Raupachetal96-canopymixinglayer

Raupachetal96-canopymixinglayer - COHERENT EDDIES AND...

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COHERENT EDDIES AND TURBULENCE IN VEGETATION CANOPIES: THE MIXING-LAYER ANALOGY M. R. RAUPACH, J. J. FINNIGAN and Y. BRUNET CSIRO Centre for Environmental Mechanics, GPO Box 821, Canberra, ACT 2601, Australia (Received in final form 6 February, 1996) Abstract. This paper argues that the active turbulence and coherent motions near the top of a vegetation canopy are patterned on a plane mixing layer, because of instabilities associated with the characteristic strong inflection in the mean velocity profile. Mixing-layer turbulence, formed around the inflectional mean velocity profile which develops between two coflowing streams of different velocities, differs in several ways from turbulence in a surface layer. Through these differences, the mixing-layer analogy provides an explanation for many of the observeddistinctive features of canopy turbulence. These include: (a) ratios between components of the Reynolds stress tensor; (b) the ratio KH/KM of the eddy diffusivities for heat and momentum; (c) the relative roles of ejections and sweeps; (d) the behaviour of the turbulent energy balance, particularly the major role of turbulent transport; and (e) the behaviour of the turbulent length scales of the active coherent motions (the dominant eddies responsible for vertical transfer near the top of the canopy). It is predicted that these length scales are controlled by the shear length scale L, = U(h)/U’(h) (where h is canopy height, U(z) is mean velocity as a function of height Z, and U’ = dU/dz). In particular, the streamwise spacing of the dominant canopy eddies is A, = mL,, with m = 8.1. These predictions are tested against many sets of field and wind-tunnel data. We propose a picture of canopy turbulence in which eddies associated with inflectional instabilities are modulated by larger-scale, inactive turbulence, which is quasi-horizontal on the scale of the canopy. 1. Introduction Vegetation-atmosphere transfer of momentum and scalar entities (heat, water vapour, CO2 and so on) influences numerous environmental variables and pro- cesses. Familiar examples include canopy microclimates; the energy and water balances of vegetated surfaces; vegetation and soil surface temperatures; the depo- sition and re-entrainment of dust and other particles; and wind damage to forests and crops. Spurred by such applications, knowledge of canopy turbulence has advanced steadily over the last three decades. Perhaps the main development over this time has been the recognition (as in turbulence research in general) that canopy turbu- lence is far from random, with major contributions to the turbulent motions arising from coherent eddies of canopy scale. This can lead to phenomena such as locally counter-gradient fluxes, thereby precluding the use of simple gradient-diffusion theory (K-theory) to describe vertical turbulent transfer in canopies. Furthermore, the coherent eddy structure within and just above the canopy is somewhat differ- ent from the eddy structure in the surface layer well above the canopy. This is
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Raupachetal96-canopymixinglayer - COHERENT EDDIES AND...

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