Analysis of Precipitation and Regionalization of Torrential Rainfall in Bulgaria

The increasing frequency of extreme rainfall events that cause severe damage is considered a clear sign of climate change. Therefore, analyzing these events and gaining a better understanding of the circulation patterns that form precipitation regimes and trigger torrential rainfall are crucial for developing adaptation strategies. This study aims to present a comprehensive picture of precipitation regimes in Bulgaria under contemporary climate conditions, investigate the connections between precipitation and atmospheric circulation patterns, and propose a regionalization of torrential rainfall. We used daily precipitation data collected in the period 1991–2020, along with data on hazardous rainfall warnings issued by the National Institute of Meteorology and Hydrology. To identify the circulation patterns associated with both rainy days and hazardous rainfall in Bulgaria, we applied the automated Jenkinson–Collison classification. To identify precipitation patterns, we conducted a principal component analysis in T-mode with varimax rotation and k-means clustering of component scores on both monthly normals and a dataset of 166 selected torrential rainfall days. The results, examined in the context of the existing regionalization of precipitation, highlight the climatic diversity of precipitation regimes in Bulgaria. Our findings indicate that torrential rainfall is associated with low-pressure systems and airflows mainly from the east or northeast, as well as with weak-gradient pressure fields.