Turbulence anisotropy carried by streaks in the neutral atmospheric surface layer

We¹ investigate the relationships between coherent structures and turbulence anisotropy in the neutral planetary boundary layer by means of empirical orthogonal function (EOF) analysis of large-eddy simulation (LES) data. The simulated flow contains near-surface transient streaks similar to those revealed by recent observations. The EOF analysis extracts recurrent patterns from the signal based on its second-order spatial correlations. The scale and direction of streaks estimated subjectively from an examination of the LES flow field do correspond to that of the EOFs. The EOF streamlines bear more resemblance with those of optimal perturbations of an Ekman boundary layer than with unstable normal modes. We find that two characteristics of the turbulence depend in an anisotropic way on the horizontal wave vector of the velocity fluctuations: (i) the vertical extent up to which the turbulent kinetic energy (TKE) is concentrated and (ii) the ratio of the vertical TKE to the horizontal TKE. Although still present in the complete signal, this anisotropy is strongly emphasized by the EOF structures. Furthermore at horizontal wave vectors where recurrent patterns are dominant, the vertical/horizontal TKE ratio is representative of same ratio based on the total flow. This ratio is appreciably far from its isotropic value.