Numerical investigation of coherent structures in a forest edge flow

It has been demonstrated that over homogeneous canopies, large coherent eddy structures are generated as in plane mixing-layer flows. The inflection point in the mean wind velocity profile at canopy top leads to the development of Kelvin-Helmholtz instabilities. These instabilities first roll over to form transverse vortices that are then transformed through secondary instabilities into three-dimensional coherent structures. The latter are characterized by cycles of strong “sweeps” (gusts) into the canopy and weak “ejections” (bursts) which are responsible for a substantial part of the turbulent transfer of momentum, heat and mass between the canopy and the atmosphere. Better knowledge of the dynamics of coherent structures is therefore of primary importance to understand such transfer processes. To this purpose, the structure of coherent eddies has been investigated for years over homogeneous vegetation canopies, using in-situ and wind-tunnel experiments, and to a lesser extent large-eddy simulations (LES). Various techniques of detection and visualisation have been developed, based on two-point velocity statistics or the detection of ramplike patterns in timeseries, but still information on their dynamics and topology remain limited, especially over heterogeneous canopies. Here we analyze turbulent vortices developing from a forest leading edge, using numerical simulations performed with the Advanced Regional Prediction System (ARPS), modified to simulate turbulent flow at very fine scale (2 m) within and above heterogeneous vegetation canopies with a LES approach. Standard average fields simulated downwind from the forest leading edge are first validated against a wind-tunnel experiment in a companion presentation. Here, it will be shown from vorticity fields that turbulent structures start to roll up from the edge with transverse vorticity; further downstream (at around 5 canopy heights), instabilities lead to the development of streamwise and vertical vorticity components. Two-point velocity statistics and wavelet analysis of time series are also performed at various distances from the leading edge. We observe that the mean streamwise separation between coherent structures is rapidly established from the leading edge, unlike the spatial scales associated with their structure, which require a larger distance.