Spatial Variability of Turbulent Fluxes in the Roughness Sublayer of an Even-Aged Pine Forest |
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| Authors: | Gabriel Katul Cheng-I Hsieh David Bowling Kenneth Clark Narasinha Shurpali Andrew Turnipseed John Albertson Kevin Tu Dave Hollinger Bob Evans Brian Offerle Dean Anderson David Ellsworth Chris Vogel Ram Oren |
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| Institution: | (1) Present address: School of the Environment, Duke University, Durham, NC, 27708-0328, U.S.A.;(2) Environmental, Population, and Organismic Biology, University of Colorado, Bolder, CO, 80309, U.S.A.;(3) School of Forest Resources and Conservation, University of Florida, Gainesville, FL, 32611, U.S.A.;(4) Department of Geography, Indiana University, Bloomington, IN, 47405, U.S.A.;(5) Department of Environmental Sciences, University of Virginia, Charlottesville, VA, 22903, U.S.A.;(6) United States Department of Agriculture Forest Service, 271 Mast Rd., Durham, NH, 03824, U.S.A.;(7) M.S.413, Federal Center, United States Geological Survey, Denver, CO, 80225, U.S.A.;(8) University of Michigan Biological Station, 9008 Biological Rd, Pellston, MI, 49769, U.S.A. |
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| Abstract: | The spatial variability of turbulent flow statistics in the roughness sublayer (RSL) of a uniform even-aged 14 m (= h) tall loblolly pine forest was investigated experimentally. Using seven existing walkup towers at this stand, high frequency velocity, temperature, water vapour and carbon dioxide concentrations were measured at 15.5 m above the ground surface from October 6 to 10 in 1997. These seven towers were separated by at least 100m from each other. The objective of this study was to examine whether single tower turbulence statistics measurements represent the flow properties of RSL turbulence above a uniform even-aged managed loblolly pine forest as a best-case scenario for natural forested ecosystems. From the intensive space-time series measurements, it was demonstrated that standard deviations of longitudinal and vertical velocities ( u, w) and temperature (T) are more planar homogeneous than their vertical flux of momentum (u*
2) and sensible heat (H) counterparts. Also, the measured H is more horizontally homogeneous when compared to fluxes of other scalar entities such as CO2 and water vapour. While the spatial variability in fluxes was significant (>15 %), this unique data set confirmed that single tower measurements represent the  canonical structure of single-point RSL turbulence statistics, especially flux-variance relationships. Implications to extending the moving-equilibrium hypothesis for RSL flows are discussed. The spatial variability in all RSL flow variables was not constant in time and varied strongly with spatially averaged friction velocity u*, especially when u* was small. It is shown that flow properties derived from two-point temporal statistics such as correlation functions are more sensitive to local variability in leaf area density when compared to single point flow statistics. Specifically, that the local relationship between the reciprocal of the vertical velocity integral time scale (Iw) and the arrival frequency of organized structures ( /h) predicted from a mixing-layer theory exhibited dependence on the local leaf area index. The broader implications of these findings to the measurement and modelling of RSL flows are also discussed. |
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| Keywords: | Canopy turbulence Moving equilibrium hypothesis Planar homogeneity Roughness sublayer Spatial variability Turbulent fluxes |
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