The Piedmont flood of November 1994: a testbed of forecasting capabilities of the CNR-ISAC meteorological model suite
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The Piedmont flood of November 1994: a testbed of forecasting capabilities of the CNR-ISAC meteorological model suite Silvio Davolio 1 Andrea Buzzi 1
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& Piero Malguzzi & Oxana Drofa & Daniele Mastrangelo &
Received: 23 March 2020 / Accepted: 15 July 2020/ # Springer Nature Switzerland AG 2020
Abstract
The celebration of the 25th anniversary of the Piedmont flood was organized by the University of Piemonte Orientale in Alessandria, a town that was severely affected in November 1994. It has been an opportunity to reexamine the meteorological event and assess the potential of CNR-ISAC models, after more than 20 years of development, to accurately simulate the heavy precipitation at different lead times. The predictability of this extreme event has been studied on a wide range of space and time scales, from subseasonal to convection resolving, using a variety of model setups and initial conditions. The subseasonal experiment produces a precipitation anomaly that, even if underestimated, indicates some predictability beyond the second forecast week. At shorter ranges, results indicate that there is a consistent improvement in the precipitation forecast going from low-resolution hydrostatic to high-resolution nonhydrostatic models. It is only at very high resolution (500 m) that the convective activity extending to the north of the divide line of the Ligurian Apennines, which was responsible for the flood of the Tanaro River, is predicted with an intensity comparable with rain gauge observations. The main mesoscale phenomena that played a role in the different phases of the event have been also identified. Keywords Heavy precipitation . Piedmont flood . Orography . ISAC models . High resolution
1 Introduction The infamous Piedmont flood of 1994 affected a relatively vast area in northwestern Italy, almost entirely contained in the administrative region of Piedmont (Fig. 1). Several rivers * Silvio Davolio [email protected]
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National Research Council of Italy, Institute of Atmospheric Sciences and Climate, CNR-ISAC, Via Piero Gobetti 101, 40129 Bologna, Italy
Bulletin of Atmospheric Science and Technology
overflowed their banks, some having their catchment basins in the western Alps, where the chain exhibits its largest concavity, corresponding to northwestern Piedmont. A severe flooding affected also the Tanaro River, whose catchment is located in a different area, between the Ligurian Apennines and the Maritime Alps in the southern portion of Piedmont (see also Fig. 1 in Buzzi et al. 1998 and Fig. 3). Flooding of the former group of rivers was associated with the huge orographic rainfall maximum present on the southeastern flank of the Alpine chain as visible in Fig. 1, with accumulated maxima exceeding 500 mm in 48 h. Flooding of the Tanaro River was associated with the secondary maximum between the Alps and the Apennines, also visible in Fig. 1, with corresponding observed values of 48 h accumulated precipitation around 250 mm (Cassardo et al. 2002). Although the Piedmont flood was not a flash-t
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