Scale interactions and anisotropy in stable boundary layers

TL;DR

This paper investigates how different flow regimes in stable boundary layers influence turbulence and anisotropy, revealing that sub-mesoscale wind variability significantly impacts turbulence structure and persistence.

Contribution

It introduces a novel analysis of scale interactions and anisotropy in stable boundary layers using FEM-BV-VARX clustering, identifying distinct flow regimes and their effects on turbulence.

Findings

Flow regimes with strong sub-mesoscale influence show highly anisotropic, one-component turbulence.

Periods of strong sub-mesoscale influence exhibit more persistent and anisotropic Reynolds stresses.

External sub-mesoscale forcing explains a significant portion of turbulence variability in stable boundary layers.

Abstract

Regimes of interactions between motions on different time-scales are investigated in the FLOSSII dataset for nocturnal near-surface stable boundary layer (SBL) turbulence. The non-stationary response of turbulent vertical velocity variance to non-turbulent, sub-mesoscale wind velocity variability is analysed using the bounded variation, finite element, vector autoregressive factor models (FEM-BV-VARX) clustering method. Several locally stationary flow regimes are identified with different influences of sub-meso wind velocity on the turbulent vertical velocity variance. In each flow regime, we analyse multiple scale interactions and quantify the amount of turbulent variability which can be statistically explained by external forcing by the sub-meso wind velocity. The state of anisotropy of the Reynolds stress tensor in the different flow regimes is shown to relate to these different signatures of scale interactions. In flow regimes under considerable influence of the sub-mesoscale wind variability, the Reynolds stresses show a clear preference for strongly anisotropic, one-component states. These periods additionally show stronger persistence in their dynamics, compared to periods of more isotropic stresses. The analyses give insights on how the different topologies relate to non-stationary turbulence triggering by sub-mesoscale motions.