An interesting case of tornado in Friuli-Northeastern Italy

On March 26, 2001 a tornado was observed on the Friuli plain (Northeastern Italy), close to the foothills. The event took place in the afternoon, 15:45 UTC and it lasted for about 15 min. Fortunately no significant damages and injuries have been recorded since the tornado hit an uninhabited area with little vegetation and cultivation owing to the season. The whole event was observed and photographed by witnesses at a distance of about 1 km from the tornado outset to its end. Radar and mesonetwork data revealed the presence of a supercell mesocyclone having a radius of 4.5±0.5 km that lasted for about half an hour and it was moving 30±8° right to the average tropospheric flow. The event occurred in a synoptic situation dominated by a strong jet in the upper troposphere associated with a well-defined baroclinic zone (front). Mesoscale data revealed modest instability, but unusually high for the early spring period in the area. Vertical wind shear was very close to the 10−2 s1 that is significantly high for stormy days in Friuli. The surface conditions reveal that a thermal boundary was present that could have played an important role in the formation of the mesocyclone. The thermal boundary was the result of earlier stormsʹ outflows and their interaction with the complex orography of the region. Comparison between horizontal relative vorticity parallel to the low level thermal boundary and the vertical relative vorticity of the mesocyclone suggests that such a boundary could have been a factor for the onset of the supercell. The lightning activity recorded during the event was characterized by a decrease of negative cloud-to-ground lightning (CG) frequency and an increase of positive (CG) lightning frequency during the observation of the event.