Study of the hailstorm of 17 September 2007 at the Pla d’Urgell. Part two: meteorological analysis

On 17 September 2007 a heavy hailstorm hit the Pla d’Urgell (Urgell Plain), with hailstones larger than 5 cm in diameter and 81 hailpads hit; unusual, given that hail in the area is usually smaller in diameter and size (7 hailpads affected on average). The first part of the study (Farnell et al., 2009) deals with the effects of the hailstorm on the surface, based on extensive fieldwork and the data obtained from hailpads in the area. This second part of the hailstorm study examines the episode from a synoptic point of view and from the observations recorded with the various tools available to the Servei Meteorològic de Catalunya (Meteorological Service of Catalonia): Meteosat images, data from the radar network and lightning records obtained from the atmospheric electrical strikes detection network. At the synoptic level, it is noteworthy that the episode occurred in conditions prone to produce graupel/hail in the area, with the presence of a wide trough at high levels associated with a low pressure system in Scandinavia. Analysis of the Meteosat images indicates that the storm core that produced the hailstorm was part of a mesoscale convective system. Originating in the center of the Iberian Peninsula, it moved to the NE, affecting Catalonia in the afternoon and later moving to southern France, where it also left heavy hailstorms. Study of the radar data reveals the presence of a hook-shaped storm during the hail. Despite this, this storm can not be defined as a supercell (a mesocyclone is not detected). On the other hand, it is observed that the maximum vertical developments occurred in the area where larger hailstones were detected. From the monitoring of the atmospheric electrical activity of the episode, we can highlight the high frequency of lightning generated by the storm. These results reinforce the conclusions of the analysis of the Meteosat and radar images on the severe nature of the storm.