Environmental conditions responsible for the type of precipitation in summer convective storms over Bulgaria

Environmental conditions of 155 days with precipitation over a part of the Upper Thracian lowland in Bulgaria from May to September of 2002–2006 are studied in order to determine meteorological variables that may be used to specify the type of precipitation (rain or hail) on the ground. The cases have been divided into two samples — days with frontal convective clouds (125) and days with free convection (30). Analyses reveal that the mean values and the thresholds of CAPE and LI during the days with hail are similar to their corresponding values determined for thunderstorm development in other regions in Europe. Results show that none of the analyzed instability indices or environmental parameters alone is able to determine the type of precipitation on the ground. The best classification function of the cases according to the type of precipitation (rain or hail) is derived when the in-cloud characteristics simulated by 1-D numerical cloud model are included together with the instability indices and the environmental parameters as variables in the stepwise discriminant analysis. The analysis reveals that the simulated characteristics of the vertical velocity are an important ingredient in the classification function of the type of precipitation on the ground, due to the incorporation of the non-linear impact of environmental conditions on the formation of precipitation.