Canopy-Boundary Layer Turbulent Kinetic Energy Transfer in Convectively Unstable Flows

Recent Large-Eddy Simulations (LES) of atmospheric flows over a vegetation canopy have enabled the detailed investigation of the structure of the flow and the link between different co-existing turbulent coherent eddies over a wide range of scales (Patton et al, Journal of Atmospheric Sciences, 2016). Even mild departure from the neutral stability regime leads to the formation of large-scale roll-like eddies in the Atmospheric Boundary Layer (ABL) which transition into Rayleigh Benard-like cells spanning the depth to the PBL when the free-convective regime is reached. It has been recently shown by Patton et al. (2016) that these ABL-scale eddies strongly interact with wind and temperature fields at canopy-level. In particular, it has been demonstrated that ABL-scale eddies induce high-momentum penetrating fluid motions or low-momentum fluid ejections in the near-canopy region and modulate the vertical shear of the horizontal wind. This interaction and its impact on the canopy flow depends on the stability regime of the ABL. At canopy top, increasing instability has been found to reduce the mean vertical shear of the horizontal wind and the magnitude of the velocity skewness, while modifying the integral length scales of turbulent motions in and slightly above the canopy. Preliminary analysis of the nature of the interaction between ABL- and canopy-scales conducted in near-neutral regime (Perret & Patton, AMS, 21st BLT, 2014) have demonstrated that the underlying mechanisms are both linear and non-linear, confirming the influence of the PBL-scale structures on the canopy-top velocity skewness. This non-linear nature also suggests that turbulent kinetic energy transfers between scales take place at canopy top between these two well-separated scales of motion.

The present study investigates the transfer of kinetic energy between the ABL scales and the canopy scales based on high-resolution large LES of atmospheric flow interacting with a vegetation canopy in different stability conditions (Patton et al., 2016). A scale-decomposition procedure based on the analysis of the bi-dimensional spectra computed in horizontal planes at various heights enables estimation not only of the mean energy transfer between the different part of the flow, but also enables analysis of instantaneous transfer. The magnitude of the mean transfer rate and an analysis of the relative importance of the contributions of the different terms involving the three velocity components will be presented. Existence of instantaneous forward- and back-scatter energy transfer among scales is demonstrated and its importance investigated. Implications of the present results showing the importance of accounting for the role of the ABL-scales in the more local canopy flow could be important both for model parameterizations and for interpretation of flux tower measurements of the momentum transfer between the ABL and vegetated surfaces.