Two cases of severe weather in Catalonia (Spain). A diagnostic study

Abstract

A diagnostic study of two cases of severe thunderstorms in Catalonia (the north-eastern region of Spain) is presented. The first occurred on 24 August 1993. A squall line, with bow echo configuration in the radar images, crossed Catalonia from west to east. Strong winds, large hail up to 7 cm in diameter, and heavy rain producing floods were observed. The meteorological situation was characterized by an upper-level trough approaching Spain from the north-west. This upper trough developed a low, which displaced the warm, dry air located over the Iberian Peninsula towards the east and north-east. The warm dry air overran the cold, humid Mediterranean air producing a strong inversion (LID) capped by an elevated mixed layer over the western Mediterranean and also over the Ebro valley. Handdrawn analyses permitted the identification of a secondary trough; a combination of this trough and a jet streak located over Spain was responsible for the mesoscale lifting mechanism that was able to break the LID and maintain the deep convection. The second case occurred on 31 August 1994. A tornado developed over south Catalonia and large hail, up to 5 cm in diameter, also fell. The meteorological situation was also characterised by an upper-level trough advancing from the west towards Spain. Accurate hand-drawn analyses identified a small thermal low over Catalonia leading to strong convergence at low levels. Over the same area, two jet streaks overlapped their left exit and right entrance zones producing an enhancement of the ascending motion forced by the low-Level convergence. However, the typical environment for supercell development was not present over Catalonia. The tornado was probably produced by a non-supercell thunderstorm.