The impact of a dense canopy on the wind profile and evolution of the boundary layer

It is well known that the mean wind profile within and above tall canopies does not conform to the standard Monin-Obukhov flux-gradient relationships. However there exist a number of applications, for instance operational numerical weather prediction, where such simple relationships and the simple parameterisations that they allow are required. Furthermore, most eddy flux measurements made at long term flux stations in programs like FLUXNET are made in the roughness sub-layer above tall plant canopies so that a consistent theory for this region is urgently required. Here we address this problem by reconsidering simple one-dimensional, mixing-length based models for the flow within and above a dense canopy in light of the mixing layer analogy for the flow at the canopy top.

Two components of the flow are considered. Firstly, within the dense canopy, stress divergence is balanced by aerodynamic drag and the flow takes the well-known exponential form. Secondly, the flow above the canopy is that of an atmospheric surface layer modified by both diabatic stability and roughness sub-layer effects. A novel form for the roughness sub-layer correction is presented. This is formulated by hypothesising that the observed deviations from the standard surface layer M-O flux-gradient relationships are the result of coherent structures arising from the mixing layer instability occurring at the canopy top. These structures are characterised by a single length scale-the vorticity thickness-which then forms the basis for the vertical scaling in the roughness sub-layer parameterisation. The two components of the flow are coupled by enforcing continuity of the flow and flux at canopy top.

Predictions of the mean flow profile from this unified Canopy-RSL-Surface Layer theory are shown to match observations well over a range of canopy types and diabatic stabilities. A consequence of the theory is that key surface exchange parameters, such as the displacement height and roughness length, are not invariant with stability and vary significantly as the Obukhov length changes. This dependence may have significant implications for modelling the evolution of the boundary layer, particularly in stable conditions. Finally, we also consider the profiles of scalars above tall canopies and the impact that the mixing layer analogy and roughness sub-layer have on relating measured eddy fluxes and concentrations in the roughness sublayer to those higher in the boundary layer.