Beyond Intensity: How Rainfall Accumulation Patterns Drive Flood Disaster Characteristics in Southern Brazil

This paper investigates the role of extreme rainfall in major flood disasters in Southern Brazil. Disaster Information System Brazil (S2ID) from 1991 to 2024 was consulted to identify the key variables best representing the magnitude of damage, through a hierarchical ranking of the First Principal Component. We hypothesize that accumulated extreme rainfall, rather than isolated intense events, constitutes the primary driver of large-scale flood disasters in the region. We investigated precipitation characteristics and extreme events occurring in the 15 largest flood disaster events. We used historical daily precipitation series data from 13 weather stations. with quality control performed by CoRain software for rain series comparison. Accumulated extreme rainfall contributed to 67% of major flood disasters in southern Brazil. The remaining 33% is linked to isolated extreme rainfall events. These isolated extreme events intensified hydrological disasters through rapid runoff generation and soil saturation feedbacks. While the total rainfall volume showed a decreasing trend in 2 municipalities and an increasing trend in 5, all studied municipalities exhibited an increasing trend in the frequency of extreme rainfall events. These results are consistent with intensification patterns observed in other subtropical regions globally. Our findings indicate that the sequential occurrence of rainfall events, culminating in significant accumulated volumes, emerges as the principal conditioning factor for large-scale hydrological disasters in the region. This scenario is associated with quasi-stationary frontal systems and persistent cyclonic activity, particularly the prolonged influence of the South Atlantic Polar air masses. These findings contribute to understand mid-latitude flood generation mechanisms, with implications for early warning systems and adaptation strategies in subtropical regions worldwide.