Multiscale Shear Forcing of Turbulence in the Nocturnal Boundary Layer: A Statistical Analysis
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Multiscale Shear Forcing of Turbulence in the Nocturnal Boundary Layer: A Statistical Analysis Vyacheslav Boyko1
· Nikki Vercauteren1
Received: 21 October 2019 / Accepted: 23 October 2020 © The Author(s) 2020
Abstract The lower nocturnal boundary layer is governed by intermittent turbulence which is thought to be triggered by sporadic activity of so-called sub-mesoscale motions in a complex way. We analyze intermittent turbulence based on an assumed relation between the vertical gradients of the sub-mean scales and turbulence kinetic energy. We analyze high-resolution nocturnal eddy-correlation data from 30-m tower collected during the Fluxes over Snow Surfaces II field program. The non-turbulent velocity signal is decomposed using a discrete wavelet transform into three ranges of scales interpreted as the mean, jet and sub-mesoscales. The vertical gradients of the sub-mean scales are estimated using finite differences. The turbulence kinetic energy is modelled as a discrete-time autoregressive process with exogenous variables, where the latter ones are the vertical gradients of the sub-mean scales. The parameters of the discrete model evolve in time depending on the locally-dominant turbulence-production scales. The three regimes with averaged model parameters are estimated using a subspaceclustering algorithm which illustrates a weak bimodal distribution in the energy phase space of turbulence and sub-mesoscale motions for the very stable boundary layer. One mode indicates turbulence modulated by sub-mesoscale motions. Furthermore, intermittent turbulence appears if the sub-mesoscale intensity exceeds 10% of the mean kinetic energy in strong stratification. Keywords Clustering analysis · Intermittent turbulence · Nocturnal boundary layer · Non-stationary autoregressive models · Sub-mesoscale motions
1 Introduction The atmospheric boundary layer in conditions of neutral or weak stability is well described using similarity theory (Grachev et al. 2013), but its modelling becomes arduous in increased stratification (Fernando and Weil 2010). In the nocturnal or polar boundary layer, for example in a very stable boundary layer (vSBL), the intermittency of turbulence challenges the existing
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Vyacheslav Boyko [email protected] Nikki Vercauteren [email protected]
1
Berlin, Germany
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V. Boyko, N. Vercauteren
parametrizations used in weather or climate models. An intermittent event of the turbulence kinetic energy (TKE) is often identified as an infrequent burst with a sharp peak followed by a rapid decay. A sequence of these bursts during a night can enhance mixing and change the structure of the boundary layer (Acevedo et al. 2006). Usually, in weak stability, a welldefined boundary layer exists in which turbulence is continuous and decreases with height according to Monin–Obukhov similarity theory, therefore, providing a predictable level of mixing. Excursions from this regime into the vSBL with less understood turbulence can occur in a variety of scenarios. Intermittent turbulence
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