Coherent structures detection within a dense Alpine forest

Abstract

The influence of coherent vortices on the turbulence structure above and below a dense plant canopy is investigated using turbulence measurements collected at five levels on top and inside a coniferous forest on an Alpine Plateau. Five different stability regimes, from free convection to very stable stratification, were identified and considered in the analysis. Coherent structures are detected by fitting the vertical velocity auto-correlation functions with a theoretical oscillating function. This allowed to evaluate the coherent vortices characteristic timescales and to discriminate between turbulent data subsets characterized by fine-scale turbulence and subsets in which turbulence is dominated by coherent structures generated at the canopy top. An original methodology to fit cross-correlation function and to single out the most energetic frequency in presence of periodic behaviour is presented and applied to the turbulent momentum flux. The analysis shows how not far from neutral conditions the dominant time scale of the momentum flux is mainly determined by the longitudinal wind velocity component, rather than vertical one, while in free convection and very stable conditions coherent vortices do not seem to influence the momentum transport, dominated by large-scale structures. This is confirmed by the fine-scale turbulence and coherent structures efficiency in turbulence transport throughout the stability regimes.