Turbulent kinetic energy budgets from a large-eddy simulation of airflow above and within a forest canopy

The output of a large-eddy simulation was used to study the terms of the turbulent kinetic energy (TKE) budget for the air layers above and within a forest. The computation created a three-dimensional, time-dependent simulation of the airflow, in which the lowest third of the domain was occupied by drag elements and heat sources to represent the forest. Shear production was a principal source of TKE in the upper canopy, diminishing gradually above tree-top height and more sharply with depth in the canopy. The transfer of energy to subgrid scales (dissipation) was the main sink in the upper part of the domain but diminished rapidly with depth in the canopy. Removal of resolved-scale TKE due to canopy drag was extremely important, occurring primarily in the upper half of the forest where the foliage density was large. Turbulent transport showed a loss at the canopy top and a gain within the canopy. These general features have been found elsewhere but uncertainty remains concerning the effects of pressure transport. In the present work, pressure was calculated directly, allowing us to compute the pressure diffusion term. Well above the canopy, pressure transport was smaller than, and opposite in sign to, the turbulent transport term. Near the canopy top and below, pressure transport acted in concert with turbulent transport to export TKE from the region immediately above and within the upper crown, and to provide turbulent energy for the lower parts of the forest. In combination, the transport terms accounted for over half of the TKE loss near the canopy top, and in the lowest two-thirds of the canopy the transport terms were the dominant source terms in the budget. Moreover, the pressure transport was the largest source of turbulent kinetic energy in the lowest levels of the canopy, being particularly strong under convective conditions. These results indicate that pressure transport is important in the plant canopy turbulent kinetic energy budget, especially in the lowest portion of the stand, where it acts as the major driving force for turbulent motions.