Regional and yearly variations of hail frequency and intensity in France

Abstract

In addition to the ground seeding hail prevention project, the Association Nationale d'Etude et de Lutte contre les Fléaux Atmosphériques (ANELFA) operates hailpad networks in four of the most hailed regions of France: Atlantic, Pyrenean, Central and Mediterranean. During the past 22 years of continuous measurements, more than five thousand point hailfalls have been recorded at 922 stations (mean annual value) installed in a 66,500 km² area. At the scale of a region and of a hail season, hail is found to be the product of two nearly independent parameters, the frequency and the mean intensity of hailfalls. The frequency is highest in the Pyrenean region, while the intensity is highest both in the Pyrenean and Central regions. This can be explained, for the frequency, by the proximity of Spain, and, for the intensity, by the mean hailstone size distributions which are different in the continental and maritime regions. The time variations and trends of hail during the 1989–2009 period are computed from the data at 457 stations which never changed during this period. The annual frequency is subject to cyclic variations, while the yearly mean intensity is affected by irregular severe hail events. The frequency did not change significantly during the period, while the intensity increased by 70%. The mean monthly distribution of hail is bimodal, with two maximums in May and July, suggesting that two types of meteorological conditions are at the origin of hailstorms. April and May are solely responsible for the mean hail increase observed during the period. A computation of the year-to-year correlation between hail intensity and mean minimum surface temperature for each month suggests that the large hail increase in April and May is at least partially due to the observed concomitant increase in temperature, and then may be a consequence of global warning.