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.
Highlights
The extreme precipitation that affected the Piedmont region, in Northern Italy, in November 1994 led to a destructive flood with significant socioeconomic impacts
#WA number of warning areas with spatial daily average precipitation exceeding the 99th percentile of the respective climatological distribution, Area total area exceeding the 99th percentile of daily precipitation [103 km2 ], mmkm2 mean area daily precipitation intensity [mm/24 h km2 ], IVTe mean zonal component of integrated water vapour transport (IVT) [kg s−1 m−1 ], IVTn mean meridional component of IVT [kg s−1 m−1 ], Taudmax, maximum daily value of Tau [hours], CAPEdmax daily maximum value of CAPE [J/kg], TCWV daily mean of total column water vapour [kg/m2 ], θe850 daily mean of equivalent potential temperature at 850 hPa [K], Dtmin daily minimum of Δθe [K] (Δθe = θe500–θe850), Cat the Extreme precipitation events (EPEs) category
We have shown that this event may be considered an archetype for southern Alpine Category 2 (Cat2) EPEs which are able to produce very high river discharges and widespread flooding on small and large river basins due to the combined presence of stratiform precipitation and deep convection
Summary
The extreme precipitation that affected the Piedmont region, in Northern Italy, in November 1994 led to a destructive flood with significant socioeconomic impacts. 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 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).
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