Hail Research

This paper is an overview of hailpad research from its origin until today using as a search criterion the bibliographic references on the subject in the Web of Science database (ISI). The search was carried out on 1st September 2008. Among the more than 3•10 7 scientific documents included in the Science Citation Index Expanded (SCI-EXPANDED) from 1945 to the present, the search engine identified 41 containing the word "hailpad⁎" (the asterisk is a wildcard for any letter or group of letters). The results have been analyzed according to various criteria: countries with hailpad networks or studies on hailpads; date of the first article; evolution of the number of articles per decade; concepts studied and research lines followed in each article; and finally, a bibliometric analysis of the journals where these articles have been published. It was found that 70% of the articles on hailpads were published in 2 journals: Journal of Applied Meteorology and Atmospheric Research. These two journals also concentrate most of the citations. The number of articles on hailpads and the documents that cite them have grown steadily, and it is expected to grow still further in the present decade if we take into account the trends found.

This paper is an overview of hailpad research from its origin until today using as a search criterion the bibliographic references on the subject in the Web of Science database (ISI). The search was carried out on 1st September 2008. Among the more than 3•10 7 scientific documents included in the Science Citation Index Expanded (SCI-EXPANDED) from 1945 to the present, the search engine identified 41 containing the word "hailpad⁎" (the asterisk is a wildcard for any letter or group of letters). The results have been analyzed according to various criteria: countries with hailpad networks or studies on hailpads; date of the first article; evolution of the number of articles per decade; concepts studied and research lines followed in each article; and finally, a bibliometric analysis of the journals where these articles have been published. It was found that 70% of the articles on hailpads were published in 2 journals: Journal of Applied Meteorology and Atmospheric Research. These two journals also concentrate most of the citations. The number of articles on hailpads and the documents that cite them have grown steadily, and it is expected to grow still further in the present decade if we take into account the trends found.

This paper is an overview of hailpad research from its origin until today using as a search criterion the bibliographic references on the subject in the Web of Science database (ISI). The search was carried out on 1st September 2008. Among the more than 3•10 7 scientific documents included in the Science Citation Index Expanded (SCI-EXPANDED) from 1945 to the present, the search engine identified 41 containing the word "hailpad⁎" (the asterisk is a wildcard for any letter or group of letters). The results have been analyzed according to various criteria: countries with hailpad networks or studies on hailpads; date of the first article; evolution of the number of articles per decade; concepts studied and research lines followed in each article; and finally, a bibliometric analysis of the journals where these articles have been published. It was found that 70% of the articles on hailpads were published in 2 journals: Journal of Applied Meteorology and Atmospheric Research. These two journals also concentrate most of the citations. The number of articles on hailpads and the documents that cite them have grown steadily, and it is expected to grow still further in the present decade if we take into account the trends found.

Since 1990 the Laboratory for Atmospheric Physics at the University of Leo Ân has made use of a network of 250 hailpads spread over an area of 1000 km 2 in the province of Leo Ân (north-western Spain). By the end of 1995, the network had detected 25 hailfalls of more than 15 km 2 in area, registered on 13 different days during the summer months.

The physical properties of hailstones registered by a hailpad network (size distribution, mass, kinetic energy) are essential data for the establishment of a regional hail climatology. Nevertheless, when comparing these data to the same properties of hailstones inside the cloud, the melting process must be taken into account. This paper presents a brief theoretical study of the changes effected on hailstone size distribution due to the melting process. The paper is based on previous studies dealing with the melting of hailstones before they reach the ground. The aim is to analyze the influence of this melting process on hailstone size distribution. An initial melting simulation was carried out in order to achieve this aim. Despite the common assumption that hailstone size distribution on the ground is exponential, it was found that when the in-cloud size distribution is exponential, on the ground, there are fewer small hailstones than what would be expected in an exponential distribution.

Hail is an important economic problem in several countries, and there would be a need for climatological studies not limited to general data such as space and time frequencies of ordinary hailfalls. A large network of hailpad stations, which operated continuously in southwestern France since 1988, gives the first opportunity to compute return periods of very severe point hailfalls characterized either by the total kinetic energy of hailstones or by the diameter of the largest hailstones. The Gumbel distribution has been used to represent the probability density function of the maximum annual value of these two parameters. Preliminary results indicate that the area located just north of the central Pyrenees is three times more exposed to damaging hailfalls than the Atlantic border. The results show that the computation of an areal return period depends upon the hailpad network density, which makes data normalization necessary for inter-network comparisons. This study also offers the possibility of determining return periods of severe hailfalls at a point. As an example, a provisional estimation indicates that a hailfall with hailstones of 3 -4 cm diameter occurs every 22 years at any point in the hail core region north of the Pyrenees. The hope is that this study may be progressively expanded to other European hailed regions in which similar hailpad networks are in operation. D

