Roberto Fraile

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.

Hailstone size distributions in southwestern France

Abstract A hailpad network, intended to draw up a large-scale, long-term physical climatology of hailfalls, is being installed in southwestern France, one of the severely hailed regions in the world. The first five years of operation of this network have enabled the organization of a suitable standardization of the data. The classical exponential form is well adapted to represent the mean size distribution of the hailstones sampled at several locations, but we suggest the replacement of the usual concentration parameter of this form—the number of zero diameter hailstones—by the number of the smallest hailstones really counted on the pads, so as not to amplify, with the regression, a relationship between this parameter and the slope parameter. There is a significant correlation between the concentration parameter and the altitude of the 0°C isotherm level, and a regression enables the determination of a concentration parameter at the 0°C level. By contrast, the correlation between the slope parameter and the altitude of the 0°C level is weak. When these results are compared with Ludlams model of hailstone melting, they suggest that hail mainly falls within downdrafts. With the limited amount of data presently available, no correlation appears between the distribution parameters and the storm thermodynamics, but the concentration parameter is larger while the slope parameter is smaller in the frontal storms than in the intramass ones, a result explaining why the most severe hailstorms of southwestern France are frontal.

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.

Radiative forcing of haze during a forest fire in Spain

Intense fires occurred in northwestern Spain on 6 September 2000, filling a valley with smoke haze. Aerosol size distribution measurements were performed during 1 day with a thermal inversion, so the aging process of the smoke aerosol could be closely monitored. In 3.5 h, the fine aerosol increased up to 0.06 μm in the geometric median diameter of the fine mode. This aging process enhanced the scattering ability of aerosols. On the basis of several hypotheses on the data obtained, shortwave radiative forcing at surface level, at top level, and in the atmosphere was estimated: instantaneous surface forcing reached up to between −80.4 and −67.4 W/m2, top of the atmosphere (TOA) instantaneous forcing reached up to between −23.4 and +4.9 W/m2, and instantaneous atmosphere forcing reached up to between +44.2 and +85.3 W/m2. The study reveals not only the absorption of solar radiation in the atmosphere by smoke aerosols but also an aerosol-induced case study, where TOA cooling forcing shifts to warming for specific aerosol single scattering albedo. The daily mean heating rate of the smoke haze was estimated at 5.9 ± 0.6 K/d.

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.

Fitting an Exponential Distribution

Abstract Exponential distributions of the type N = N0 exp(−λt) occur with a high frequency in a wide range of scientific disciplines. This paper argues against a widely spread method for calculating the λ parameter in this distribution. When the ln function is applied to both members, the equation of a straight line in t is obtained, which may be fit by means of linear regression. However, the paper illustrates that this is equivalent to a least squares fit with a weight function that assigns more importance to the higher values of t. It is argued that the method of maximum likelihood should be applied, because it takes into account all of the data equally. An iterative method for determining λ is proposed, based on the method of moments for cases in which only a truncated distribution is available.

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 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.