Mj. Dwyer et al., TURBULENT KINETIC-ENERGY BUDGETS FROM A LARGE-EDDY SIMULATION OF AIR-FLOW ABOVE AND WITHIN A FOREST CANOPY, Boundary - layer meteorology, 84(1), 1997, pp. 23-43
The output of a large-eddy simulation was used to study the terms of t
he turbulent kinetic energy (TKE) budget for the air layers above and
within a forest. The computation created a three-dimensional, time-dep
endent simulation of the airflow, in which the lowest third of the dom
ain was occupied by drag elements and heat sources to represent the fo
rest. Shear production was a principal source of TKE in the upper cano
py, diminishing gradually above tree-top height and more sharply with
depth in the canopy. The transfer of energy to subgrid scales (dissipa
tion) 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 t
o canopy drag was extremely important, occurring primarily in the uppe
r half of the forest where the foliage density was large. Turbulent tr
ansport showed a loss at the canopy top and a gain within the canopy.
These general features have been found elsewhere but uncertainty remai
ns 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 turbule
nt transport to export TKE from the region immediately above and withi
n 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-third
s 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 pa
rticularly strong under convective conditions. These results indicate
that pressure transport is important in the plant canopy turbulent kin
etic energy budget, especially in the lowest portion of the stand, whe
re it acts as the major driving force for turbulent motions.