Regional and yearly variations of hail frequency and intensity in France
Research Highlights
► Point hailfalls are recorded on several hailpad networks in France. ► Hail intensity increased by 70% during the 1989–2009 period. ► Hail intensity and mean minimum surface temperature are correlated. ► Hail intensity increase may be a consequence of global warning.
Introduction
In France, hail has been the subject of scientific and economic attention since the beginning of modern meteorology (Plumandon, 1901; Genève, 1961). More recently, Vinet (2000) compiled meteorological observations and insurance data, and published the first comprehensive climatology of hail in France, with special emphasis on geographical and agricultural aspects. A summary of his work has also been published in English (Vinet, 2001). This author used the first hailpad data available 10 years ago but, with a much longer time of recording (nearly doubled), and a more extended network, the hail climatology can be seriously refined.
In 1987, the Association Nationale d'Etude et de Lutte contre les Fléaux Atmosphériques (ANELFA) developed its hailpad project in order to improve the control of the seeding project, check and adjust hail forecast, and establish a physical climatology of hail in France. As the number of hail seasons with measurements has increased in the regional hailpad networks, several studies have progressed, among them those of the daily variability of hail (Dessens et al., 2001), of the return periods of severe hailfalls at a given point (Fraile et al., 2003), and of hailfall classification (Dessens et al., 2007). To our knowledge, it is the first time an extended hailpad network has been continuously operated over a long period in the same area. Now, with 22 years of measurements and 5332 point hailfalls recorded, it becomes possible to examine the regional and yearly variations of hail in France, and to compute correlations between hail and other meteorological elements like surface temperature.
A physical knowledge of hail is obviously important for preventive actions and insurance purposes. It is also the necessary starting point for a survey of possible changes with time. According to results derived from models using empirical relationships, a future increase in hail is in fact possible (Cubasch and Meehl, 2001), and the problem is to know if either the frequency or the intensity, or both, will change. Based on the trends already observed these past two decades, and on the measured correlations between hail and temperature, a discussion of hail in the context of global warming will conclude the paper.
Section snippets
Hailpad data
The method chosen for hailfall measurement at the ground was developed in 1987 by Dr. Pierre Admirat and the Société d'Applications Atmosphériques from the results of the First International Workshop on Hailfall Measurements (Lozowski et al., 1978). The hailpad material, calibration, and data processing are described in Dessens et al. (2001). In short, a hailpad station consists of a 42 × 30 cm, 2 cm thick, plate of extruded polystyrene (Styrofoam) posted 1 m above the ground near a volunteer's
Regional variations
The following mean hailpad characteristics and hailfall measurements for the 22 years of the 1988–2009 period will be considered:
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Ninst, number of hailpad stations per year
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Nimp, number of hailfalls per year
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F, frequency, F = Nimp/Ninst
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Ntot, total number of hailstones per hailfall
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Dmean, mean hailstone diameter computed from the hailstone size distribution
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Dmax, mean diameter of the largest hailstone per hailfall
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Mh, total hailstone mass per hailfall
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Ms, total hailstone mass per year and per station
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Eh,
Time variation and trends
The year-to-year correlations between regions noticed in the previous section should justify the aggregation of a national series of yearly hail values in France in order to examine how hail has changed during the period of observation. However, because the Central and the Mediterranean networks have joined the project after the Pyrenean and the Atlantic ones, it is better to examine the hail evolution only in the last two networks grouped together. Moreover, in these two networks, there are
Correlations with temperature
Based on insurance and temperature data, Dessens (1995) has shown that, due to correlations between hail and mean surface temperature, and particularly to minimum temperature, hail has increased in France in relation with global warming. The physical hail data will allow checking this observation by correlating temperature with hail then by determining if the temperature trends may explain the hail trends.
The F, Eh and Es values used for computation are again those of the 457 permanent stations
Main results and conclusion
After 22 years of measurement in several hundred stations located in 4 hailed regions of France, the following mean characteristics of hail are observed.
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In a given region, hail is characterized by two independent parameters, which are the hailfall frequency and the mean intensity at a station. The total hail amount per station is the product of these two parameters.
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The Pyrenean region is ranked number one for frequency, probably due to its proximity to Spain, followed by the Mediterranean
Acknowledgements
More than a thousand voluntary and very motivated people are in charge of the hailpad stations. Their continuous support has allowed this first physical study of hailfalls for more than two decades. We also want to particularly acknowledge Mr. Didier Dutrey, who has carefully processed the impacted pads since the beginning of the project.
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