Trend analysis and climatology of hail in Croatia

Abstract

This study analyzed the hail climatology across Croatia using data from a national network of 199 stations, which recorded a total of 8551 hail cases. The spatio-temporal analyses were made for: (i) several different periods, (ii) five additional 121 year long data series and (iii) investigation of the connection between weather types and occurrence of hail. Due to high complexity of Croatian territory (which includes the northern lowland, the central mountainous area and the long Adriatic coast with numerous islands and the Istrian peninsula), the spatial and temporal variability of hail as well as the limited number of hail data, three types of stations were defined. The types represent sets of stations with similar annual cycles of hail occurrence. Trend analysis using Mann–Kendall trend significance tests with Theil–Sen trend estimates was performed for three periods: from 1900 to 2020, from 1964 to 2019, and from 1995 to 2019. The first two periods showed a negative and significant trend in the number of hail days, while the most recent one, 25-year-long trend showed a change in sign toward a positive but not significant trend.The northernmost parts of the coast (i.e., inner Istria) and lowland of Croatia had greater hail activity during summer. In contrast, along the southern and central Croatian coast, the highest hail activity was present in the colder part of the year. There was also a transitional area located in between, that recorded the most hail in spring and fall. Diurnal cycle of hail showed a shift in the daily maximum from morning to afternoon hours from the coast toward Croatian lowland.The coastal part of Croatia generally recorded higher hail frequencies than the continental part. The climatology of hail duration revealed a log-normal distribution pattern, further suggesting that most hail cases last between 1 and 5 min with peek duration of about 4 min. The most dominant air masses (over 80% of time) responsible for hail come from the SW, W and NW directions, and 83% of hail is associated with cyclonic influence in the region.