terrain in which the horizontally homogeneity of turbulence holds. The orography around the ATTO site makes no exception. The measurement site is located on a narrow plateau SW-NE oriented, surroundend by lower hills. It is then natural to investigate the spatial variability of the turbulent field around the ATTO and the INSTANT towers to understand the role played by gentle topography. This work investigates the topography-induced changes in the flow field as a function of wind speed, wind direction and atmospheric stability. To this purpose, the Parallelized Large-Eddy Simulation Model (PALM, https://palm.muk.uni-hannover.de/trac/wiki/palm) model was used. A 3.6 km x 3.6 km squared area centered at the ATTO tower with a horizontal resolution of 5 m was considered. The real topography was used, while an horizontally homogeneousleaf area density (LAD) vertical profile was assumed for the canopy over the whole area. Hence, the flow variability can be totally ascribed to spatial orography gradients. Two preliminary simulations are presented in which the atmosphere was neutrally stratified and all buoyancy effects were neglected. 2 ms-1 geostrophic winds, westward and eastward oriented, were considered as top boudary conditions. The dependence of the velocity vertical profile with orography at the latitude of the tall tower is presented, together with a comparison of the wind profiles at the ATTO and the INSTANT towers.