Observational analysis of a Mediterranean 'hurricane' over south-eastern Italy

This article documents the presence of a mesoscale vortex in the Mediterranean Sea. The cyclone appeared in the early morning of 26 September 2006 over the Ionian Sea and moved north-east, affecting southeastern Italy. Compared with similar events described in the literature, the movement of the cyclone inland is quite an unusual feature. The fact that the area affected by the cyclone is covered with two radars and a high-resolution surface station network made possible a more complete investigation of the event. A sodar, belonging to ISAC-CNR in Lecce, and a radiosonde station in Brindisi are present in the area. Unfortunately, the rainfall affected the quality of the data collected by sodar, and no sounding was taken at Brindisi during the brief passage of the vortex. The observations reveal that the vortex possessed features typical of tropical cyclones, such as the spiral shape, the eye, the deep, rapid sea-level-pressure falls, the evidence of a warm core (as shown in Figure 2), the strong surface wind and the fact that the strongest winds corresponded to the eye-wall (Lagouvardos et al., 1999; Reale and Atlas, 2000). Also, the synoptic analysis shows that the mesoscale low developed in correspondence with a cold upper-level pressure low and a large-scale surface cyclone centred in the Tyrrhenian Sea. Mediterranean 'hurricanes', in analogy with polar lows, usually develop under deep upper-tropospheric troughs (Emanuel, 2005), owing to the unusually deep, cold air associated with the trough in combination with the relative warmth of the underlying sea (Rasmussen et al., 1992). In the case shown, the vortex developed in a region where the surface sensible heat flux, coming from the warm sea surface (23-24 °C, a warm anomaly of +0.5 to +1 degC with respect to 1971-2000), together with the latent heat flux interacted with the upper-level cold air coming from the north and triggered the present observed small-scale cyclone. A deeper understanding of the event requires a numerical investigation, however, and a sensitivity study of the mechanisms responsible for the development and the maintenance of the cyclone that will be discussed in a further study.