The measurement of the physical characteristics of hailstones reaching the ground is usually carried out by means of hailpads, on which the impact of hailstones leaves dents. Hailstone dents provide information about parameters, such as the number N of hailstones, their size M, and their kinetic energy E. In the case of intense hailfalls, however, the dents often overlap and the final measurement may not be totally reliable. This paper presents a computerized simulation with the aim of assessing measurement errors caused by dent overlap. The simulated dents represent several random hailfalls with both exponential size distributions and mono- dispersed size distributions. The simulated hailpads were measured following the procedure employed in the case of hailpads exposed to authentic hailfalls, and it was thus possible to assess the error due to dent overlap. The results show that dent overlap makes it impossible to measure all the dents, which means that in a real hailfall the number of hailstones registered will often be lower than the number of hailstones that actually hit the ground (up to 25% may go undetected). Consequently, the energy and mass of the hailstones are also underestimated (they may be up to 50% higher than the values registered on a hailpad). The maximum size registered, however, does not depend on the degree of overlapping and neither does the slope parameter lambda of the exponential distribution, except when l takes higher values. Finally, the authors suggest a heuristic correction of the data obtained by real hailpads based on the results of the simulations. An example is pro- vided that applies these corrections to the 228 hailfalls registered by the Italian hailpad network over a period of 10 yr. The results show that, on average, the correction applied because of overlapping increases the number of hailstones in 3.2%, the mass in 1.9%, and the energy in 5.4%. However, there are cases in which these corrections reached much higher values of up to 6.9% in N and M, and up to 25.2% in E. It is therefore advisable to correct dent overlap before carrying out a regional climatic study of hail, since this study would certainly be affected by the errors accumulated by all the hailpads.

The physical properties of hailstones registered by a hailpad network (size distribution, mass, kinetic energy) are essential data for the establishment of a regional hail climatology. Nevertheless, when comparing these data to the same properties of hailstones inside the cloud, the melting process must be taken into account. This paper presents a brief theoretical study of the changes effected on hailstone size distribution due to the melting process. The paper is based on previous studies dealing with the melting of hailstones before they reach the ground. The aim is to analyze the influence of this melting process on hailstone size distribution. An initial melting simulation was carried out in order to achieve this aim. Despite the common assumption that hailstone size distribution on the ground is exponential, it was found that when the in-cloud size distribution is exponential, on the ground, there are fewer small hailstones than what would be expected in an exponential distribution.
The data registered by the hailpad network in Leo ́n (Spain) for 1 year were used to estimate the size of every hailstone before the melting process. The results show that the hailstone size distribution simulated inside the cloud resembles more closely an exponential distribution than the hailstones on the ground.
The type of hailstone size distribution inside the cloud will be the starting point for calculating the hailstone size distribution on the ground. Several equations describing the melting processes are used to calculate a new probability density function that initially corresponds to an exponential distribution that undergoes a partial melting process. The result is a function that is not monotonously decreasing like the exponential function, but rather a function that has a peak for a given size. This new function fits better the data found than the exponential function and actually resembles the gamma function.

This paper is an overview of hailpad research from its origin until today using as a search criterion the bibliographic references on the subject in the Web of Science database (ISI). The search was carried out on 1st September 2008. Among the more than 3·107 scientific documents included in the Science Citation Index Expanded (SCI-EXPANDED) from 1945 to the present, the search engine identified 41 containing the word “hailpad*“ (the asterisk is a wildcard for any letter or group of letters).
The results have been analyzed according to various criteria: countries with hailpad networks or studies on hailpads; date of the first article; evolution of the number of articles per decade; concepts studied and research lines followed in each article; and finally, a bibliometric analysis of the journals where these articles have been published. It was found that 70% of the articles on hailpads were published in 2 journals: Journal of Applied Meteorology and Atmospheric Research. These two journals also concentrate most of the citations. The number of articles on hailpads and the documents that cite them have grown steadily, and it is expected to grow still further in the present decade if we take into account the trends found.

This paper is a comparative study between the two most common hailpad calibration systems: one annual calibration of a whole consignment of material, and the individual calibration of each plate after a hailfall. Individual calibration attempts to minimize errors due to differences in sensitivity to the impact of hailstones between plates from the same consignment, or due to differences in the inking process before the actual measurement.
The comparison was carried out using calibration data from the past few years in the hailpad network in south-western France, and data from an individual calibration process on material provided by the hailpad network in Lleida (Spain). The same type of material was used in the two cases.
The results confirm that the error in measuring hailstone sizes is smaller in the case of an individual calibration of hailpads than when one single calibration process was carried out for a whole consignment. The former is approximately 80% of the latter. However, this error could have been higher if it had not been the same person carrying out the single calibration process and the measuring of the dents: it has been found that differences in the inking process may account for up to 20% of the error in the case of small hailstones. Calibration errors affecting other variables, e.g. energy or parameter λ of the exponential size distribution are generally higher (5% and 18%, respectively) than errors due to the spatial variability of the hailstones. However, the calibration method does not influence the maximum size, since the relative error attributed to the spatial variability is about 8 times the calibration error.
In conclusion, if errors in determining energy or parameter λ are to be reduced to a minimum, it is highly advisable to be consistent in applying the measuring procedure (if possible with the same person carrying out the measurements all the time), and even to use individual calibration on each plate, always bearing in mind that technicians have to be trained appropriately in order to achieve the highest possible degree of uniformity.