Meteorological and Hidrological Service of Croatia - Croatia
The number of haildays in the warm period of the year, June-September, on meteorological stations is used for the evaluation of hail suppression activities in northwestern Croatia.
The mean number of haildays on the station is connected with the mean number of haildays in the region and the mean area covered by individual hailfalls (Long, 1980). The assumptions of this analysis are that regional frequency of haildays cannot be changed with hail suppression, and that the changes of mean area covered by individual hailfalls are the consequence of hail suppression.
Multivariate linear regression is chosen as a method of analysis. The frequency of haildays on stations is a dependent variable, and the year, the number of days with thunder, and amounts of precipitation greater than 10, 20 and 50 mm are independent variables. These variables were chosen because they are correlated with the number of haildays. Their purpose is to take over the short-term and long-term variations of conditions for outbreak of storms. Although no proof is given in this work, it is assumed that more stormy weather means more hail days in the region and that through the correlants the changes of the number of hail days in the region are taken into account. The effect of the hail suppression is represented with step function, which has a value one for years with hail suppression and zero otherwise. Regression coefficient of this function, according to assumptions of analysis, reflects the effect of hail suppression.
Data from seven meteorological stations in northwestern Croatia are used in the analysis. The area of the region is about 11000 km2. Operational hail suppression system started work in this part of Croatia in 1971, and spread over the whole area until 1974, so there were years in which some stations were under the influence of hail suppression and others not. The number of stations has changed over the years. The oldest station started to work in 1861 and the youngest one in 1982. Analyses were done for all years with at least one working station, and for the two periods, 1936-2000. and 1945-2000. The common characteristic of all cases is very low percent of explained variance, 14 - 18%, and negative regression coefficient of step function representing the effect of hail suppression. According to the calculated value of the regression coefficient the yearly mean number of days taken over all stations was 20% lower due to the effect of hail suppression.
Conditions for storm development are farther investigated by CAPE calculated from upper air soundings taken at the station Zagreb-Maksimir, placed in the southern part of the region, for period 1971-1994. The number of days with CAPE greater than chosen threshold energies is found for several values of threshold energy reaching from 1000 to 2000 J/kg. Generally, the number of days with CAPE greater than threshold value was diminishing with years for all threshold values, but correlation coefficients are not significant.