Hailfalls in southwest Europe: EOF analysis for identifying synoptic pattern and their trends

One of the main challenges in studies of hail precipitation is the estimation of its trends in the context of global warming. The strong spatiotemporal variability of hailfalls, together with their small scale, makes it difficult to record all such events and build long and homogeneous databases. For this reason, obtaining direct observation trends yields large uncertainties. The study of atmospheric parameters at various scales that influence deep convection and its temporal change has been used to estimate possible trends in the frequency of hailfalls. In this article, we analyze atmospheric variability at synoptic scale during the warm season in Southwest Europe, using empirical orthogonal function analysis and hailfall records from dense hailpad networks in northeastern Spain and southern France. These networks have uninterrupted operation during the warm season and reliable data. This method allowed us to obtain spatial modes of variability and temporal changes, facilitating estimation of changes in hail frequencies. The results show that the main modes of variability of 500-hPa geopotential height facilitate the definition of patterns that favor hailfalls throughout the warm season, whereas lapse-rate patterns between 850 and 500 hPa are shown to be important in the central months of the hail season. In addition, temporal changes in patterns related to hailfall variability suggest changes in the hailfall distribution during the warm season, with a decrease in hail frequency during June and increase during July.