Coherent Eddy Structures over Plant Canopies

Abstract

Formore than two decades, coherentflowpatterns have been reported at the interface between vegetation canopies and the lowest levels of the atmospheric surface layer, a region called the roughness sublayer. Statistical analyses provided the first indications of such organization. Later, instrumented towers in forested lands revealed ramplike patterns in time traces of scalar quantities.When coupledwith the velocity field measured by fast-response anemometry, a picture was formed of a repeated downstream ejection and upstream sweep combination creating streamwise convergence and the formation of scalar microfronts. Large-eddy simulation (LES) has been shown to reproduce both the statistics of canopy and roughness sublayer flow, and the turbulence structures detected in the field. Further, LES has allowed elucidation of the vortical nature of canopy flow structures and revealed a combination of head-up and head-down vortices, the former associated with the ejection and the latter with the sweep. Recent simulations that resolve flow within a forest canopy at the base of a full boundary layer under thermally unstable conditions illustrates the impact of surface heating on roughness sublayer structure and on mechanisms for heat, mass and momentum exchange at the surface. With increasing buoyant instability, momentum and scalar fluxes become increasingly disassociated in space. Convective cellular patterns produce regions of enhanced canopy-top shear and increased canopy drag beneath large-scale downdrafts. In updraft areas, canopy-top plumes coalesce to form the walls of the open-cell mesoscale structure, and diminished within-canopy drag.