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Hydrologic Discovery Through Physical Analysis Honoring the Scientific Legacies of Wilfried H. Brutsaert and Jean-Yves Parlange >
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Title: E3. Stability and Properties of Near-Surface Turbu-lent Shear Flows: Enhancing Our Understanding of Passive Scalar Fields
Authors: Fitton,George F.
Tchiguirinskaia, Ioulia
Schertzer, Daniel
Lovejoy, Shaun
Issue Date: May-2012
Publisher: Internet-First University Press
Abstract: We analyse two field experiments of near-surface atmospheric turbulence, particularly the applicability of the concept of the stability parameter (Brutsaert, 1999) in the context of intermittency. The first field experiment consisted of a single mast located in Corsica, France. Three sonic anemometers were installed on the mast at 22, 23 and 43m; measuring three-dimensional wind velocity data at 10Hz. Complex terrain and buoyancy forces were observed to have influenced the measurements. The second (GROWIAN) field experiment took place in Germany. It consisted of an array of propeller anemometers measuring wind speed inflow data at 2.5Hz over flat terrain. The propeller anemometers were positioned vertically at 10, 50, 75, 100, 125 and 150m with four horizontal measurements taken at 75, 100 and 125m. The spatial measurements meant we could calculate the horizontal and vertical shear structure functions of the horizontal wind allowing us to test Taylor’s hypothesis over a wide range of scales. To statistically characterise the stability, we used the probability distributions of the gradient Richardson number — large negative values indicate unstable conditions, large positive values indicate stable conditions and values close to zero are indicative of neutral conditions — this implies therefore anti-symmetric distributions correspond to either stable or unstable conditions. Since the empirical probability distributions follow power law behaviour the departure from neutral to (un)stable conditions is quantified with the ratio of the corresponding power law exponents. Finally, under the universal multifractal (UM) framework, we study and compare the scaling properties of near-surface atmospheric turbulence. We found in both experiments the multifractality parameter, α ≈ 1.5, and the intermittency parameter, C1 ≈ 0.2. The scaling non-conservativeness parameter, H, of the vertical shears of the horizontal wind varied from Kolmogorov to Bolgiano-Obukhov depending on the condition of stability. These results give new insights into the 23/5-dimensional model of stratified turbulence (Schertzer and Lovejoy, 1985, Lilley et al., 2006, Fitton et al. 2011) thus greatly enhancing our understanding of the multifractal properties of passive scalar fields, e.g., water vapour.
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