Hail Climatology and Trends in Romania: 1961–2014

Abstract

AbstractA climatology and trend of hail events in Romania are presented using hail data spanning the years 1961–2014. Hail observations from weather stations and model reanalysis data were used to document the spatial and temporal distributions, variabilities, and environments of hail events. The results show that hail occurs most frequently in mountainous areas, while the smallest average number of hail days per year is found in the southeast. Herein, the convective season was defined as April–September, given that 94.2% of all mean monthly hail days were identified in this period. During the convective season the hail events prevail with most of these occurring in the afternoon and evening hours between 1000 and 1800 UTC. The severe hail events occur, overall, between 1400 and 1600 UTC, while in the southwest severe hail occurs later between 1600 and 1900 UTC. The spatial distribution of the convective parameters is consistent with the spatial distribution of hail days, revealing that hail is not favored in southeastern Romania, but in the rest of the country. The trend analysis of mean hail days per year disclose that 55.2% of all stations show a statistically significant upward trend, 3.8% show a statistically significant downward trend, while 40.9% show no statistically significant trend. A correlation between the variability of hail days per year and the variability in the occurrence of low pressure systems of Atlantic origin exists, the latter generating low pressure systems over the Mediterranean Sea that supply eastern Europe with the moist air needed for convection.