Role of coherent vortices on scalar transport and exchange processes within and above the Amazon forest

Exchange processes between canopies and the Atmosphere are relevant in many environmental applications, from urban heat islands to the net ecosystem exchange in forest. Both above and within canopies the dynamics of scalars is dominated by non-local coherent vortices generated at the canopy-Atmosphere interface and themselves evolving depending on thermal stratification. In the present work, observations taken at seven levels (14 m – 325 m) during a 19-days campaign at the Amazon Tall Tower Observatory (ATTO) site were used to study the vertical evolution of the turbulent field in the roughness sublayer and in the surface layer above it in different stability conditions. An extremely stable regime, characterized by low wind speeds, the almost completely suppression of turbulence and a dominance of submeso motions, was identified. The shear length scale, responsible of the coherent vortices generation, increases with decreasing stability, presenting two asymptotes for large unstable and stable stratification and a linear behaviour close to neutral stratification. An original method based on 5-minutes autocorrelation functions was applied to the detection of coherent structure time and length scales. The characteristic timescales of vertical velocity, temperature and scalar fluxes are presented, together with their relationship to the scalar fluxes characteristic time-scales.