J.L. Sánchez

Analysis of mesoscale convective systems with hail precipitation

Abstract Severe thunderstorms hit the Ebro Valley (Northeastern Spain) during the summer months, and hail precipitation is frequently registered. In this area, the spatial scale of storm cells is usually of 10 to 40 km. Nevertheless, in conditions of deep convection there may be precipitation systems on spatial scales from 40 to 500 km or larger. These types are known as mesoscale convective systems (MCS). Occasionally, there are long-lived mesoscale convective systems, termed mesoscale convective complexes (MCC), that cause intense precipitation and may produce hail. There are middle latitude classification criteria used to identify MCS and MCC from enhanced IR images, which may be derived from METEOSAT. This paper presents an analysis based on a classification of METEOSAT images for hail events in the Ebro Valley. The database consists of 72 of these hail events including 28 cases of MCS and 5 of MCC. In the remaining 36 cases, the convective air masses produced hail precipitation, but did not fulfill the classification criteria to be considered MCs. In addition, the paper includes a characterization of preconvective conditions according to different sounding variables. A logistic regressive analysis has been applied and the results show the difficulties encountered in forecasting the formation of MCs on the basis of preconvective variables.

Day-of-the-week variability of hail in southwestern France

A study of the day-of-the-week variability of hailfall has been made using hailpad data, collected for 11 years in a large area of southwestern France. In the Atlantic region of this area, point hailfall frequency and intensity are not different on weekends and weekdays. In the inland region, the frequency is the same during both week periods, but the mean kinetic energy per hailfall is about twice as important on weekends as on weekdays. An analysis of the corresponding mean hailstone size distributions for the two periods shows that both intercept and slope parameters of exponential size distributions, which fit with the observed ones are significantly different, and that the reduction in hail severity on weekdays is due to a shift from large to small hailstones. The day-of-the-week variability of air pollution measured in the inland region surrounding Toulouse suggests that anthropogenic emissions of ice-forming particles may be responsible for the observed hail change.

Analysis of hailstone size distributions from a hailpad network

Abstract In the province of Leon, a network of 250 hailpads has been installed in an area of 1000 km 2 . After the individual calibration of every plate, the dents are measured by a manual method which stores data in files that can be analyzed by computer. Once the hailstones are classified according to their size, difficulties may arise when fitting linearly this distribution to a function of the type log N = log N 0 - β x , where N is the number of hailstones in the size class x . A discussion is presented on the universal validity of parameters N 0 and β, on the problem of empty classes (to which it is impossible to apply logarithms), and on the discrimination of the smallest hail classes when making such a fitting. In conclusion, statistical methods are proposed for fitting the exponential or gamma distribution. The latter of these distributions assumes the former as a particular case and offers a better fit to the experimental data.

Return periods of severe hailfalls computed from hailpad data

Abstract 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.

Discriminant analysis applied to the forecasting of thunderstorms

SummaryThe combination of several thermodynamic variables based upon the data provided by a radiosounding can be useful for the forecasting of thunderstorms. As a matter of fact, there are many indices that allow the establishment of a storm risk prediction once they have been gauged. The problem comes when not all indices lead to the same prediction. In these cases, it is necessary to establish one single function based on the information provided by all the variables employed, which should be able to determine a two-fold prediction: risk or no risk. This article presents a statistic model for the short tem prediction of thunderstorms in the region of León (Spain). To reach this aim 15 meteorological variables were selected. These variables were easy to handle by non-expert staff, and they allowed the characterisation of the preconvective environment early in the morning on thunderstorm days. The variables have been properly combined and gauged with the help of a dense network of meteorological observers. The result has led to the construction of a reliable model. The discriminant quadratic model has been easily applied to determine in an objective and binary way the risk/no risk for the occurrence of thunderstorms.

CAPE values and hailstorms on northwestern Spain

A study has been carried out in Leon (in the northwest of the Iberian Peninsula) on atmospheric convection during summer periods, by analyzing the values of the Convective Available Potential Energy (CAPE) around 07:00 UTC. The project analyzed the data provided by a network of voluntary observers, a hailpad network, and a meteorological station on a sample of 224 days. The CAPE values found were not high: they never reached 2000 J/kg, not even on hail days, i.e. on days with high convective activity. These values are much lower than the ones measured in convective situations in tropical regions, but they are within the usual values found in Europe. The same happens with the wet bulb potential temperature measured in Leon. The frequency distribution of the CAPE values shows a clear prevalence of very low or zero values. The group of days with the lowest CAPE value is that which included days with no storm. The mean value increases on storm days, and it is even higher for the days with recorded hailfalls. All these differences are significantly marked. Nevertheless, the differences across the years are not significant enough to be able to speak of an influence of the climatic change on the CAPE. The correlation of the CAPE with some of the variables previously used for hail forecasts was analyzed, and the correlation found was higher for the lifted index. The possible use of the CAPE as a thunderstorm and hailstorm forecasting method was considered. The results were encouraging, especially for hail forecasting, although the CAPE should not be used as the only variable, but combined with other parameters. Moreover, the relationships between the CAPE and the wet bulb potential temperature and between the CAPE and the physical parameters of the hailstones were also analyzed. A relationship was observed with the parameters of the hailstone size distribution. Nonetheless, these results are provisional, and they should be confirmed by analyzing a more representative sample. With a more detailed analysis of these and other relationships, the present forecast model used by the Laboratory for Atmospheric Physics at the University of Leon is expected to be greatly improved by including the CAPE in this model.

