Vertical propagation of submeso and coherent structure in a tall and dense Amazon Forest in different stability conditions PART I: Flow structure within and above the roughness sublayer

Abstract

Understanding the processes that govern the mixing and transport of scalars within and above the Amazon Forest is of great importance for many environmental applications. The impact of atmospheric stability on the roughness sublayer (RSL) as well as the influence on it by the processes in the overlying atmosphere are investigated using measurements collected at the Atmospheric Tall Tower Observatory. Five different stabilities are defined according to the turbulent fluxes’ behaviour. Ejections dominate the transport in the RSL. In near neutral and unstable conditions coherent structures propagate up to 2–3 times the canopy height (h) and intermittently penetrate in the lowest part of the forest where sweeps drive the transport processes. In the unstable regime a weakening of the wind inflection at the canopy top and a transition to a convective regime above z = 2 h are observed. In stable conditions three regimes were defined characterised by a progressive lowering of the RSL and the weakening of the mixing-layer type coherent structures. In the ‘weakly stable’ regime the intense momentum and scalar fluxes appear driven by the coherent structures being able to penetrate inside the canopy intermittently coupling the flow above and within the forest. The ‘very stable’ regime is characterized by weak winds, a weakening of coherent structures and a decrease of the turbulent fluxes inhibited by buoyancy. The definition of a ‘super stable’ regime allowed the identification of a peculiar condition characterized by low-wind and weak coherent structures confined close to the canopy top and producing negligible transport. Submeso motions dominate the flow dynamics in this regime both above and inside the RSL. Multiresolution analysis highlights the ability of submeso motions to propagate inside the canopy and to modulate the exchange, particularly of scalars, fully driving the large positive CO2 flux observed inside the forest in the super stable regime.