Cavalleri et al. (2026) Hourly Precipitation Patterns and Extremization over Italy using convection-permitting reanalysis data

Identification

• Journal: Natural hazards and earth system sciences
• Year: 2026
• Date: 2026-01-19
• Authors: Francesco Cavalleri, Cristian Lussana, Francesca Romana Viterbo, Michele Brunetti, Riccardo Bonanno, Sergio Pisani, Matteo Lacavalla, Maurizio Maugeri
• DOI: 10.5194/nhess-26-279-2026

Research Groups

• Environmental Science and Policy Department (ESP), University of Milan, Milan, Italy
• Sustainable Development and Energy Resources Department, Research on Electric Systems (RSE), Milan, Italy
• Division for Climate Services, the Norwegian Meteorological Institute, Oslo, Norway
• Institute of Atmospheric Sciences and Climate, National Research Council (CNR-ISAC), Bologna, Italy

Short Summary

This study investigates hourly precipitation patterns and extremes over Italy using convection-permitting reanalysis data, revealing significant upward trends in hourly extreme precipitation occurrences, particularly in Alpine regions during summer and southern coastal areas in autumn. The research provides a methodological framework and a new dataset to understand evolving precipitation patterns and inform climate resilience strategies.

Objective

• To investigate hourly precipitation patterns, extremes, and their temporal increase over Italy using convection-permitting reanalysis data.
• To propose a methodological framework for characterizing hourly precipitation structures across space and seasons, and to detect the potential extremization of precipitation over time.
• To contribute to the scientific 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.

Study Configuration

• Spatial Scale: Italian domain (5.84 to 18.96° E and 35.37 to 48.25° N) with approximately 4 km horizontal resolution and 56 vertical levels. Analysis performed within a 0.5° moving window with 0.1° increments.
• Temporal Scale: 37-year period from 1986 to 2022, with hourly resolution.

Methodology and Data

• Models used:
  • MERIDA HRES (MEteorological Reanalysis Italian DAtaset – High RESolution) convection-permitting reanalysis.
  • Weather Research and Forecasting (WRF) model for dynamical downscaling.
• Data sources:
  • Hourly precipitation fields from MERIDA HRES.
  • ERA5 global reanalysis (for initial and boundary conditions).
  • SYNOP surface air temperature observations (assimilated into WRF).
  • HOPSS-X (HOurly Precipitation Spatial Structures and eXtremes) dataset (generated in this study).

Main Results

• Approximately 160,000 Hourly Precipitation Spatial Structures (HPSSs) are extracted per year over Italy, with an interannual variability of about 10 %.
• The HOPSS-X dataset provides detailed climatological analysis of hourly precipitation frequency, intensity, and spatial extent, showing higher median intensities and heavier tails in summer and autumn.
• Hourly Precipitation Extremes (HPEs) are identified as HPSSs whose maximum precipitation value exceeds the local average of annual maxima of hourly precipitation (RX1hour). HPEs constitute about 4.8 % of all HPSSs, with higher fractions in summer (11 %) and autumn (7 %).
• Significant upward trends in HPEs occurrences are detected:
  • Summer: +20 % to +30 % per decade in several Alpine and Prealpine regions, and parts of Calabria.
  • Autumn: +30 % to +40 % per decade in the southern Apennines, and various coastal and maritime areas (e.g., eastern Ligurian coast, eastern Sardinia, southern Adriatic Sea, Ionian Sea).
• HPEs generally exhibit smaller spatial extents (typically 10-20 km in summer, up to 50 km or more in autumn in northern Italy) and higher intensities (MeanInt ranging from 5 to 15 mm/h, PeakInt from 20 to 50 mm/h).
• Trends in the spatial extent, mean intensity, and peak intensity of HPEs are weak (below 10 % per decade) and not statistically significant, suggesting the increase in occurrences is driven by more frequent exceedances rather than a significant change in the magnitude of individual events.
• Most HPEs are short-lived, rarely persisting for more than one hour, indicating that the impact of temporal double-counting on trend estimates is limited.

Contributions

• Development of a novel methodological framework for identifying and characterizing hourly precipitation spatial structures and their extremes using convection-permitting reanalysis data.
• Creation of the publicly available HOPSS-X dataset, an extensive archive of nearly 6 million hourly precipitation structures over Italy for the period 1986–2022.
• Identification of statistically significant increasing trends in the occurrence of hourly extreme precipitation events over specific Italian regions and seasons, which are corroborated by independent observational studies.
• Provision of a robust, reanalysis-based approach to assess short-lived, intense precipitation events, offering valuable insights for climate change impact assessments and infrastructure resilience planning.

Funding

• PhD of Francesco Cavalleri: PNRR funds (EU Next Generation programme) and R.S.E. s.p.a.
• Research Fund for the Italian Electrical System: Three-Year Research Plan 2025–2027 (MASE, Decree no. 388 of 6 November 2024).
• Italian Ministry for University and Research: PRIN 2022 – CN4RWK–CCHP-ALPS Climate Change and HydroPower in the Alps (funded by the European Union, Programme Next Generation EU).
• Veronica Manara: Ministero dell'Università e della Ricerca of Italy [grant FSE – REACT EU, DM 10/08/2021 no. 1062].

Citation

@article{Cavalleri2026Hourly,
  author = {Cavalleri, Francesco and Lussana, Cristian and Viterbo, Francesca Romana and Brunetti, Michele and Bonanno, Riccardo and Pisani, Sergio and Lacavalla, Matteo and Maugeri, Maurizio},
  title = {Hourly Precipitation Patterns and Extremization over Italy using convection-permitting reanalysis data},
  journal = {Natural hazards and earth system sciences},
  year = {2026},
  doi = {10.5194/nhess-26-279-2026},
  url = {https://doi.org/10.5194/nhess-26-279-2026}
}