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Inertia-dominated wall-sheared turbulent flows are composed of an inner and outer layer, where the former is occupied by the well-known autonomous inner cycle while the latter is composed of coherent structures with spatial extent comparable to the flow depth. In arbitrary streamwise–wall-normal planes, outer-layer structures instantaneously manifest as regions of quasi-uniform momentum – relative excesses and deficits about the Reynolds average – and for this reason are termed uniform momentum zones (UMZs). By virtue of this attribute, the interfacial zones between successive UMZs exhibit abrupt wall-normal gradients in streamwise momentum; these interfacial gradients cannot be explained by the notion of attached eddies, for which the vertical gradient goes as $(x_3^+)^{-1}$ in the outer layer, where $x_3^+$ is inner-normalized wall-normal position. Using data from direct numerical simulation (DNS) of channel turbulence across inertial regimes, we recover vertical profiles of Kolmogorov length a posteriori and show that $\eta ^+ \sim (x_3^+)^{1/4}$ , thereby requiring that ambient wall-normal gradients in streamwise velocity must scale as $(x_3^+)^{-1/2}$. The data reveal that UMZ interfaces are responsible for these relatively larger wall-normal gradients. The DNS data afford a unique opportunity to interpret inner- and outer-layer structures simultaneously: we propose that UMZs – and the associated outer-layer dynamics – can be explained as the product of inner-layer bluff-body-like interactions, wherein wakes of quasi-uniform momentum emanate from the inner layer; wake-scaling arguments agree with observations from DNS. [ABSTRACT FROM AUTHOR]

The inadequacy of conventional gradient diffusion in closure modelling of turbulent heat fluxes within the convective atmospheric boundary layer is often alleviated by accounting for non-local transport effects, such as Deardorff's counter-gradient models, Wyngaard's transport asymmetry closures or mass-flux parametrization. This concept of large-eddy flux transport is examined here with the principal aim of unifying these seemingly different models. Using large-eddy simulation (LES) runs for the atmospheric boundary layer, spanning weakly to strongly convective conditions, a generic diagnostic framework that encodes the role of third-order moments in non-local transport is developed and tested. The premise is that these non-local effects are responsible for the inherent asymmetry in vertical transport and hence the necessary non-Gaussian nature of the joint probability density function (JPDF) of vertical velocity and potential temperature must account for these effects. Conditional sampling (quadrant analysis) of this JPDF and the imbalance between the flow mechanisms of ejections and sweeps are used to characterize this asymmetry, which is then linked to the third-order moments using a cumulant-discard method for the Gram-Charlier expansion of the JPDF. While the concept of ejection-sweep events used here is not a simple extension of that commonly used in the surface layer, their connection to third-order moments shows that the concepts of bottom-up/top-down diffusion or updraught/downdraught models are accounted for by various quadrants of the JPDF. An analogy between mass-flux models and the relaxed eddy accumulation method reveals that there is a seemingly implicit assumption of a Gaussian JPDF in the former. [ABSTRACT FROM AUTHOR]

The mean velocity profile (MVP) in thermally stratified atmospheric surface layers (ASLs) deviates from the classic logarithmic form. A theoretical framework was recently proposed (Katul et al. Phys. Rev. Lett., vol. 107, 2011, 268502) to link the MVP to the spectrum of turbulence and was found to successfully predict the MVP for unstable stratification. However, the theory failed to reproduce the MVP in stable conditions (Salesky et al. Phys. Fluids, vol. 25, 2013, 105101), especially when ζ >0.2 (where ζ is the atmospheric stability parameter). In the present study, it is demonstrated that this shortcoming is due to the failure to identify the appropriate length scale that characterizes the size of momentum transporting eddies in the stable ASL. Beyond ζ ≈0.2 (near where the original theory fails), the Ozmidov length scale becomes smaller than the distance from the wall z and hence is a more stringent constraint for characterizing the size of turbulent eddies. An expression is derived to connect the Ozmidov length scale to the normalized MVP (ϕm), allowing ϕm to be solved numerically. It is found that the revised theory produces a prediction of ϕm in good agreement with the widely used empirical Businger-Dyer relation and two experimental datasets in the stable ASL. The results here demonstrate that the behaviour of ϕm in the stable ASL is closely linked to the size of momentum transporting eddies, which can be characterized by the Ozmidov scale under mildly to moderately stable conditions (0.2<ζ <1-2). [ABSTRACT FROM AUTHOR]