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Weather and Climate Dynamics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/wcd-2020-1
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/wcd-2020-1
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 14 Jan 2020

Submitted as: research article | 14 Jan 2020

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This discussion paper is a preprint. It is a manuscript under review for the journal Weather and Climate Dynamics (WCD).

The role of large-scale dynamics in an exceptional sequence of severe thunderstorms in Europe May/June 2018

Susanna Mohr1,2, Jannik Wilhelm1, Jan Wandel1, Michael Kunz1,2, Raphael Portmann3, Heinz Jürgen Punge1, Manuel Schmidberger1, and Christian M. Grams1 Susanna Mohr et al.
  • 1Karlsruhe Institute of Technology (KIT), Institute of Meteorology and Climate Research (IMK-TRO), Karlsruhe, Germany
  • 2Center for Disaster Management and Risk Reduction Technology (CEDIM), Karlsruhe, Germany
  • 3Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland

Abstract. Over three weeks in May and June 2018, an exceptionally large number of thunderstorms hit vast parts of western and central Europe, causing precipitation of up to 80 mm and several flash floods. During this time, the large-scale atmospheric circulation, which was characterized by a blocking situation over northern Europe, influenced atmospheric conditions relevant for thunderstorm development. Initially, the southwesterly flow on the western flank of the blocking anticyclone induced the advection of warm, moist, and unstably stratified air masses. Due to a low-pressure gradient associated with the blocking anticyclone, these air masses were trapped in western and central Europe, remained almost stationary and prevented a significant air mass exchange. In addition, the low-pressure gradient led to weak flow conditions in the mid-troposphere and thus to low vertical wind shear that prevented thunderstorms from developing into severe organized systems. Most of the storms formed as local-scale, relatively slow-moving single cells. However, due to the related weak propagation speed, several thunderstorms were able to produce torrential heavy rain that affected local-scale areas and triggered several flash floods.

Atmospheric blocking also increased the upper-level cut-off low frequency on its upstream regions, which was up to 10 times higher than the climatological mean. Together with filaments of positive potential vorticity (PV), the cut-offs served as trigger mechanisms for a majority of the thunderstorms. For the 22-day study period, we found that more than 50 % of lightning strikes can be linked to a nearby cut-off low or PV filament. The exceptional persistence of low stability combined with weak wind speed in the mid-troposphere over three weeks has not been observed during the past 30 years.

Susanna Mohr et al.
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Susanna Mohr et al.
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Short summary
We investigated an exceptional thunderstorms episode in 2018. In this context, atmospheric blocking provided the large-scale environmental conditions favouring convection in its vicinity. In addition, blocking was accompanied by high cut-off frequency on its upstream side (10 times higher than climatology), which often served as trigger for the thunderstorms. The exceptional persistence of low stability combined with weak wind speed in the mid-troposphere has never been observed during the past.
We investigated an exceptional thunderstorms episode in 2018. In this context, atmospheric...
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