Noteworthy C-band radar parameters of storms on hail days in northwestern Spain

Abstract The data provided by a meteorological C-band radar have been employed in order to analyze the behavior of 224 storms that occurred on hail days in the provinces of Leon and Zamora, in the northwest of Spain. A network of more than 700 voluntary observers supplied the necessary information to distinguish hailstorms from storms that did not produce hail precipitation. The observers also reported the size of the hailstones found. Among the variables studied are the maximum reflectivity factors, the vertical developments, and the motion speed of the storm for hailstorms as well as for storms with no hail. The altitude at which the precipitation processes take place inside the storm cell and its average lifetime are the variables that determine the subsequent evolution of hailstorms and no-hail storms in the same day. Furthermore, these variables have been analyzed taking into account the type of storm: unicellular, multicellular or supercellular. Finally, a correlation between the characteristics of the storms and the size of the hailstones registered simultaneously on the ground has been searched for using the RHI detected on the radar exactly at the time of the precipitation.

The 14 July 2001 hailstorm in northeastern Spain: diagnosis of the meteorological situation

Hail producing thunderstorms developed over the Ebro valley (NE Spain) during the evening of 14 July 2001, affecting mainly the Lerida province. Hail stones as large as 3 cm in diameter produced damage on 2979 ha of fruit trees, vineyard and cornfields. The thunderstorms developed ahead of a cold front, which was moving from the Gulf of Biscay towards inland Spain. Meteosat images and radar data demonstrate that the storms formed over the central part of the Ebro valley and moved towards the east attaining their maximum development in Lerida province. A diagnosis, using data from ECMWF, shows that at surface there was a cyclonic circulation over northeastern Spain and at medium levels (500 hPa) a trough with cold air located towards northwestern Spain. The Q vector diagnosis demonstrates that the forcing for upward vertical motions was rather weak at both low and medium levels over the area where the thunderstorms developed. However, a significant frontogenesis contribution is identified over the Ebro valley. A more detailed handmade analysis shows that over the Ebro valley there was a thermal mesolow, which favoured the inland entrance of humid air from the Mediterranean. Frontogenesis and the humid air intrusion coexisted where remote-sensing observations indicated that the storms developed. A numerical study of the event using the MM5 model has been carried out. In a control experiment, the model is able to develop the thermal mesolow and reproduce, quite well, the convergence produced by the front as well as the timing of the event. In order to study the genesis and influence of the thermal mesolow, another simulation has been performed without consideration of solar radiation. The results indicate that the thermal mesolow does not develop, the convergence ahead of the cold front is significantly weakened and the front itself becomes increasingly progressive. As a result, thunderstorms do not develop and very little precipitation falls in the area.

The influence of melting on hailstone size distribution

AbstractThe 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 processmust be taken into account. This paper presents a brief theoretical study of the changes effected onhailstone size distribution due to the melting process. The paper is based on previous studies dealingwith the melting of hailstones before they reach the ground. The aim is to analyze the influence ofthis melting process on hailstone size distribution. An initial melting simulation was carried out inorder to achieve this aim. Despite the common assumption that hailstone size distribution on theground is exponential, it was found that when the in-cloud size distribution is exponential, on theground, 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 thesize of every hailstone before the melting process. The results show that the hailstone sizedistribution simulated inside the cloud resembles more closely an exponential distribution than thehailstones on the ground.The type of hailstone size distribution inside the cloud will be the starting point for calculating thehailstone size distribution on the ground. Several equations describing the melting processes are usedtocalculate anew probability densityfunction thatinitiallycorresponds to anexponential distributionthat undergoes a partial melting process. The result is a function that is not monotonously decreasinglike the exponential function, but rather a function that has a peak for a given size. This new functionfits better the data found than the exponential function and actually resembles the gamma function.D 2003 Elsevier B.V. All rights reserved.

Concentration of ice nuclei in continental and maritime air masses in León (Spain)

Among the objectives of the Plan de Actuacion de Lucha Antigranizo, (PALA, Hail Suppression Activity Plan), is the on-going analysis of the mechanisms operating during the formation of storms. There is a relationship between the processes at work in the interior of cloud masses and the concentration of natural ice nuclei (IN) at ground level. Through the rigorous analysis of 954 measurements made with an isothermal cloud chamber, it has been possible to establish that a dependence exists between the background concentration of IN and the type of air mass. The work shows that the concentration of IN in the samples analyzed in Leon was, on average, higher for continental than for maritime air masses, with a substantial presence of continental air masses originating from the Sahara desert. Furthermore, from the analysis of 4-day back trajectories, it has been observed that slow-moving maritime air masses gradually become altered during their passage over the peninsula, which causes an increase in the number of active IN.