Spatial observations of large eddies and cross-canopy coupling with joint fiber-optic distributed sensing and eddy covariance flux measurements

Air flows above forest canopies are typically governed by large coherent eddies generated mechanically by inflected mean wind velocity profile or thermally by buoyancy in the convective regime. A significant body of research have been devoted to the role of these eddies on ecosystem scalar (gases and heat) exchange since they are likely related to the energy balance closure problem observed at the eddy covariance (EC) stations and turbulent flux divergence under stable stratification. Here we utilize fiber-optic distributed sensing on a tall mast to observe the turbulent fluctuations of air temperature with high spatial (25 cm) and temporal resolution (1 Hz) from the forest floor up to 120 m above the ground. These unique measurements resolved the continuous vertical profile of scalar turbulence and hence enabled us to study the topology (height – time space) of the turbulent eddies in different stability regimes. For example, the inclination angle of the eddies changed with stability and the scalar ramps often observed in canopy flows were evident only close to the canopy top, whereas higher up thermal eddies dominated the flow. Furthermore, the measurements permitted the identification of coupled air layers and hence analysis on the dynamics of below-canopy decoupling. During stable conditions with wind shear large eddies and the related inverted ramps in the temperature time series were observed at the top of the decoupling layer, however when the wind shear decreased the flow switched to submeso regime with canopy waves. These analyses were then combined with concurrent turbulence measurements with 3D sonic anemometers at several heights and EC gas flux measurements at one height to gain new insights on the role of these eddies on gas (e.g. carbon dioxide) transport. The measurements were conducted during summer 2019 at the Hyytiälä SMEAR II station located in central Finland and the permanent ICOS measurements at the site were utilized to the fullest.