Effect of stability on mixing in open canopies

Abstract

In open canopies, the within-canopy flux from the ground surface and understory can account for a significant fraction of the total flux above the canopy. Here, “open” canopies refer to sufficient spacing between vegetation elements to allow significant clear-sky radiative heating and cooling of the ground or understory surface, which in turn significantly influences the subcanopy turbulence. This study incorporates the important influence of within-canopy stability on turbulent mixing and subcanopy fluxes into a first-order closure scheme. Toward this goal, we analyze within-canopy eddy-correlation data from the old aspen site in the Boreal ecosystem–atmosphere study (BOREAS) and a mature ponderosa pine site in Central Oregon, USA. A formulation of within-canopy transport is framed in terms of a stability-dependent eddy diffusivity, which approaches Monin–Obukhov similarity theory above the canopy roughness sublayer. The new simple formulation for the subcanopy is an improvement upon the usual neglect of the influence of stability on turbulent mixing. However, frequent well-defined cold air drainage within the pine subcanopy inversion reduces the utility of simple models for nocturnal transport. Other shortcomings of the formulation are discussed.