Modelling the turbulence structure in the canopy layer

Abstract

The large-eddy simulation (LES) is used to investigate the canopy flow structure and the transfer of TKE, momentum and heat within and above a tall canopy in full leaf. This paper reports on a comparison of three simulations performed, one in neutral conditions and two with different thermal loads on the canopy layer. The instantaneous flow structures observed in the upper crown region, the horizontally averaged statistics of the flow, the second-order equations of the Reynolds stress, TKE and heat fluxes, and the quadrant analyses of the Reynolds stress and heat flux are examined for three atmospheric conditions. The LES results of vertical profiles, temperature, momentum and heat fluxes, velocity variances and skewnesses generally agree well with observations. The effects of buoyancy forces on the velocity variances and skewnesses are discussed. LES results of instantaneous turbulent flow fields reveal basic characteristics of turbulence structures particularly near the top of the canopy layer. The LES simulation results show that near the treetop, shear production constitutes the main source of TKE. The pressure transport, together with the canopy drag are important energy source and sink respectively throughout most of the canopy. In the bottom two-third of the canopy layer, the pressure transport term exceeds other production terms. The turbulent transport term shows an export of TKE from the upper canopy to the region above the canopy and to the deeper regions of the forest. Thermal effects inside the canopy layer are generally small in that budget in unstable conditions, except for the buoyancy production term which peaks at about two-thirds of the layer. This is in contrast with the corresponding term in the Reynolds stress budget where buoyancy production is of almost the same magnitude as the turbulent transport term in unstable conditions, corresponding to about 15% of the shear production term near the canopy top. In the Reynolds stress budget of the upper canopy region, pressure destruction is balanced by shear production. Turbulent transport and subgrid-scale effects are largest near the canopy top. Thermal effects impact mostly on buoyancy production and on the turbulent transport term. These effects are most pronounced near the top of the canopy layer. In the vertical heat flux budget, gradient production and pressure covariance are the main production and destruction of heat flux terms respectively and the gradient production is the largest source term producing an upward heat flux in the crown layer and a downward heat flux in the lower canopy. Unlike its corresponding terms in the TKE and Reynolds stress budgets, this term becomes large and negative in the lower canopy. The importance of the turbulent transport term increases substantially with increasing thermal instability. The magnitude of the pressure covariance, gradient production, turbulent transport and buoyancy production increases with increased thermal load on the canopy layer. Quadrant analysis obtained from our LES results both for momentum and heat fluxes show that above and below the treetop, the contribution to momentum and to a lesser extent to heat transfer, is chiefly controlled by the sweep/ejection mechanisms in the cases studied. In these regions, the ejections contribution increases with unstable conditions while the sweeps contribution to the Reynolds stress decreases. Results are similar although more subdued in the case of the heat flux. For both momentum and heat flux, in the lower canopy, thermal stability significantly alters the structure of the flow mostly by shifting the predominance of the inward interaction quadrant in neutral conditions to the outward interaction quadrant in unstable conditions.