Visualising Exceptional Rainfall in Portugal: The Severity Heat Map Approach (1981/1982–2022/2023)

This study develops a Severity Heat Map to analyse the spatiotemporal dynamics of exceptional rainfall in Portugal over 42 hydrological years (1981/1982–2022/2023) using the ERA5-Land reanalysis dataset. By employing four upper quantiles (Q95, Q99, Q99.5, Q99.9), this research identifies exceptional rainfall events and analyses their occurrences, cumulative impacts, and intensity. The ERA5-Land data, validated against local meteorological records, provide a robust foundation for assessing regional variations and temporal changes. The results reveal significant regional variations and temporal changes in exceptional rainfall. While overall cumulative rainfall has decreased, the most exceptional events have intensified in severity. This trend is particularly evident when comparing two subperiods: 1981/1982–2001/2002 and 2002/2003–2022/2023. The innovative Severity Heat Map visualises these changes, illustrating combined characteristics of exceptional rainfall. It shows significant increases in frequency and intensity, particularly in coastal and urban areas. The Severity Heat Map was created using a 2×2 matrix, considering two variables: the mean annual number of occurrences of exceptional rainfall and the mean annual cumulative rainfall above the threshold. Higher severity was assigned to areas where both variables exhibited increased values in the more recent subperiod. This visualisation highlights regions experiencing heightened severity, offering a comprehensive view of the evolving landscape of exceptional rainfall in Portugal. This tool is crucial for understanding the broader impacts of climate change on regional hydrological systems. The findings indicate that despite a decrease in overall rainfall, the intensity and severity of exceptional events have increased, reflecting the broader impacts of climate change. These insights are vital for informing risk management and sustainable development strategies aimed at mitigating the impacts of exceptional weather events in Portugal. The Severity Heat Map approach provides valuable insights for informed decision making in risk management and sustainable development, tailored to the changing patterns of exceptional rainfall in the country.