EGU2020-13233
https://doi.org/10.5194/egusphere-egu2020-13233
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Rossby wave packets associated with extreme precipitation events over Northern-Italy

Federico Grazzini1,3, Georgios Fragkoulidis2, Franziska Teubler2, Volkmar Wirth2, and George Craig1
Federico Grazzini et al.
  • 1LMU, Meteorologisches Institut, Atmosphärische Dynamik, München, Germany (federico.grazzini@lmu.de)
  • 2Institute for Atmospheric Physics, Johannes Gutenberg University Mainz, Mainz, Germany
  • 3ARPAE SIMC Emilia-Romagna, Italy

Several studies on extreme precipitation events (EPEs) in the alpine area reported, as the main triggering factor, a meridionally elongated upper-level trough (i.e., a breaking Rossby wave) as part of an incoming Rossby wave packet (RWP). In this work, we investigate a vast number of EPEs occurring between 1979 and 2015 in northern-central Italy. The EPEs are subdivided into three categories (Cat1, Cat2, Cat3) according to thermodynamic conditions over the affected region. The three categories do not only differ locally but also in the evolution of precursor RWPs. These differences cannot be solely explained by the apparent seasonality of the flow; therefore, the relevant physical processes in the RWP propagation of each case are further investigated. In particular, we show that RWPs associated with the strongest EPEs, namely the ones falling in Cat2, undergo a substantial amplification over the western N. Atlantic due to anomalous ridge-building two days before the event; arguably due to diabatic heating sources. This type of development induces a downstream trough which is highly effective in focusing water vapour transport towards the main orographic barriers of the Apennines and the Alps. Finally, we identify an increasing trend of water vapour transport over the western N. Atlantic which is likely associated with the observed increase in Cat2 and Cat3 events

How to cite: Grazzini, F., Fragkoulidis, G., Teubler, F., Wirth, V., and Craig, G.: Rossby wave packets associated with extreme precipitation events over Northern-Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13233, https://doi.org/10.5194/egusphere-egu2020-13233, 2020

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Presentation version 1 – uploaded on 01 May 2020
  • CC1: Potential impact of North Atlantic extratropical transition, Jacopo Riboldi, 06 May 2020

    Very interesting work! I was interested by the diabatic amplification of EP2 - related wave packets: it reminded me of a study I collaborated to, about extreme precipitation events downstream of recurving hurricanes in the North Atlantic. We identified a cluster of cases characterized by ridge amplification and anomalous IVT transport ahead of the downstream trough, with a rather south-north orientation that would be consistent with heavy rain over North Italy. Seasonality would also match, as EP2 and hurricanes are most frequent in autumn.

    I copy the link to the paper, in case you are interested (I am talking of cluster 3):

    Pohorsky, R., M. Röthlisberger, C.M. Grams, J. Riboldi, and O. Martius, 2019: Mon. Wea. Rev., 147, 1513–1532,

    • CC2: Reply to CC1, Federico Grazzini, 06 May 2020

      Hi Jacopo, thanks for your comment.

      Indeed the amplification of the upstream Atlantic ridge in Cat2 is very similar to the "downstream development" pathway that you find in cluster 3 and 4 if I remember correctly. I have read that (nice) paper that you were suggesting and I'm commenting on that on the paper that I have submitted a few weeks ago on the topic. It seems that in category of cases IVT over the Atlantic plays a key role for ridge building, more perhaps than other contributions (like baroclinic and quasi-barotropic component)

      • CC3: Reply to CC2, Adriano Barbi, 06 May 2020

        You say that an increasing trend of moisture transport over the western N. Atlantic is likely associated with the observed increase in Cat2 and Cat3 events. Could you better specify this hypothesis? Is the trend related to the observed period 1979-2015? 
        Thanks and compliment.

        Adriano Barbi

        • CC4: Reply to CC3, Federico Grazzini, 06 May 2020

          Hi Adriano,

          For space limitation, I didn't show the full story which we are going to address in the paper on the EPE dynamic precursors which is now under review. We notice that there is an increasing trend of occurrence of Cat2 and Cat3 EPEs over N-Italy (observed over the period 1979-2015) and the same time there is a statistically significant trend of integrated water vapour transport (IVT) over the western Atlantic. Through the argumentation that IVT is one the key player in ridge building in the central Atlantic, and strong ridge building is a characteristic of Cat2 EPEs, we infer that could be a link of causality. More moisture fluxes (essentially due to increasing water vapour), more ridge amplification, more likely to have Cat2 or Cat3 events.