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Boundary-Layer Meteorology

, Volume 121, Issue 3, pp 433–457 | Cite as

Large-eddy Simulation of Turbulent Flow across a Forest Edge. Part II: Momentum and Turbulent Kinetic Energy Budgets

  • Bai YangEmail author
  • Andrew P. Morse
  • Roger H. Shaw
  • Kyaw Tha Paw U
Original Paper

Abstract

Momentum and turbulent kinetic energy (TKE) budgets across a forest edge have been investigated using large-eddy simulation (LES). Edge effects are observed in the rapid variation of a number of budget terms across this vegetation transition. The enhanced drag force at the forest edge is largely balanced by the pressure gradient force and by streamwise advection of upstream momentum, while vertical turbulent diffusion is relatively insignificant. For variance and TKE budgets, the most important processes at the forest edge are production due to the convergence (or divergence) of the mean flow, streamwise advection, pressure diffusion and enhanced dissipation by canopy drag. Turbulent diffusion, pressure redistribution and vertical shear production, which are characteristic processes in homogeneous canopy flow, are less important at the forest transition. We demonstrate that, in the equilibrated canopy flow, a substantial amount of TKE produced in the streamwise direction by the vertical shear of the mean flow is redistributed in the vertical direction by pressure fluctuations. This redistribution process occurs in the upper canopy layers. Part of the TKE in the vertical velocity component is transferred by turbulent and pressure diffusion to the lower canopy levels, where pressure redistribution takes place again and feeds TKE back to the streamwise direction. In this TKE cycle, the primary source terms are vertical shear production for streamwise velocity variance and pressure redistribution for vertical velocity variance. The evolution of these primary source terms downwind of the forest edge largely controls the adjustment rates of velocity variances.

Keywords

Forest edge Large-eddy simulation Momentum budget Turbulent kinetic energy budget 

List of Symbols

 −  (overbar)

mean quantities or averaging operation

′ (single prime)

departures from mean quantities

u,v, w or u1, u2, u3

components of resolved scale velocity in streamwise (x or x 1), spanwise (y or x 2) and vertical (z or x 3) directions, respectively

ρ0

air density

p

resolved scale static pressure

h

canopy or tree height

u*

friction velocity

Fd

canopy drag force

τ ij

subgrid-scale stresses

Cd

element drag coefficient

a

leaf area density

V

wind speed

δij

Kronecker delta

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Copyright information

© Springer Science+Business Media B.V. 2006

Authors and Affiliations

  • Bai Yang
    • 1
    • 2
    Email author
  • Andrew P. Morse
    • 3
  • Roger H. Shaw
    • 2
  • Kyaw Tha Paw U
    • 2
  1. 1.Environmental Sciences DivisionOak Ridge National LaboratoryOak RidgeUSA
  2. 2.Department of Land, Air and Water ResourcesUniversity of CaliforniaDavisUSA
  3. 3.Department of GeographyUniversity of LiverpoolLiverpoolUK

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