Flow over a forested hill: Canopy-atmosphere interactions

Currently much of the work on the flow within and above a forest canopy has concentrated on flat, homogeneous canopies, however many forests are in hilly or mountainous terrain. Recent theoretical and modelling work has shown how such complex terrain can significantly alter the flow. In such cases traditional roughness length parameterisations may no longer be appropriate for modelling the flow over the canopy, with significant mean flow into and out of the canopy being predicted.

Here results are presented from numerical simulations using a first order canopy turbulence closure scheme. Particularly for small hills, the different balance between terms in the momentum equation within and above the canopy leads to flow into the forest over the upwind slope and flow out of the forest and enhance separation over the lee slope. These differences in flow also give rise to an increase in the pressure drag over a forested hill. The importance of these interactions for canopy-atmosphere transport is investigated through the use of experiments with passive tracers. Results from large eddy simulations (LES) confirm the general results obtained using the first order closure scheme and provide further insight into the structure of the turbulence in and above the canopy.