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When studying hail falls, it is common to mention their variability, but not so much to present results in support of this statement and do so in detail. This is because of the difficulty of accurately understanding hail falls in spatial and temporal terms. It is even more difficult to ascertain the characteristics of hailstones precipitated by storms. In this paper, we use hailpad databases, which are known to give detailed information on hail falls, to determine spatial and temporal variability across two networks in the Middle Ebro Valley (Northeast Spain), just a few tens kilometers apart. These results demonstrate the importance of having sound ground-based information obtained directly. The databases used have 2865 hailpads that measured 197,076 hailstone impacts (≥ 5 mm). When classifying the severity of storms, the maximum hail diameter is typically used. Among all days with hail events, we selected 71 hailstorms that were tracked over their entire duration, having formed and dissipated within either of the two hailpad networks. We present a methodology and application to determine a mathematical equation that expresses the maximum diameter as a function of meteorological variables, analyzing possible uncertainties to make short-term predictions. This equation facilitates improvement of our knowledge of factors that influence the appearance of storms and the classification of predicted severity. • Hailpad databases, to determine spatial and temporal was used. • 71 hailstorms events were tracked over their entire duration • A equation that expresses the maximun diameter as a function of meteorological variables was developed. • The findings improve the knowledge of factors that influence the classification of predicted storms severity. [ABSTRACT FROM AUTHOR]

Abstract: Hailstone size distribution has been an integral part of hailfall measurement in several research projects. The exponential distribution is usually assumed to be an adequate representation of hailstone size spectra. This paper presents the results of the fitting to the exponential distribution of databases obtained from several hailpad networks set up in southern France (3 networks with 1,081 hailpads), Spain (2 networks with 269 hailpads), and Argentina (3 networks with 567 hailpads). These networks use the same type of plate, method of calibration, and semi-automatic measurement procedure. Results show that the characteristic parameters of the fittings are different in each area, even in areas that lie geographically quite close to each other. The values of the slopes obtained in the French networks are higher than the ones found in Spain and Argentina. Statistical techniques were applied to detect “breakpoints” in the hail size distribution, using piece-wise regression models (or “broken-stick” models). These points were used to estimate critical thresholds associated with transitions between two hailstone size distributions. In 4 networks in Spain and Argentina, breakpoints appear in large hail. The results show that, in 2 of the networks, the distribution is exponential (statistically significant) again from the breakpoint on. [Copyright &y& Elsevier]

Abstract: Microphysical measurements inside cumulonimbus clouds (Cb) are technically complex and imply some risks. However, data such as the characteristics of the hydrometeors in a cloud, both in liquid and solid phases, their spectrum or the liquid water content are essential to understand microphysical processes inside severe storms as well as the internal structure of the storm. These data can also be used to validate numerical models to characterize severe storms beyond the information provided by the measurements. During a campaign for gathering microphysical data, measurements were carried out through a vigorous cloud turret rising on the flank of a Cb that produced hailstones, at an altitude with temperatures around −5°C. Hydrometeor spectra and the liquid water content were determined. The droplet concentration, measured by an FSSP 100, varied a lot depending on the sampling area. The droplet concentration varied inversely when compared to the concentration of larger hydrometeors registered by an OAP 2D2-C. In addition, in some areas supercooled large drops were found to form accumulation zones. The microphysical results have been used to calculate the minimum magnitude of a sustained updraft required to produce supercooled water at an altitude of −16°C in adverse conditions for the increase in liquid water due to the presence of ice particle populations. [Copyright &y& Elsevier]