Abstract
Between the 4th and the 6th of November 1994, Piedmont and the western part of Liguria (two regions in north-western Italy) were hit by heavy rainfalls that caused the flooding of the Po, the Tanaro rivers and several of their tributaries, causing 70 victims and the displacement of over 2000 people. At the time of the event, no early warning system was in place and the concept of hydro-meteorological forecasting chain was in its infancy, since it was still limited to a reduced number of research applications, strongly constrained by coarse-resolution modelling capabilities both on the meteorological and the hydrological sides. In this study, the skills of the high-resolution CIMA Research Foundation operational hydro-meteorological forecasting chain are tested in the Piedmont 1994 event. The chain includes a cloud-resolving numerical weather prediction (NWP) model, a stochastic rainfall downscaling model, and a continuous distributed hydrological model. This hydro-meteorological chain is tested in a set of operational configurations, meaning that forecast products are used to initialise and force the atmospheric model at the boundaries. The set consists of four experiments with different options of the microphysical scheme, which is known to be a critical parameterisation in this kind of phenomena. Results show that all the configurations produce an adequate and timely forecast (about 2 days ahead) with realistic rainfall fields and, consequently, very good peak flow discharge curves. The added value of the high resolution of the NWP model emerges, in particular, when looking at the location of the convective part of the event, which hit the Liguria region.
Highlights
At the beginning of November 1994, a heavy precipitation event (HPE) caused extensive flooding in Piedmont and in Liguria, two regions in northern Italy
The Weather Research and Forecasting (WRF) model outputs of the hindcast simulations run in operational mode, i.e. forced with the Integrated Forecasting System (IFS)-FC products, are shown in Fig. 5 for all four configurations
In terms of differences produced by the choice of the microphysical parameterization, only the WSM6 scheme (FC-W, panel a) stood out, producing a more intense precipitation field, which can not be ascribed to the fact that WSM6 is a single-moment scheme
Summary
At the beginning of November 1994, a heavy precipitation event (HPE) caused extensive flooding in Piedmont and in Liguria, two regions in northern Italy. The Mediterranean area is frequently hit by HPEs that can trigger severe floods and flash floods, which are responsible for massive damages and, sometimes, numerous casualties (Llasat et al 2013). Such HPEs are usually characterised by the presence of four factors: a moist low-level jet coming from the relatively warmer Mediterranean Sea; a conditionally unstable air mass; a mesoscale lifting mechanism, such as an orographic barrier; and a synoptic pattern that slowly evolves in time (Nuissier et al 2008; Dayan et al 2015). From a forecaster standpoint, since type II events are often controlled by local forcing factors that initiate the upward motion (e.g. surface wind convergence lines, gradients in the surface heat fluxes), they are harder to predict (Fiori et al 2017)
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