Eddy Scale-wise Topology Underlying Turbulence Anisotropy Illuminates the Dissimilar Transport of Momentum, Heat, and Moisture in a Stably Stratified Katabatic Flow

Abstract

The backdrop for this study is a knowledge gap about how turbulence anisotropy relates to the dissimilar transport of momentum and scalars. We use single-level measurements of turbulence over an alpine glacier for exploring the dissimilar transport of momentum, heat, and moisture in stably stratified katabatic flows. Our study is motivated by the need of addressing their flux dissimilarity from a fresh perspective of anisotropic motions of turbulence. Its objective is to promote new understanding of boundary-layer turbulence anisotropy as one possible factor in dissimilar behaviours between momentum and scalar transport over a sloping terrain. Specifically, the momentum–heat flux correlation (${R}_{{F}_{uT}}$) and the heat–moisture flux correlation (${R}_{{F}_{Tq}}$) coefficients vary across three different bulk states of kinetic anisotropy. Those states, identified using the barycentric Lumley map, suggest the predominance of two-component turbulence (being axisymmetric or not) and miscellaneous turbulence (whose topological shape is less salient). Miscellaneous turbulence typically bears a higher degree of the flux similarity between momentum and heat (i.e., ${R}_{{F}_{uT}}$ > 0.6) but a lower degree of that between heat and moisture (i.e., $\left|{R}_{{F}_{Tq}}\right|$ < 0.7). The multi-resolution decomposition technique is then applied to identify larger-scale eddies of two-component topology, intermediate-scale eddies of oblate topology, and smaller-scale eddies of isotropic topology. Further analysis shows that an explicit change in eddy scale-wise topology is correlated not only with variations in ${R}_{{F}_{uT}}$ and $\left|{R}_{{F}_{Tq}}\right|$ but with the dissimilar transport of momentum and scalars, so explaining a deviation from the Reynolds and the Lewis analogies in fluid mechanics.