The impact of atmospheric stability on the flow dynamics within the roughness sublayer (RSL) and the influence on it by the processes in the overlying atmosphere are investigated using measurements collected at the Atmospheric Tall Tower Observatory (ATTO) in the Amazon. Observations were taken at seven levels within and above the forest along an 81-meter and a 325-meter towers allowing a unique investigation of the vertical evolution of the turbulent field in the RSL and in the surface layer above it. The vertical structure of the turbulent flow and the evolution of the mixing-layer type coherent structures (CS) are investigated in five stability regimes defined according to the turbulent fluxes’ behavior. CS are identified using an original method based on a non-linear fit of the 5-min autocorrelation function of the turbulent variables with a decaying, oscillating exponential function. The oscillating frequency of the fitting function is associated to the most energetic frequency in the spectrum and hence to the characteristic time scale of the coherent vortices. The shear length scale at the canopy top together with the CS time and separation length scales are evaluated to determine the influence of stability on the RSL and to verify the applicability of the mixing layer theory in stability conditions far from neutrality. Original parameterizations of these parameters are proposed. The definition of an intense stable regime allows 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 a large positive CO2 flux observed inside the forest in the intense stable regime.
Characterisation of the flow dynamics in the Roughness Sublayer within and above a dense Amazonian Forest / D. Cava, L. Mortarini, D. Brondani, U. Giostra. ((Intervento presentato al 5. convegno Congresso Nazionale dell’Associazione Italiana di Scienze dell’Atmosfera e Meteorologia (AISAM) : 5-8 febbraio tenutosi a Lecce nel 2024.
Characterisation of the flow dynamics in the Roughness Sublayer within and above a dense Amazonian Forest
L. MortariniCo-primo
;
2024
Abstract
The impact of atmospheric stability on the flow dynamics within the roughness sublayer (RSL) and the influence on it by the processes in the overlying atmosphere are investigated using measurements collected at the Atmospheric Tall Tower Observatory (ATTO) in the Amazon. Observations were taken at seven levels within and above the forest along an 81-meter and a 325-meter towers allowing a unique investigation of the vertical evolution of the turbulent field in the RSL and in the surface layer above it. The vertical structure of the turbulent flow and the evolution of the mixing-layer type coherent structures (CS) are investigated in five stability regimes defined according to the turbulent fluxes’ behavior. CS are identified using an original method based on a non-linear fit of the 5-min autocorrelation function of the turbulent variables with a decaying, oscillating exponential function. The oscillating frequency of the fitting function is associated to the most energetic frequency in the spectrum and hence to the characteristic time scale of the coherent vortices. The shear length scale at the canopy top together with the CS time and separation length scales are evaluated to determine the influence of stability on the RSL and to verify the applicability of the mixing layer theory in stability conditions far from neutrality. Original parameterizations of these parameters are proposed. The definition of an intense stable regime allows 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 a large positive CO2 flux observed inside the forest in the intense stable regime.
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