Climatology and formation environments of large hail in Russia

Large hail is a hazardous convective phenomenon that causes significant socioeconomic damage. Although climate change has led to an increase in severe convective events in several Russian regions, the statistics of large hail occurrences in Russia remain understudied. For the first time, this work compiles a comprehensive database of hail events in Russia from 1906 to 2024. Sources include ESWD data, routing meteorological observations, news and social network information, and existing scientific literature. The database contains over 12,000 hail reports, including more than 4,000 reports of large hail. A comparative analysis of hail events and lightning activity, based on WWLLN data from 2016 to 2024, reveals a statistically significant difference in the characteristics of lightning between hail and non-hail cases. The skill score analysis enables the determination of the optimal frequency of lightning flashes for identifying severe hail occurrences. Lightning data were used to specify the time and place for some hail cases in the presented database.Analysis of the collected database revealed an exponential distribution of hail sizes, with a median diameter of 2 cm and a 95th percentile of 5 cm. The spatial distribution of hail generally reflects population density, with the highest frequencies (including hail larger than 10 cm in diameter) occurring in southern and central European Russia and southern Western Siberia. The temporal distribution of hail reports revealed a pronounced annual cycle, with a peak in June, as well as a diurnal cycle, with a peak around 16:00 local time.To evaluate the atmospheric conditions associated with hail formation, thermodynamic and dynamic indices were calculated for hail events during the warm season (May–August) between 2010 and 2020, using ERA5 reanalysis data at an hourly temporal resolution and a 25 km spatial resolution. Statistically significant relationships were found between index values and the occurrence and size of hail. The most informative indices and their threshold values were identified for diagnosing hail of various sizes.The study was supported by the Russian Science Foundation (grant no 24-17-00357).