What can anisotropy tell us about the nature of atmospheric boundary layer flows*

Anisotropy is one of the fundamental characteristics of all turbulent flows in nature, engineering, and technology. At large scales, turbulence rarely attains an energy state that is equipartitioned between the three velocity components or a state where turbulent stresses disappear. This deviation from isotropy underscores turbulence's essential role in promoting momentum transfer.

 

Here, we explore the nature of turbulence anisotropy in high Reynolds number stratified turbulent boundary layer flows. For this purpose, we use data from a comprehensive range of atmospheric turbulence measurement datasets over canonical and highly complex surfaces, together with anisotropy invariant analysis.  The results show that the degree of departure from isotropy explains the observed deviations from the classical surface-layer scaling laws. This departure is largest for large-scale quasi-isotropic turbulence, which contrary to expectations, is shown to be characterized by larger than expected efficiency of transport of momentum and heat. We further use simplified Reynolds stress budgets and scaling analysis to examine the drivers of this quasi-isotropic turbulence, both over canonical and highly complex surfaces, and highlighting the change of anisotropy as stratification becomes progressively more dominant, as well as terrain progressively more complex. Our findings provide avenues for enhanced turbulence modelling and highlighting limitations of conventional approaches under strong stratification or complex environments.