Tornadoes in Italy: an underestimated threat?

Ten years of tornadoes (TR) and waterspouts (WS) in Italy are analyzed in terms of geographical, seasonal, monthly, diurnal, and rating distribution [1]. A comprehensive dataset is developed for the period 2007- 2016, which includes 707 WS and 371 TR. The category of WS includes many weak events but also some intense vortices, able to produce significant damage as they make landfall. WS form mainly near the Tyrrhenian and Apulia region of the Ionian coast; the majority of them develops in autumn, followed by summer. The average density is 0.9 events per 100 km of coastline per year, although peaks of around 5 events per 100 km of coastline per year are reported along the Tyrrhenian coast. TR originate from WS in about half the cases; the average density of TR is 1.23 events per 104 km2 per year, which is comparable with other Mediterranean regions. TR are more frequent in summer, followed by autumn; however, limiting the analysis only to TR originated inland, the peak activity occurs in summer and in late spring. Thus, there is a distinction between "continental" cases, mainly affecting northern Italy in late spring and summer, and "maritime" cases, which affect mainly the peninsular regions in late summer and autumn, usually originated as WS. The highest density of TR is reported along the coasts of Lazio and Tuscany, in the Venetian plain, in the southern part of Apulia: in these regions, the density of events is comparable with that of the U.S.A. states with the highest TR rates. In contrast, the probability of significant TR in any Italian region is much smaller than that of the U.S.A. states with the highest risk. Among the significant Italian tornadoes in the last few years, the supercell that spawned the EF3 multi-vortex tornado affecting the city of Taranto on 28 November 2012 [2] is analyzed by means of the WRF model. Numerical simulations are able to reproduce the track, the change in intensity, and the evolution of a supercell thunderstorm similar to the actual one [3]. The genesis of the simulated supercell is due to a combination of mesoscale features: warm low-level air advected toward the Ionian Sea, combined with mid-level cooling due to an approaching trough; intense vertical shear, favoring the possibility of supercell development; the uplift induced by the orography of the Calabria region, whose role was verified in a sensitivity experiment where the mountain height was reduced. Another set of sensitivity experiments, using modified sea surface temperatures (SST), shows that the thermodynamic changes induced by the positive SST anomaly enhanced lower tropospheric instability and favored deep convection, inducing dramatic changes in updraft helicity and vertical velocity even for SST variations of only + /-1 K [4]. The latter result appears relevant in the framework of future climate projections of SST.