Quantification of Global Intermittency in Stably Stratified Ekman Flow
In the atmospheric boundary layer, some turbulence-like structures are maintained up to a stable density stratification several times higher than the above-mentioned linear stability analysis predicts. Nonetheless, the cessation of turbulence in such flows exposed to stable density stratification is a well-recognized problem. For non-rotating configurations, namely stably stratified channel- and free-shear flows, it has been shown that this cessation does not occur as an on–off process but is rather a complex transition from a turbulent to a laminar state. When stratification increases gradually, this transition begins with the localized absence of turbulent eddies in an otherwise turbulent flow, and has recently been shown to also occur in stably stratified Ekman flow. This localized absence of turbulence bears a striking resemblance to the absence of turbulence on some or all scales even close to the surface which is sometimes observed in the atmosphere and has been termed Global Intermittency. We propose here a method based on the intermittency factor together with high-pass-filtered flow fields that successfully distinguishes between turbulent and non-turbulent patches in Ekman flow.
KeywordsDirect Numerical Simulation Richardson Number Intermittency Factor Localize Absence Stratify Boundary Layer
Support from the Max Planck Society through its Max Planck Research Groups program is gratefully acknowledged. Computational time was provided by the Jülich Supercomputer Centre under that grant HHH07.