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Berthet C.,Anelfa | Wesolek E.,Keraunos | Dessens J.,Anelfa | Sanchez J.L.,University of León
Atmospheric Research | Year: 2013

The concentration of hail damage over a very few days of each warm season is a main characteristic of hail climatology in Southwestern France. The hailpad networks set up there in 1988 by the ANELFA hail prevention project have allowed the development of a new extreme hail day climatology which consists in examining the hail day data in the Atlantic region mainly under maritime conditions, and in the Pyrenean region, under continental and mountain influences. A pyramidal representation of each daily mean hailfall parameter (frequency per station, kinetic energy per hailfall, and kinetic energy per station), ranked by order of magnitude, allows a comparison of the mean and extreme values in the two regions, and reveals the strongest hail days in each one. During the 23-year period under study, 13 extreme situations (7 in the Atlantic region, 6 in the Pyrenean one) are highlighted, keeping the first three day ranking for each parameter. For each day, the synoptic situation, cell type, storm trajectories, and hail damages are examined, and numerical simulations with the WRF model, initialized by the CDAS/NCEP-NCAR reanalysis program, are considered. The study confirms that the Pyrenean region (20,000km2) has experienced the major hail situations, with record values of each parameter for the two regions. On the frequency record day (16 April 2007), 17% of the hailpad stations were impacted, and on the kinetic energy record day (16 July 2009), the total hailstone energy reached 33J/m2 per station. Two main types of synoptic situations have been identified according to the location of the pressure low generating the upper level flow, either near Portugal or near Ireland. A third type, with the pressure low above the Mediterranean, has been observed only once. The WRF-NMM simulations show that all the severe hailstorms that occurred in the twelve situations (one situation is common to both regions) developed in the areas where the mesoscale environment was characterized by a strong dynamical forcing and by high values of the low-level and deep shears. After calibration with this sample of extreme hail days, the WRF model appears promising for severe hail forecasting. © 2012 Elsevier B.V.

Dessens J.,ANELFA | Sanchez J.L.,University of León | Berthet C.,ANELFA | Hermida L.,University of León | Merino A.,University of León
Atmospheric Research | Year: 2016

The science of hail suppression by silver iodide (AgI) cloud seeding was developed during the second half of the 20th century in laboratory and tested in several research or operational projects using three delivery methods for the ice forming particles: ground generators, aircraft, and rockets. The randomization process for the seeding was often considered as the imperative method for a better evaluation but failed to give firm results, mostly because the projects did not last long enough considering the hazardous occurrence of severe hailfalls, and also probably due to the use of improper hail parameters. At the same time and until now, a continuous long-term research and operational field project (1952-2015) using ground generator networks has been conducted in France under the leadership of the Association Nationale d'Etude et de Lutte contre les Fléaux Atmosphériques (ANELFA), with a control initially based on annual insurance loss-to-risk ratios, then on hailpad data. More recently (2000-2009), a companion ground seeding project was developed in the north of Spain, with control mostly based on microphysical and hailpad data. The present paper, which focuses on hail suppression by ground seeding, reviews the production of the AgI nuclei, their dispersion and measurement in the atmosphere, as well as their observed or simulated effects in clouds. The paper summarizes the results of the main historical projects in Switzerland, Argentina, and North America, and finally concentrates on the current French and Spanish projects, with a review of already published results, complemented by new ones recently collected in Spain. The conclusion, at least for France and Spain, is that if ground seeding is performed starting 3. hours before the hail falls at the ground with a 10-km. mesh AgI generator network located in the developing hailstorm areas, each generator burning about 9. g of AgI per hour, the hailfall energy of the most severe hail days is decreased by about 50%. © 2015 Elsevier B.V.

Dessens J.,ANELFA | Berthet C.,ANELFA | Sanchez J.L.,University of León
Atmospheric Research | Year: 2015

In France, the melting level height varies, during the hail season, from about 1 km in the spring to more than 4 km in the summer, and this parameter appears to have a connection with the mean characteristics of hailfalls. In fact, a large 25-year sample of hail size distribution measurements, made with hailpad networks along the southern Atlantic coast, reveals strong correlations between the melting level height and the mean hailstone number in different hailstone diameter ranges. On average, an increase in the melting level height is associated with a decrease in the number of small hailstones (5 to 7 mm diameter), has no apparent effect in the middle range (7 to 9 mm), and is concomitant with an increase in the number of larger hailstones, particularly in the 11 to 21 mm ranges. When the relationships giving the hailstone number in each diameter range as a function of the melting level height are applied to a given hailfall, an increase in the melting level height is then found to go together with changes in the total hailstone number and kinetic energy. From this observation, it is possible to estimate the potential effect of global warming on hailfalls with different hailstone size distributions. The trends in the monthly surface temperature and in the hail day melting level height observed at Bordeaux during the 1988-2012 period and the global warming projections made for the present century suggest that an increase in the melting level height of 500 m is forecast to occur in this region between 2000 and 2040. The consequences of such an increase for a standard hailfall in the region will be a slight decrease (-12%) in the total number of hailstones, and a significant increase (40%) in total kinetic energy, but with no significant change in the hail frequency. This analysis is compared to the already observed increase in hail intensity in the area under study, and to observations, measurements, and numerical models in other parts of the world. © 2014 Elsevier B.V. All rights reserved.

