Long-term variability of winter thunderstorms in Poland (1951–2020)

Due to anthropogenic climate change, warmer and almost snowless winters are increasingly observed in mid-latitudes, including Poland. As a result, air masses can transport more moisture, making them more unstable, potentially leading to more convective phenomena, including thunderstorms. Although thunderstorms occurring in the cold half of the year, especially in winter, are extremely rare and not as dangerous as summer thunderstorms, they can be classified as extreme weather phenomena. At the same time, the increase in their frequency is another signal of climate change. This study presents changes in the occurrence of thunderstorms in the cold half of the year and in winter in Poland from 1951 to 2020. The research was based on data from 46 synoptic stations of the Institute of Meteorology and Water Management – National Research Institute (IMGW-PIB), and the basic indicator was a day with a thunderstorm. Studies have shown a clear spatial differentiation of thunderstorms occurring in the cold half of the year, which in some regions of the country accounted for even more than 8% of all cases in the year. The strongest influence on the occurrence of these phenomena in the cold season is air circulation and local conditions (especially the impact of the Baltic Sea in the north and varied topography in the south). In Poland in the period 1951–2020, there were on average 13.1 days with a thunderstorm from October to March (days where at least one station recorded a thunderstorm), including 3.4 days during meteorological winter. The number of such days at individual stations ranged from 0.5 to 1.8 and 0.01 to 0.59, respectively. In the period under study in Poland, the number of thunderstorm days in the cool season slowly increased, and the increase amounted to 0.35 days per decade. Most of the thunderstorms considered were associated with atmospheric fronts. The strongest occurred during the passage of dynamic cold fronts, which are characterised by low potential energy and high wind shear values.