Hail in Southwestern France. I: Hailfall Characteristics and Hailstrom Environment

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Abstract
In the context of an experiment of hail prevention in the Aquitaine region, 12 869 reports of damaging hail have been compiled during 29 years in an area of 88 980 km2. These observations, together with crop insurance data, have led to a unique hail climatology. The data presented in this paper concern the geographical distribution of hail damage, the yearly, 5-day and hourly frequencies of hailfall, and the distributions of hailstone size and of hailfall duration. Most of these data are well explained by the fact that hail in the surveyed area is the result of almost any rather severe thunderstorm: large hail, however, is produced by a few isolated long-lived hailstorms traveling downwind of the central part of the Pyrenees with the strong upper level winds. Study of the mean characteristics of 30 of the most severe storms which have damaged the Aquitaine in the last three decades leads to the following description: a typical long-traveling hailstorm moves at 15 m s−1 for 1.5 h, dropping a hail strip 86 km long and 6 km wide. The direction of propagation is from the southwest, with an angular deviation of 28° to the right of the mean tropospheric wind. This wind is characterized by an increase in velocity up to 10 km (mean maximum: 32.6 m s−1) without any change in direction above 3 km. In some circumstances, these long-traveling hailstorms produce only hailspots along their path, although the convective and wind conditions are the same as for the major hailstorms. The insurance data complete the observers reports because they take into account the severity of the hailfalls and also because they give the economical impact of the hail. A decrease in the mean annual percentage of loss is observed during the last two decades in Aquitaine. The significance of this change will be discussed in Part II which is related to a hail prevention project in the same region.

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