Preprints
https://doi.org/10.5194/wcd-2022-4
https://doi.org/10.5194/wcd-2022-4
 
25 Jan 2022
25 Jan 2022
Status: a revised version of this preprint was accepted for the journal WCD and is expected to appear here in due course.

The role of cyclones and PV cutoffs for the occurrence of unusually long wet spells in Europe

Matthias Röthlisberger1, Barbara Scherrer1, Andries Jan de Vries1,a, and Raphael Portmann1 Matthias Röthlisberger et al.
  • 1Institute for Atmospheric and Climate Science, ETH Zürich, Zürich, Switzerland
  • anow at: Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, Switzerland

Abstract. The synoptic dynamics leading to the longest wet spells in Europe are so far poorly investigated, despite these events’ potentially large societal impacts. Here we examine the role of cyclones and PV cutoffs for unusually long wet spells in Europe, defined as the 20 longest uninterrupted periods with at least 5 mm daily accumulated precipitation at each ERA-Interim grid point in Europe (this set of spells is hereafter referred to as δ20). The δ20 occur predominantly in summer over the eastern continent, in winter over the North Atlantic, in winter or fall over the Atlantic coast, and in fall over the Mediterranean and European inland seas. Four case studies reveal distinct archetypal synoptic storylines for long wet spells: (a) A seven-day wet spell near Moscow, Russia, is associated with a single slow-moving cutoff-cyclone couple; (b) a 15-day wet spell in Norway features a total of nine rapidly passing extratropical cyclones and illustrates serial cyclone clustering as a second storyline; (c) a 12-day wet spell in Tuscany, Italy, is associated with a single but very large cutoff-complex, which is replenished multiple times by a sequence of recurrent anticyclonic wave breaking events over the North Atlantic and western Europe; and (d) a 17-day wet spell in the Balkans features intermittent periods of diurnal convection in an environment of weak synoptic forcing and recurrent passages of cutoffs and thus also highlights the role of diurnal convection for long wet spells over land. A systematic analysis of cyclone and cutoff occurrences during the δ20 reveals considerable spatial variability in their respective role for the δ20. For instance, cyclones and cutoffs are present anywhere between 10 % and 90 %, and 20 % and 70 % of the δ20 time steps, respectively, depending on the geographical region. However, overall both cyclones and cutoffs appear in larger number and at a higher rate during the δ20 compared to climatology. Furthermore, in the Mediterranean, the cutoffs and cyclones are significantly more persistent during the δ20 compared to climatology. Our study thus documents for the first time the palette of synoptic storylines accompanying unusually long wet spells across Europe, which is a prerequisite for developing an understanding of how these events might change in a warming climate and for evaluating the ability of climate models to realistically simulate the synoptic processes relevant to these events.

Matthias Röthlisberger et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on wcd-2022-4', Anonymous Referee #1, 28 Feb 2022
  • RC2: 'Comment on wcd-2022-4', Anonymous Referee #2, 03 Mar 2022
  • AC1: 'Response to the reviewers of wcd-2022-4', Matthias Röthlisberger, 05 Apr 2022

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on wcd-2022-4', Anonymous Referee #1, 28 Feb 2022
  • RC2: 'Comment on wcd-2022-4', Anonymous Referee #2, 03 Mar 2022
  • AC1: 'Response to the reviewers of wcd-2022-4', Matthias Röthlisberger, 05 Apr 2022

Matthias Röthlisberger et al.

Matthias Röthlisberger et al.

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Short summary
We examine the palette of synoptic storylines accompanying unusually long wet spells in Europe. Thereby, we identify a hitherto not documented mechanism for generating long wet spells, which involves recurrent Rossby wave breaking and subsequent cutoff replenishment. Understanding the synoptic processes behind long wet spells is relevant in light of projected changes in wet spell characteristics, as it is a prerequisite for evaluating climate models with regard to such events.