Turbulence Statistics Above And Within Two Amazon Rain Forest Canopies |
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| Authors: | B Kruijt Y Malhi J Lloyd A D Norbre A C Miranda M G P Pereira A Culf J Grace |
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| Institution: | (1) Institute of Ecology and Resource Management, University of Edinburgh, Darwin Building, Mayfield Road, Edinburgh, EH9 3JU, U.K.;(2) Environmental Biology Group, Research School of Biological Sciences, Institute of Advanced Studies, Australian National University, Canberra, ACT, 0200, Australia;(3) Present address: Max Planck Institut für Biogeochemie, Postfach 100164, D-07701 Jena, Germany;(4) Instituto Nacional de Pesquisas da Amazonia, CPU78, 69011 Manaus, AM, Brazil;(5) Departamento de Ecologia, Universidade de Brasilia, DF-70910, Brazil;(6) Institute of Hydrology, Wallingford, Oxon, OX10 8BB, U.K. |
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| Abstract: | The turbulence structure in two Amazon rain forestswas characterised for a range of above-canopystability conditions, and the results compared withprevious studies in other forest canopies and recenttheory for the generation of turbulent eddies justabove forest canopies. Three-dimensional wind speedand temperature fluctuation data were collectedsimultaneously at up to five levels inside and abovetwo canopies of 30–40 m tall forests, during threeseparate periods. We analysed hourly statistics, jointprobability distributions, length scales, spatialcorrelations and coherence, as well as power spectraof vertical and horizontal wind speed.The daytime results show a sharp attenuation ofturbulence in the top third of the canopies, resultingin very little movement, and almost Gaussianprobability distributions of wind speeds, in the lowercanopy. This contrasts with strongly skewed andkurtotic distributions in the upper canopy. At night,attenuation was even stronger and skewness vanishedeven in the upper canopy. Power spectral peaks in thelower canopy are shifted to lower frequencies relativeto the upper canopy, and spatial correlations andcoherences were low throughout the canopy. Integrallength scales of vertical wind speed at the top of thecanopy were small, about 0.15 h compared to avalue of 0.28 h expected from the shear lengthscale at the canopy top, based on the hypothesis that theupper canopy air behaves as a plane mixing layer. Allthis suggests that, although exchange is not totallyinhibited, tropical rain forest canopies differ from other forests in that rapid, coherentdownward sweeps do not penetrate into the lowercanopy, and that length scales are suppressed. This isassociated with a persistent inversion of stability inthat region compared to above-canopy conditions. Theinversion is likely to be maintained by strong heatabsorption in the leaves concentrated near thecanopy top, with the generally weak turbulence beingunable to destroy the temperature gradients over thelarge canopy depth. |
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| Keywords: | Tropical forest Skewness Turbulence spectra Coupling Length scales Canopy inversion |
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