Hermida L.,University of León | Lopez L.,University of León | Merino A.,University of León | Berthet C.,Anelfa | And 3 more authors.
Atmospheric Research | Year: 2015

Precipitation in southwestern France was analyzed. Monthly rainfall data from the Global Precipitation Climatology Centre (GPCC) were used to obtain three clusters of average precipitation from May through September for the period 1901-2010. We attempt to understand the role of hailfalls in total precipitation using three different variables related to hail, the number of hail days, hail frequency, and intensity. We examine trends via the Mann-Kendall test and periodicities by wavelet analysis. The results show cluster classification of precipitation to be somewhat consistent with hailpad networks in the study area. In the Pyrenees, there was a non-significant negative trend of precipitation and a substantial increase of hail days and intensity of hail, which were close to the limit of significance. Finally, a wavelet coherence analysis shows a strong influence of the North Atlantic Oscillation (NAO) in the Atlantic region. © 2015 Elsevier B.V.

Merino A.,University of León | Wu X.,Lanzhou University | Gascon E.,University of León | Berthet C.,Anelfa | And 2 more authors.
Atmospheric Research | Year: 2014

The characterization of atmospheric conditions at different scales and their relationships to meteorological phenomena is a basic tool for improving the understanding and prediction of severe atmospheric events. Hailstorms are relatively common in southern Europe during summer, causing significant adverse impacts to property and infrastructure. This paper provides a spatiotemporal characterization of hail falls in southwestern France between 2000 and 2010, using the hail pad network operated there by the Association Nationale d'Etude et de Lutte contre les Fléaux Atmosphériques (ANELFA). This area is greatly affected by hailstorms. It was observed that the greatest incidence and severity of hail was in the central Pyrenees during May and July, with decreasing frequency and intensity toward the Atlantic coast. We selected 100 events in which severe hailstorms were reported, to study atmospheric parameters responsible for their occurrence. We performed mesoscale simulations with the WRF model, using parameterizations and fields reported in previous studies. By applying principal component analysis (PCA) and cluster analysis, we obtained three configurations to help establish relationships with the spatiotemporal incidence of hailstorms. The method and results obtained improve knowledge of the conditions favorable for hailstorms in southwestern France. This allows better hail prediction by relating atmospheric conditions with characteristics of hail precipitation on the ground. © 2014 Elsevier B.V.

Berthet C.,Anelfa | Dessens J.,Anelfa | Sanchez J.L.,University of León
Atmospheric Research | Year: 2011

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. © 2010 Elsevier B.V.

Hermida L.,University of León | Sanchez J.L.,University of León | Lopez L.,University of León | Berthet C.,Anelfa | And 3 more authors.
The Scientific World Journal | Year: 2013

Hail precipitation is characterized by enhanced spatial and temporal variability. Association Nationale d'Etude et de Lutte contre les Fléaux Atmosphériques (ANELFA) installed hailpad networks in the Atlantic and Midi-Pyrénées regions of France. Historical data of hail variables from 1990 to 2010 were used to characterize variability. A total of 443 stations with continuous records were chosen to obtain a first approximation of areas most affected by hail. The Cressman method was selected for this purpose. It was possible to find relationships between spatial distributions of the variables, which are supported by obtained Pearson correlations. Monthly and annual trends were examined using the Mann-Kendall test for each of the total affected hailpads. There were 154 pads with a positive trend; most were located between Tarbes and Saint-Gaudens. We found 177 pads with a negative trend, which were largely south of a pine forest in Landes. The remainder of the study area showed an elevated spatial variability with no pattern, even between relatively close hailpads. A similar pattern was found in Lérida (Spain) and Southeast France. In the entire area, monthly trends were predominantly negative in June, July, and August, whereas May had a positive trend; again, however, there was no spatial pattern. There was a high concentration of hailpads with positive trend near the Pyrenees, probably owing to orographic effects, and if we apply cluster analysis with the Mann-Kendall values, the spatial variability is accentuated for stations at higher altitude. © 2013 Lucía Hermida et al.

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