EGU2020-2315
https://doi.org/10.5194/egusphere-egu2020-2315
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Moisture sources of extreme Vb-floods in Central Europe

Amelie Krug1, Franziska Aemisegger2, Michael Sprenger2, Cristina Primo1, and Bodo Ahrens1
Amelie Krug et al.
  • 1Institute for Atmospheric and Environmental Sciences, Goethe Universität Frankfurt, Frankfurt am Main, Germany
  • 2Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland

River floods are the most common and devastating natural hazard in Europe. In this study, we focus on a specific flood type which is associated with so-called Vb-cyclones. These extratropical cyclones are defined by their pathway from the western Mediterranean Sea north-eastward over northern Italy along the eastern fringe of the Alps towards Central Europe. Prominent examples of Vb-floods are the July 1954 and the August 2002 floods in the Elbe and Danube catchments as well as the Odra flooding during May/June 2010.

Only a few Vb-cyclones cause extreme flooding in Central Europe, even though about 2-5 follow the Vb pathway on average per year. The processes which intensify these flood triggering Vb-cyclones are only partly understood. One potential mechanism could be the soil-precipitation feedback over the continent. Moreover, the resulting latent heat release could re-enforce the atmospheric blocking conditions, e.g., over eastern Europe, that foster cyclones to follow the Vb-like pathway.

Our study aims to increase knowledge about potential feedback mechanisms by quantifying the role of specific moisture sources. We analysed the moisture uptake for selected extreme events in the 20th century based on backward trajectories in dynamically downscaled ERA-20C reanalysis. The downscaling was performed over Europe with a high-resolution and interactively coupled atmosphere-ocean model setup (COSMO-CLM+NEMO). The Mediterranean Sea contributed to rainfall in the affected river catchments often at the event start. Throughout the events, other main moisture uptake regions were the European continent pointing towards an important role of the soil-moisture precipitation feedback, but also other oceanic sources such as the North Atlantic, the North Sea, and the Baltic Sea were identified. The large variety of the identified sources highlights the complex dynamical interplay of different airmasses leading to convergence of moisture during particularly severe flood producing heavy precipitation events.

How to cite: Krug, A., Aemisegger, F., Sprenger, M., Primo, C., and Ahrens, B.: Moisture sources of extreme Vb-floods in Central Europe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2315, https://doi.org/10.5194/egusphere-egu2020-2315, 2020

Comments on the presentation

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Presentation version 1 – uploaded on 01 May 2020
  • CC1: Comment on EGU2020-2315, Francisco Pastor, 04 May 2020

    Dear Dr. Krug

    Congrats on your excellent work. I have one question about the role of the Mediterranean on the rain events you studied. You state that the Med sea has a role at the start of an event, how important is the Med SST to the onset of the rain event? Does it affect the actual event start or just the event intensity?

    Thanks and best regards

    • AC1: Reply to CC1, Amelie Krug, 05 May 2020

      Dear Dr. Pastor,

      Thanks for your comment. The event start - in terms of Vb-cyclones  - is triggered by various processes, for example Alpine lee cyclogenesis (see Aebischer and Schär 1998, https://doi.org/10.1175/1520-0469(1998)055<0186:LLPVAC>2.0.CO;2) or atmospheric blocking conditions.

      Nonetheless, previous studies like Hofstätter et al. 2016 (https://doi.org/10.1002/2016WR019146) or Messmer et al. 2017 (https://doi.org/10.5194/esd-8-477-2017) indicate that high Med SSTs and thus high latent heating are an essential prerequisite for intenser Vb-cyclones. This is in accordance with our figure on slide 11 showing that higher SSTs in the Mediterranean Sea are associated with higher precipitation amounts, especially in the Danube catchment.

       

      Kind regards,

      Amelie (without Dr.)

      • CC2: Reply to AC1, Francisco Pastor, 05 May 2020

        Hi Amelie,  (Dr. will come for sure :-) )

        Yes, from the moisture uptake maps the Odra case gets an important moisture amount from, mainly, the Adriatic and Ligurian seas. Nice to see works linking SST and air-sea heat exchange to "remote" catchments, not just for coastal rain events. Thanks for the references.