Miquel Gayà

Tornadoes and waterspouts in the Balearic Islands: phenomena and environment characterization

Abstract In the Balearic Islands, located in the Western Mediterranean, 27 tornadoes and 54 waterspouts have been recorded during the period 1989–1999. A climatology focusing, which focuses on path length, F-scale velocity, season and time of occurrence, is presented. September and October appear to be the months with the highest frequency of appearance. The environment in which thunderstorm producing tornadoes and waterspouts developed has been analysed. Main thermodynamic stability indices reveal that no specific conditions are required for the tornado and waterspout genesis. It is also found that these events form in air masses colder than that indicated by the climatology of the region. Analyses of helicity and CAPE demonstrate that, in most of the cases, the environments were not favourable for mesocyclone formation and supercell development.

Tornadoes over complex terrain: an analysis of the 28th August 1999 tornadic event in eastern Spain

Abstract On 28 August 1999, a tornadic storm developed during the afternoon over the Gudar range, near the border between Teruel and Castellon provinces ( Sistema Iberico , eastern Spain). The area has a characteristic complex terrain with peaks up to 2000 m. At least one tornado developed, which attained F3 intensity, producing severe damage in the forest that covers the mountains. The region is well known as a summer convective storm nest and, not surprisingly, a range in the Sistema Iberico is called Sierra del Rayo (lightning range). The meteorological situation on 28 August 1999 shows the presence of a thermal low over the Iberian peninsula, producing warm and moist air advection towards inland Castellon at low levels. Meanwhile, a cold trough crossed the Iberian Peninsula from west to east at upper levels. Deep convection is identified on the Meteosat images during the afternoon, when the upper level trough reached the area where warm and humid Mediterranean air was concentrated. The radar images reveal signals indicating the supercellular character of the tornado-producing storm. Weak echo region, frontal and rear inflow notches are identified on the low-level CAPPIs images. In order to study the roles of the orography and the solar radiation on the ingredients associated with the tornadic storm development, numerical simulations of the event were performed, using the non-hydrostatic MM5 model. Very fine grid resolution, up to 2 km in the horizontal, allows us to determine the role of the complex terrain in favoring environmental conditions associated with the onset of severe convection. In fact, interaction of certain low-level flows with particular topographic features with scales of about 20 to 50 km is found to generate intense small-scale circulations over the storm nesting region. A more detailed analysis of the orographic effect shows that fine scale terrain features (2–5 km) are responsible for the intensification of the convective storms, whereas modification of the low-level flow by the large-scale features (20–50 km) is able to trigger the convective systems. The effect of the solar-induced surface heating is also analyzed and it turns to be crucial not only by intensifying the Iberian thermal low and the easterly warm and moist air advection towards the area but also by promoting mountain breezes.

A synoptic and mesoscale diagnosis of a tornado outbreak in the Balearic Islands

A tornadic event occurred over the Balearic Islands (Western Mediterranean) during the evening of 11 September 1996 and the following night. A total of six tornadoes were observed, affecting populated areas, with an economical damage of more than 6 million Euro. The meteorological situation in which severe weather developed was characterised at low levels by a low covering all the Western Mediterranean with well-marked warm advection towards the Balearic Islands. At mid and upper levels, a low was located to the southwest of the Iberian peninsula, producing southwesterly winds over the region. Satellite imagery shows that the first tornado, observed over the Ibiza Island, was produced by a mature thunderstorm, which presented a well-defined V-shape on the IR images. Tornadoes occurring in Majorca and Minorca islands were produced by convective systems, which were in their initial development state. Most of these tornadogenetic convective systems developed over a low-level convergence line formed as a consequence of the existence of a low moving northeastwards along the south of the Balearics and a very small and deep cyclone formed offshore in front of the Valencia coast. Positive interaction between the low-level convergence line and an upper-level jet streak for producing the lift of low-level parcels has also been identified.