Statistical analysis of hail characteristics in the hail-protected western part of Croatia using data from hail suppression stations

Abstract

In the continental part of Croatia, operational hail suppression has been conducted for more than 30 years. The current protected area is 25,177 km 2 and has about 492 hail suppression stations which are managed with eight weather radar centres. This paper present a statistical analysis of parameters connected with hail occurrence on hail suppression stations in the western part of protected area in 1981–2000 period. This analysis compares data of two periods with different intensity of hail suppression activity and is made as a part of a project for assessment of hail suppression efficiency in Croatia. Because of disruption in hail suppression system during the independence war in Croatia (1991–1995), lack of rockets and other objective circumstances, it is considered that in the 1991–2000 period, hail suppression system could not act properly. Because of that, a comparison of hail suppression data for two periods was made. The first period (1981–1990), which is characterised with full application of hail suppression technology is compared with the second period (1991–2000). The protected area is divided into quadrants (9×9 km), such that every quadrant has at least one hail suppression station and intercomparison is more precise. Discriminant analysis was performed for the yearly values of each quadrant. These values included number of cases with solid precipitation, hail damage, heavy hail damage, number of active hail suppression stations, number of days with solid precipitation, solid precipitation damage, heavy solid precipitation damage and the number and duration of air traffic control bans. The discriminant analysis shows that there is a significant difference between the two periods. Average values of observed periods on isolated discriminant function 1 are for the first period (1981–1990) −0.36 and for the second period +0.23 standard deviation of all observations. The analysis for all eight variables shows statistically substantial differences in the number of hail suppression stations (which have a positive correlation) and in the number of cases with air traffic control ban, which have, like all other variables, a negative correlation. Results of statistical analysis for two periods show positive influence of hail suppression system. The discriminant analysis made for three periods shows that these three periods can not be compared because of the short time period, the difference in hail suppression technology, working conditions and possible differences in meteorological conditions. Therefore, neither the effectiveness nor ineffectiveness of hail suppression operations nor their efficiency can be statistically proven. For an exact assessment of hail suppression effectiveness, it is necessary to develop a project, which would take into consideration all the parameters used in such previous projects around the world—a hailpad polygon.