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Clouds are known to be turbulent, but the details of their internal turbulent structure have been largely unexplored. Measurements of turbulent velocities in stratocumulus clouds presented here reveal an intermittent structure consistent with that observed in classic homogeneous isotropic turbulence. The measurements were taken close to cloud top in a 200-m-thick cloud layer over a path of approximately 6 km, using a hot-wire anemometer below a helicopter as part of the Airborne Cloud Turbulence Observation System (ACTOS) measurement system. Hot-wire signal artifacts resulting from droplet impacts are removed without significantly degrading the signal, such that high-order velocity structure functions can be evaluated. The structure function analysis for orders 2-8 show statistically significant departures from the Kolmogorov's 1941 scaling, yielding scaling exponents consistent with the Kolmogorov--Obukhov refined similarity hypothesis, with an intermittency exponent of 0.25. This is in agreement with the accepted value determined in single-phase flows under carefully controlled conditions, and no evidence is found of any departure from the large body of knowledge obtained from the laboratory on the finescale turbulence structure. This suggests that processes depending on the finescale structure of turbulence that cannot presently be measured in clouds can be explored in the laboratory setting. Since these findings pertain to clouds with relatively low liquid water content and weak turbulence, further work will be required to determine their applicability to other cloud types. DOI: 10.1175/2009JAS3200.1

The postulate of local isotropy (PLI) is tested in a wind tunnel uniform shear flow in which the Reynolds number is varied over the range 100≼R[sub λ]≼1, 100(6.7×10[sup 2]≼R[sub l]≼6.3×10[sup 4]). The high R[sub λ] is achieved by using an active grid [Mydlarski and Warhaft, J. Fluid Mech. 320, 331 (1996)] in conjunction with a shear generator. We focus on the increments of the longitudinal velocity fluctuations in the direction of the mean shear. PLI requires that odd order moments of these quantities approach zero as R[sub λ]→∞. Confirming the lower Reynolds number measurements of Garg and Warhaft [Phys. Fluids 10, 662 (1998)], we show that the skewness of ∂u/∂y decreases as R[sub λ][sup -0.5] (with a value of 0.2 at R[sub λ]∼1000). Although the decrease is slower than classical scaling arguments suggest, the result is consistent with PLI, indicating a negligible value at high R[sub λ]. However, the normalized fifth moment, <(∂u/∂y)[sup 5]>/<(∂u/∂y)[sup 2]>[sup 5/2], is of order 10, and shows no diminution with Reynolds number, while the normalized seventh moment increases with R[sub λ]. These dissipation range results are inconsistent with PLI. Within the inertial subrange we show that all the odd order moments of the increments of Δu(y) are nonzero, exhibiting scaling ranges. Here, the skewness structure function has a value ∼0.5 indicating that in the inertial subrange significant anisotropy is evident even at the third moment level. Fifth- and seventh-order inertial subrange skewness structure functions are of order 10 and 100, respectively. The results show that PLI is untenable, both at dissipation and inertial scales, at least to R[sub λ]∼1000, and suggest it is unlikely to be so even at higher Reynolds numbers. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

By generating a temperature step in grid turbulence using two new methods (a mandoline and a toaster) the evolution of some first-, second-, third-, and fourth-order moment quantities of the temperature field in a thermal mixing layer are studied. The results confirm the earlier experimental findings of LaRue and Libby [Phys. Fluids 24, 597 ( 1981 ) ] and LaRue et al. [Phys. Fluids 24, 1927 ( 1981 ) ] who used a half-heated grid to generate the thermal field. It is also shown that although second-order quantities such as the heat flux coefficient and the centerline temperature variance appear to have reached their equilibrium values and that all quantities collapse well using the half-width of the mean temperature profile, the data is not consistent with self-similarity. [ABSTRACT FROM AUTHOR]