Climatological Analysis and Trends of Sub-Daily Rainfall Extremes in Croatia

The analysis of short-term heavy rainfall is crucial for designing drainage systems resilient to intense precipitation events, and for assessing the risk of flash floods, urban flooding, and associated hazards. This study investigates the frequency, intensity, and timing of sub-daily rainfall extremes in Croatia using hourly data (1-, 3-, and 6-hour durations) from 54 meteorological stations spanning the period 1961–2020. It provides the first comprehensive insight into the climatology of sub-daily rainfall indices on monthly, seasonal, and annual scales, as well as their trends in the context of climate change. Trends were evaluated using a Bayesian approach, which offers greater robustness in handling data availability inconsistencies across stations. The results show that the highest frequencies and intensities of sub-daily rainfall extremes occur in both the northernmost and southernmost Adriatic regions. The timing of peak intensity and frequency aligns seasonally within each region, suggesting a common influence of convective and thunderstorm activity peaks. In continental Croatia, sub-daily rainfall maxima are most frequent in summer, while in mountainous and coastal Adriatic regions, they occur later, toward late summer and early autumn. Inland areas also show significant contributions of sub-daily extremes to daily totals, indicating the influence of either isolated short-duration events or prolonged storms with pronounced peaks. Furthermore, the wettest hour of the day predominantly occurs around midnight. However, during warmer months, it tends to shift to the afternoon in inland regions and to nighttime along the Adriatic coast and islands. Although data availability varied across stations, limiting the ability to conduct a fully consistent countrywide trend analysis, results indicate an overall increase in short-term heavy precipitation since 1981. This is reflected in the growing number of wet hours, higher sub-daily precipitation intensities, and longer wet spells.