An intense tornado hit the Italian island of Pantelleria on 10 September 2021, causing widespread damage and two deaths.
24 January 2023
14 September 2021
By Federico Fierli (EUMETSAT), Ivan Smiljanic (CGI) and HansPeter Roesli (Switzerland)
The tornado formed inside a severe mesoscale convective system (MCS), that was itself part of an ensemble of severe convection events in the Sea of Sardinia and Sicily (Figure 1).
The Meteosat-10 HRV animation (Figure 2) shows the evolution off Tunisia in the afternoon with important convection. This convection over the sea (most probably) created a concentric outflow boundary, which emerged slowly from under the anvil after 10:00 UTC (see situation at 12:15 UTC on the left side on Figure 3). While one arc disappears over Tunisia, another part of the arc persists south west of Pantelleria and moved eastward. It appears to intersect another outflow boundary coming from the north west after 16:00 UTC (right-hand image) right over the island. This boundary intersection could be supporting the intensity of convective processes, which eventually led to a tornado formation.
The infrared image (Figure 4) shows a so-called cold U/V signature (Setvak et al. (2010)), visible as a ring of colder infrared temperatures surrounding a relatively warmer anvil centre. This feature is indicative of the presence of overshooting top that are visible from an IR cloud top temperature reaching as low as 205K (see also the best practices for convection detection document).
The intensity, persistence, and speed of the overshooting development indicates that the tornado was generated from a convective supercell, sustained by high vertical velocities.
The combination of high-resolution visible imagery at 1km and infrared at 3km provides a good insight into the anvil-level processes of these thunderstorms. Figure 5 shows two (in this case very pronounced) signs of intense thunderstorms, namely Overshooting Top (OT) and Above Anvil Cirrus Plume (AACP).
The Severe Convection RGB, on the other hand, provides insight into microphysical signatures on top of the clouds, revealing the presence of small ice crystals (sign of strong updrafts) over the observed storms (Figure 6). As the loop approaches late afternoon the weakness of this product becomes apparent — with low Sun angles, the microphysical signal becomes very weak, almost non-apparent.
Such events are not infrequent in the Mediterranean area, however, their intensity and track may be influenced by sea surface temperature positive anomalies (Miglietta et al., Nature, 2017). The SST anomaly, from NOAA (Figure 7) indicates a warming between 2 and 3°C in the area where the intense convection developed, that was the likely additional supporter of the observed intense systems (vertical destabilisation and moisture).