Abstract
Dispersive flux terms are formed when the time-averaged meanmomentum equation is spatially averaged within the canopy volume.These fluxes represent a contribution to momentum transfer arisingfrom spatial correlations of the time-averaged velocity componentswithin a horizontal plane embedded in the canopy sublayer (CSL).Their relative importance to CSL momentum transfer is commonlyneglected in model calculations and in nearly all fieldmeasurement interpretations. Recent wind-tunnel studies suggestthat these fluxes may be important in the lower layers of thecanopy; however, no one study considered their importance acrossall regions of the canopy and for a wide range of canopy roughnessdensities. Using detailed laser Doppler anemometry measurementsconducted in a model canopy composed of cylinders within a largeflume, we demonstrate that the dispersive fluxes are onlysignificant (i.e., >10%) for sparse canopies. These fluxes arein the same direction as the turbulent flux in the lower layers ofthe canopy but in the opposite direction near the canopy top. Fordense canopies, we show that the dispersive fluxes are <5% atall heights. These results appear to be insensitive to theReynolds number (at high Reynolds numbers).
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Bohm, M., Finnigan, J. J., and Raupach M. R.: 2000, 'Dispersive Fluxes and Canopy Flows: Just How Important Are They?', in American Meteorology Society, 24th Conference on Agricultural and Forest Meteorology, 14-18 August 2000, University of California, Davis, CA, pp. 106-107.
Cheng, H. and Castro, I. P.: 2002, 'Near Wall Flow over Urban-Like Roughness', Boundary-Layer Meteorol. 104, 229-259.
Finnigan, J.: 1985, 'Turbulent Transport in Flexible Plant Canopies', in B. A. Hutchison and B. B. Hicks (eds.), The Forest-Atmosphere Interaction, Reidel, Dordrecht, pp. 443-480.
Finnigan, J.: 2000, 'Turbulence in Plant Canopies', Annu. Rev. Fluid Mech. 32, 519-571.
Kaimal, J. C. and Finnigan, J. J.: 1994, Atmospheric Boundary Layer Flows: Their Structure and Measurement, Oxford University Press, 289 pp.
Poggi, D., Porporato, A., and Ridolfi, L.: 2003, 'An Experimental Contribution to Near-Wall Measurements by Means of a Special Laser Doppler Anemometry Technique', Exp. Fluids 32, 366-375.
Poggi, D., Porporato, A., Ridolfi, L., Albertson, J. D., and Katul, G. G.: 2004, 'The Effect of Vegetation Density on Canopy Sub-layer Turbulence', Boundary-Layer Meteorol. 111, 565-587.
Raupach, M. R.: 1994, 'Simplified Expressions for Vegetation Roughness Length and Zero-Plane Displacement as Functions of Canopy Height and Area Index', Boundary-Layer Meteorol. 71, 211-216.
Raupach, M. R. and Shaw, R. H.: 1982, 'Averaging Procedures for Flow within Vegetation Canopies', Boundary-Layer Meteorol. 61, 47-64.
Raupach, M. R., Coppin, P. A., and Legg, B. J.: 1986, 'Experiments on Scalar Dispersion within a Model Plant Canopy. Part I: The Turbulence Structure', Boundary-Layer Meteorol. 35, 21-52.
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Poggi, D., Katul, G.G. & Albertson, J.D. A Note On The Contribution Of Dispersive Fluxes To Momentum Transfer Within Canopies. Boundary-Layer Meteorology 111, 615–621 (2004). https://doi.org/10.1023/B:BOUN.0000016563.76874.47
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DOI: https://doi.org/10.1023/B:BOUN.0000016563.76874.47