A second-order closure for neutrally stratified vegetative canopy flows

An existing second-order closure model is modified to include the effects o n mean and turbulent motions of form and viscous drag in vegetative canopie s. The additional physical mechanisms represented by the closure are viscou s and pressure drag on canopy elements, their role in momentum absorption, in the creation of fine scale turbulent eddies and in enhancing the total v iscous dissipation in the canopy airspace. Viscous dissipation is split int o a standard 'isotropic' contribution associated with the spectral eddy cas cade and a foliage contribution associated with work against pressure and v iscous drag on the foliage. Changes in the turbulent time scale that result from these mechanisms are included in the standard parameterisations of th ird moments and of the eddy cascade contribution to dissipation. The model is tested against a wind-tunnel 'wheat' canopy, a corn canopy and a eucalyp t canopy, a height range from 50 mm to 12.6 m. Model results show that the parameterisations of foliage interaction used in the closure are sufficient ly robust to reproduce second-moment profiles within and above vegetative c anopies to a high degree of accuracy without resorting to 'tuning' of the m odel constants. The model also shows the natural emergence of two length sc ales, one associated with the familiar eddy cascade isotropic contribution to total dissipation and the other associated with the length scales of the canopy elements.