Study of the hailstorm of 17 September 2007 at the Pla d’Urgell. Part one: fieldwork and analysis of the hailpads

The plain of Lleida is a farming area with about 200,000 ha of crops. On 17 September 2007 a hailstorm hit 889 ha of fruit crops, especially the area of the Pla d’Urgell (Urgell Plain). That hailstorm was the largest since 1990, according to the data collected by the Agrupació de Defensa Vegetal de les Terres de Ponent (ADV-TP) (Group for the protection of plants in the Terres de Ponent): hailstones larger than 5 cm in diameter and 81 hailpads hit (the average in the Lleida area in September is 7 hailpads affected). A mesoscale convective system originating in the Iberian Range was reactivated when arriving to Catalonia, resulting in a hailstorm that was particularly virulent and affected a wide area (the second part of the study focuses on the synoptic and mesoscalar study). This first part presents the analysis of the data from the network of hailpads managed by the ADV-TP and the information obtained from observers in the affected area. The conclusion drawn from this analysis is that the location of the largest hailstones does not match with the more heavily impacted area. Regarding the qualitative information obtained from the contributors, the affected area is similar to that obtained from the hailpads. However, it should be noted from the comparative study between both sources of information that observers tend to perceive only the larger hailstones when there is a mixture of diameters. To conclude, a number of recommendations are presented in order to maximize the objectivity of the reports presented by the observers.