Physical understanding of anisotropy in the Reynolds stress tensor of near-surface turbulence

Classical theories of atmospheric turbulence work well for isotropic turbulence. However, near the surface, as well as under strongly stable stratification, turbulence can be very anisotropic, due to the physical constraints of the ground and the buoyancy, respectively. This anisotropy has an impact on the mixing properties of turbulence, which need to be taken into account in parameterizations. In atmospheric boundary-layer studies, turbulence anisotropy mainly refers to the difference in intensity of velocity fluctuations along different directions. This analysis can be performed along the principal directions of the Reynolds stress tensor. By doing so, a classification of turbulence according to its anisotropy, independent of the choice of the coordinate system where turbulence is measured, can be developed. This classification is an useful tool for improving current scaling relations of near-surface turbulence. The present contribution focuses on the physical understanding of these different anisotropic states of turbulence, by exploring the possible sources which are driving  them. In addition, their relation with the variances and turbulent fluxes evaluated in the coordinate system commonly adopted in studies of near-surface turbulence is investigated. Special attention is given to results for stably stratified boundary layer, as under this condition the anisotropization of turbulence is considered one of the causes for poor performance of current parameterization at high Richardson number.