Canopy-Boundary Layer Coupling in Convectively Unstable Flows

A phenomenological theory of the coherent eddy structure that develops in and above tall plant canopies in neutrally stratified flow has existed for some years. These large eddies dominate the exchange of scalars and momentum with the overlying PBL. This theory depends on the hydrodynamic instability of the inflected mean velocity profile that develops at the canopy top. A cascade of secondary instabilities yields the characteristic canopy eddies and the eigenmodes of the primary inflexion-point instability can be used to modify Monin-Obukhov scaling in the roughness sublayer to match observations. In even mildly convective conditions, however, large scale roll-like structures appear in the Planetary Boundary Layer (PBL) above the canopy. When free convective conditions are approached, these rolls become Rayleigh Benard-like cells spanning the depth of the PBL. We have studied this situation through a range of diabatic stability from neutral to free convection, using a high resolution LES that includes a detailed canopy representation and extends above the capping inversion of the PBL.

At the canopy level, the wind fields of these PBL-scale structures modulate the wind and temperature fields so that patches of canopy with horizontal scales of 100m-1000m are alternately subjected to enhanced or reduced wind shear. In regions of strong shear the canopy eddy structure corresponds to that devolving from the inflection point instability, described above. When shear is small, however, convective plumes develop with vertical and horizontal scales of a few canopy heights. These plumes coalesce to form the ascending walls of the PBL-scale rolls or cells. The areally averaged heat and momentum transfer from the canopy is therefore effected by two distinct processes according to whether the canopy is below ascending or descending regions of the PBL-scale structures.

In this talk we will investigate the implications of the fact that the total canopy-PBL exchange consist of two spatially distinct physical processes at canopy scale, modulated by a third process at PBL scale. The results are important both for model parameterizations and for flux tower measurements of the exchange between the PBL and vegetated surfaces.