Mixing in open canopies

In open canopies, the subcanopy flux can account for a significant fraction of the total heat, moisture and carbon dioxide fluxes from the forest and therefore, the turbulent exchange coefficient within canopies becomes important. A first-order closure model is developed within and above the vegetation canopy for heat flux based on analysis of data from the old aspen site in BOREAS and a ponderosa pine site in central Oregon. A new formulation for subcanopy fluxes is developed in terms of a mixing length that depends on local stability. In the trunk space, the flux-gradient relationship for heat approximately follows Monin-Obukhov similarity theory for near neutral and unstable conditions in spite of the fact that the conditions for Monin-Obukhov similarity theory are not satisfied. At the pine site, a strong nocturnal inversion often develops in the canopy overstory, which prevents significant exchange of heat and CO₂ between the soil surface and the air above the canopy. This strong inversion is examined in terms of cold air drainage in the subcanopy trunk space.