E3. Stability and Properties of Near-Surface Turbu-lent Shear Flows: Enhancing Our Understanding of Passive Scalar Fields
Fitton,George F.; Tchiguirinskaia, Ioulia; Schertzer, Daniel; Lovejoy, Shaun
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.
Internet-First University Press