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A wavelet method to handle synoptic-scale coherent structures

Plu Matthieu

Synoptic-scale atmospheric dynamics is known to be driven by coherent structures that interact at different vertical levels of the atmosphere. For instance in the Mediterranean region, upper-level precursors coming from the Atlantic basin may govern the development of intense cyclogeneses and convective events that have a high weather impact. The synoptic-scale dynamics is subject to some balance between motion and mass fields, which enables a single field summary of the current flow distribution, using for example potential vorticity, that may be inverted to yield wind and temperature in a dry atmosphere. The potential-vorticity coherent structures are responsible for the majority of synoptic-scale events, however there is currently no satisfactory numerical tool that is able to describe and handle them.
The present work proposes a wavelet representation of the coherent structures of potential vorticity. Using a bidimensional discrete orthogonal wavelet transform and its translation-invariant information-redundant counterpart, it is possible to extract the localized coherent structures from the flow. This framework gives a means to understand better the dynamical processes and interactions between coherent structures; moreover it should give birth to new tools to deal with predictability issues. The capability and the limits of the algorithm and its application on a real-case cyclogenesis will help to show its utility for the understanding of synoptic-scale dynamics and for dealing with its predictability.