Hourly Precipitation Patterns and Extremization over Italy using convection-permitting reanalysis data

Abstract. The latest generation of convection-permitting reanalyses, capable of representing atmospheric processes at small spatial scales (≤4 km), is crucial for studying the temporal and spatial evolution of phenomena such as convective storms and orographic precipitation. Given the long (37 years) and continuous availability of the MERIDA HRES convection-permitting reanalysis datasets over Italy, this study employs its precipitation fields to investigate hourly precipitation patterns, extremes, and quantify their increase over time in this region. Precipitation clusters are extracted from hourly reanalysis fields as spatially coherent structures, yielding approximately 160 000 of them per year, each one characterized by intensity and shape indicators. The resulting HOPSS-X (HOurly Precipitation Spatial Structures and eXtremes) dataset enables a detailed climatological analysis of hourly precipitation frequency, intensity, and spatial extent across different seasons and regions. The Hourly Precipitation Extremes (HPEs) are selected from this dataset basing on the mean of local annual maxima in hourly precipitation (RX1hour). Significant upward trends in HPEs occurrences (+20 % / +30 %) are present during summer in several Alpine and Prealpine regions, as well as in parts of Calabria. In autumn, similar signals (+30 % / +40 %) emerge in the southern Apennines and in coastal and maritime areas, including the eastern Ligurian coast, eastern Sardinia, the southern Adriatic Sea, and the Ionian Sea. These spatial and seasonal patterns align with regions where convective processes predominantly drive intense, localised precipitation, potentially amplified by climate change. While these findings should be considered in light of known limitations of reanalysis products, such as spatial mismatches with observations and temporal inhomogeneities, multiple independent observational studies support the increase in HPEs during summer and autumn in specific areas. Moreover, the methodology presented here is broadly applicable in any region with access to long-term convection-permitting reanalysis data. In summary, the purpose of this study is to offer a contribution to the ongoing discussion on precipitation extremes and trends in Italy and provide guidance for leveraging reanalysis data to enhance infrastructure resilience to short-lived, intense precipitation events.