Horizontal meandering in direct numerical simulations of the stable boundary layer

Abstract

Large‐scale nonturbulent motions are investigated using high‐resolution direct numerical simulations of the turbulent Ekman layer at a moderate Reynolds number. In particular, the role of stable stratification effects in the generation and amplification of large‐scale oscillations in the wind‐speed components and buoyancy is analysed. Eulerian autocorrelation functions (EAFs) and spectral analysis help to describe and characterize such modes as meandering‐like structures. Focus is given to the strong stability cases, where vertical turbulent motions are damped and large‐scale modes drive near‐wall patterns of flow laminarization. Horizontal meandering arises in the near‐wall region when the ratio of vertical wind‐speed variance to horizontal wind‐speed variance decreases to small orders of magnitude. In the case of strong stratification, the characteristic features of meandering motion were identified as negative lobes in the EAF, with corresponding low‐frequency peaks in the horizontal wind speed and buoyancy spectra. The Ekman configuration used reproduced the development of meandering‐like structures satisfactorily in strong stable conditions, as indicated by field observations of the stable boundary layer and its dependence on the level of stratification. At strong stability, the Rotta model constant, which represents the relationship between the dissipation and return to isotropy terms in the velocity variance budget, is shown to vary with stability.