Tornadoes in Italy: an underestimated threat?

The occurrence of tornadoes and waterspouts in Italy has received little attention so far in scientific literature. However, there is a rich history of tornado activity in Italy. Here, some recent results are discussed. First, ten years of tornadoes and waterspouts in Italy are analyzed in terms of geographical, seasonal, monthly, diurnal, and rating distribution, showing that their occurrence has largely underestimated. The analysis suggests a distinction of "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. Some preliminary results on the synoptic environment favorable to their development will also be presented. Next, the supercell that spawned the EF3 tornado, affecting southeastern Italy on 28 November 2012, is analyzed by means of WRF-model numerical simulations. We show that it is possible to reproduce the track, the change in intensity, and the evolution of a simulated supercell thunderstorm similar to the actual one. 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; boundary-layer rolls over the Ionian Sea that supplied moisture and heat to convection. An unusual feature of the present case is the central role of the orography, which was verified in a sensitivity experiment where the orography of Calabria was reduced. Another set of sensitivity experiments, using modified sea surface temperatures (SST), shows that the thermodynamic changes induced by the positive SST anomaly during the event enhanced lower tropospheric instability and favored deep convection, so that dramatic changes for updraft helicity and vertical velocity, which measure the intensity of the supercell, occurred even for SST variations of only + /-1 K.