Role of the orography in the generation of a tornadic supercell in the Mediterranean

On 28 November 2012, a multi-vortex EF3 tornado occurred in southeastern Italy causing one casualty and estimated damage of 60 MEUR. At approximately 1050 LT (0950 UTC), this tornado, which initially formed in association with an apparent supercell thunderstorm over the Ionian Sea, moved inland. The environment where the tornadic supercell developed was characterized by large vertical wind shear in the lowest 1 km of the atmosphere and moderate conditional instability. The WRF-model numerical simulations show that it is possible to produce the track, change in intensity, and evolution of a simulated supercell thunderstorm similar to the actual one that spawned the tornado in Taranto, southern Italy. The genesis of the simulated supercell is due to a combination of mesoscalemeteorological features: warm low-level air advected toward the Ionian Sea, combined with a mid-level cooling due to an approaching trough, increased the potential instability; the intense vertical shear favored the possibility of supercell development; boundary layer rolls over the Ionian Sea moved in phase with the cells produced by the orography of Calabria to supply moisture and heat to convection. An unusual feature of the present case of tornadogenesis is the central role of the orography, which was verified in a sensitivity experiment where the orography of Calabria was reduced by 80%. The upper-level vertical-vorticity couplets generated in the lee of the orography are evidence of updrafts in vertical wind shear for the cells triggered by the orography, which are essential for supercell dynamics.