The 1994 Piedmont flood: an archetype of extreme precipitation events in Northern Italy
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The 1994 Piedmont flood: an archetype of extreme precipitation events in Northern Italy F. Grazzini 1,3
2
3
& G. Fragkoulidis & V. Pavan & G. Antolini
3
Received: 18 April 2020 / Accepted: 26 August 2020/ # The Author(s) 2020
Abstract
Extreme precipitation events (EPEs) are meteorological phenomena of major concern for the densely populated regions of northern and central Italy. Although statistically rare, they tend to be recurrent in autumn and share common characteristics in the large-scale dynamical evolution responsible for their generation. Past studies on EPEs have reported, as the main triggering factor, a meridionally elongated upper-level trough embedded in an incoming Rossby wave packet. In this respect, we show how the meteorological conditions leading to the devastating 1994 Piedmont flood represent a typical flow evolution for this type of extreme events. Exploiting the systematic classification of EPEs recently published by the authors and taking advantage of a new observational dataset, this article revisits the role of the large-scale flow on this and similar cases of past EPEs. Keywords Extreme precipitation . Floods . Po river . Atmospheric rivers . Rossby wave packets . Downstream development
1 Introduction The extreme precipitation that affected the Piedmont region, in Northern Italy, in November 1994 led to a destructive flood with significant socioeconomic impacts. Seventy people died, and more than two thousand had to be evacuated. Damage to public and private property was extensive, 150 bridges collapsed or were severely damaged, and more than 5000 head of livestock were lost (Buzzi et al. 1998). The heaviest precipitation occurred between 4 and 6 of November when several rain gauges in mountainous regions recorded accumulated values above 300 mm/36 h (Buzzi and
* F. Grazzini [email protected]
1
Meteorologisches Institut, Ludwig-Maximilians-Universität, Munich, Germany
2
Institute for Atmospheric Physics, Johannes Gutenberg University Mainz, Mainz, Germany
3
ARPAE-SIMC, Bologna, Regione Emilia-Romagna, Italy
Bulletin of Atmospheric Science and Technology
Tartaglione, 1995). Forty percent of the Piedmont territory received more than 200 mm of rain during the event (Arpa Piemonte 1998). The large-scale circulation was characterized by a Rossby wave with meridional extension from the British Isles to the Iberian Peninsula featuring an elongated trough over Western Europe and a blocking anticyclone over Central Europe. Extreme precipitation events (EPEs), like this one, are typically associated with a strongly confluent flow ahead of a polar cold front concentrating water vapour into a narrow plume, which then interacts with the orography (Krichak et al. 2015). Such a flow can be triggered by a breaking Rossby wave over Western Europe that takes the form of a PV streamer, an elongated filament of high potential vorticity (PV) air (Grazzini, 2007, Martius et al. 2008). Since most of the intense orographic precipitation falls in the prefrontal sector, it is essential to s
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