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Canopy Spectra and Dissipation John Finnigan CSIRO Atmospheric Research Canberra, Australia

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The turbulent kinetic energy budget in the canopy

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Canopy TKE Budget Very high production rates of TKE are balanced by very large dissipation rates

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Viscous Dissipation

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Vortex stretching by turbulent strain field Vortex stretching by mean strain field Pressure, turbulent and dispersive transport of Destruction of by viscosity Vortex stretching in the strain fields around plants and their wakes Production/destruction of by coupling between local variations in mean viscous and Reynolds stresses Canopy waving; flux of –ve vorticity from solid surfaces The Canopy Dissipation Equation +/- ++ +/- + - +/- - ++

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How can we measure and parameterize the dissipation? The problem: dissipation peaks at the Kolmogorov scale, in the atmosphere so we cant measure eddies of this scale with conventional instruments. Instead we try to represent the dissipation in terms of the much larger and measurable eddies. To do this we need a model of the processes that transfer energy from the energy-containing to the dissipation scales. This is why Kolmogorovs theory is so important for experimentalists-it relates to the eddies in the ISR.

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Kolmogorov Spectrum for free-air turbulence In the viscous subrange In the inertial subrange (ISR), Using dimensional and symmetry arguments, Kolmogorov (1941) found that in the ISR

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Spectra measured in a spruce canopy by Amiro (1990) Canopy spectra measured at high Re generally show 11 (k 1 ) rolling off more rapidly than –5/3 in the low frequency end of the ISR but not all data show this. Isotropy in the ISR requires: And this is usually violated in canopy data Do measurements show the predicted form?

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Measuring Spectra to compare with Kolmogorov and modified ISR theory

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Comparison of the theory with spectra from the Moga experiment, a 10m high uniform Eucalyptus forest.

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Canopy TKE Budget: estimating total dissipation The total dissipation is calculated as the sum of 2 terms: energy passing down the eddy cascade and work against drag by turbulence. The latter term represents the spectral short-cut. (Finnigan, 2000) Data from Brunet et al. (1996)

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Implications for Modelling In k-epsilon and higher order closure models the tke or variance equations must include the extra wake production by the mean flow working against drag. The dissipation equation must include a re- parameterized eddy cascade term, plus W D (U, Cd), the term representing the work of the turbulence against drag. The turbulent time scale used in all other parameterisations is formed from (Ayotte et al, 1999)

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Conclusions tke in canopies finds it way to dissipation scales via the eddy cascade and via interaction with the canopy elements The assumptions of Kolmogorov theory for the form of E(k) in the ISR are violated because energy is continually lost as eddies do work against drag and the anisotropy of the drag ensures that the turbulence is at most axially symmetric, not isotropic We can treat the two routes to the dissipation scales separately but we must derive a new form for E(k) in the ISR that reflects these extra processes This form appears to predict the spectral levels and anisotropy of (two) canopy experiments quite well. The total dissipation is the sum of the energy passing down the eddy cascade (reduced compared to free air) and the work of the turbulence against drag This changes the turbulent time scale with implications for closure models.

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Physics of turbulence at small scales Turbulence is a property of the flow not the fluid. 1. Can only be described statistically. 2. Dissipates energy.

Physics of turbulence at small scales Turbulence is a property of the flow not the fluid. 1. Can only be described statistically. 2. Dissipates energy.

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