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Investigating turbulence intermittency via multiscale analysis and statistics
Nikki Vercauteren
Freie Universität Berlin
Jeudi 03/03/2016, 11:00-12:00
Bât. 701, P. 17C, LSCE Orme des Merisiers
Stably stratified atmospheric flows are typically characterized by weak and intermittent, anisotropic turbulence, gravity waves, low level jets, Kelvin-Helmholtz instability and other complex phenomena of unknown origin. All these phenomena greatly complicate the modeling and measurements of the stable boundary layer (SBL). Among these, the intermittency of turbulence clearly lacks physical understanding and that leads to specific problems in the stable atmospheric boundary layer (ABL) representation in weather or climate models. 
Identifying specific physical mechanisms triggering intermittent turbulence is complicated by the fact that stably stratified atmospheric flows are home to a myriad of non-turbulent motions that can exhibit structure such as ramp-cliff convection patterns, waves or microfront. There is evidence that such motions could be among the causes for the observed intermittency of turbulence, however the scientific community clearly lacks understanding of what these motions are and of the extent to which they affect turbulent mixing in the SBL. Some case studies suggest that there is indeed interplay between larger scale atmospheric flow features (at the so-called submesoscales) and onset of 
turbulence.  In front of the difficulties to distinguish between different physical mechanisms affecting turbulent mixing, this seminar will introduce the use of recent statistical tools for analyzing interactions of scales of motions in the SBL. A recent data clustering methodology, based on a bounded variation, finite element, vector autoregressive factor method (FEM-BV-VARX), will be applied to the SnoHATS dataset of near-surface stable boundary layer turbulence. We use the FEM-BV-VARX methodology to characterize the influence of non-turbulent, submesoscale motions on the turbulence in the SnoHATS dataset. Regimes are thereby identified, two of them weakly stable and two very stable turbulence states. In each identified regime, the variability of turbulent momentum fluxes will be characterized using an extended multiresolution flux decomposition methodology. The transport properties in each regime of near-surface SBL turbulence will be assessed. The same methodology is used to investigate the scales of motion responsible for shear generation of turbulence.
Contact : Aline Govin
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