Capozzi et al. (2026) Identification of atmospheric circulation schemes that promote summer hail events in the Northern Italy

Identification

• Journal: Atmospheric Research
• Year: 2026
• Date: 2026-04-01
• Authors: Vincenzo Capozzi, Alberto Fucci, Giorgio Budillon, Giannetta Fusco, Enrico Arnone, Nicola Cortesi, Giulio Monte, Sante Laviola
• DOI: 10.1016/j.atmosres.2026.108990

Research Groups

• Department of Science and Technology, University of Naples “Parthenope”, Naples, Italy
• Department of Physics, University of Turin, Italy
• Institute of Atmospheric Sciences and Climate (ISAC-CNR), Bologna, Italy

Short Summary

This study identifies three distinct atmospheric circulation schemes that promote summer hail events in Northern Italy from 2014-2023, revealing a recent increase in hail frequency and a shift towards circulation types favoring Alpine hail.

Objective

• To identify and characterize the main atmospheric circulation and thermodynamic patterns associated with summer (June-August) hailstorms in Northern Italy during the period 2014-2023.

Study Configuration

• Spatial Scale: Northern Italy (Po Valley and Alpine-Adriatic fringe, 44°N-47°N, 6°E-14°E) for hail events; broader European domain (25°N-72°N, 45°W-60°E) for large-scale circulation analysis.
• Temporal Scale: Summer seasons (June-July-August) from 2014 to 2023.

Methodology and Data

• Models used: Principal Component Analysis (PCA), k-means clustering, Microwave Cloud Classification-Hail (MWCC-H) algorithm (probabilistic model for hail likelihood).
• Data sources:
  • Satellite: Microwave Cloud Classification-Hail (MWCC-H) dataset, 1°×1° spatial resolution, 3-hour temporal resolution, detecting hail ≥ 2 cm.
  • Reanalysis: ERA5 reanalysis dataset (ECMWF), 0.25° x 0.25° horizontal resolution, hourly data (12:00 UTC fields used). Variables include 500-hPa geopotential height (Z500), mean sea-level pressure (MSLP), 850-hPa air temperature (T850), 250-hPa winds (W250), water vapour transport (VT), bulk shear (BS) between 950 and 500 hPa, Convective Available Potential Energy (CAPE), and equivalent potential temperature (θe).

Main Results

• A total of 122 summer hail days (≥ 2 cm) were identified and classified into three distinct spatial patterns (ACSs) using PCA and k-means clustering.
• Atmospheric Circulation Scheme 1 (ACS 1, 39.1% of events): Associated with hail maxima over north-eastern Italy and the eastern Po Plain. Characterized by a negative 500-hPa geopotential height (Z500) anomaly over Northern Europe/Western Mediterranean, a ridge over Southern Italy/Balkans, strong south-westerly flow, upper-level divergence at 250 hPa over eastern Northern Italy, a thermal boundary, enhanced mid-tropospheric moisture transport (maximum between 600-800 hPa), moderate to high CAPE (1500-1700 J kg⁻¹), and bulk shear (BS) of 15-17 m s⁻¹.
• Atmospheric Circulation Scheme 2 (ACS 2, 34.4% of events): Linked to hail primarily over the western Po Plain and northern Tyrrhenian side. Features a broader and deeper cyclonic circulation (trough core over France, Z500 anomalies up to −60 gpm), negative mean sea-level pressure (MSLP) anomalies over the Italian Peninsula, south-westerly low-level flow, diffluent flow and upper-level divergence over the Po Valley/northern Tyrrhenian, positive 850-hPa temperature (T850) anomalies (+2.5 °C) over the northern Adriatic, negative T850 anomalies (−1 to −2 °C) over the western Alps, enhanced moisture transport (maximum between 700-800 hPa, approaching the 90th percentile), moderate CAPE (1000-1500 J kg⁻¹), and BS of 15-16 m s⁻¹.
• Atmospheric Circulation Scheme 3 (ACS 3, 26.2% of events): Concentrated over north-western Italy and the northern Alpine chain. Characterized by a cut-off low over the British Isles/western Europe and a strong ridge over central/eastern Europe (Z500 anomalies up to 70 gpm), weaker jet stream oscillation but diffluent flow and upper-level divergence over northwestern Italy, widespread positive T850 anomalies (>3°C), markedly enhanced mid-tropospheric moisture transport (maximum between 550-800 hPa, exceeding the 90th percentile), higher CAPE over the Alpine sector, and stronger BS (up to 18-19 m s⁻¹) across northwestern Italy/northern Alps.
• July is the most active month for hail events across all ACSs.
• A shift in the relative contribution of ACSs was observed: ACS 3 increased from 18% (2014-2018) to 32% (2019-2023), while ACS 1 and ACS 2 slightly decreased.
• All hail-prone environments exhibit elevated equivalent potential temperature (θe) in the boundary layer and mid-tropospheric moisture transport. ACS 1 and 2 show near-surface θe maxima, while ACS 3 has a more elevated and spatially extended moist layer (anomalies approaching the 90th percentile between 900-700 hPa). The Adriatic Sea is identified as a significant source of warm, moist air for ACS 1 and 2.

Contributions

• Provides the first comprehensive, systematic classification of synoptic-scale circulation regimes modulating the spatial distribution and frequency of summer hail in Northern Italy.
• Offers a process-oriented, synoptic-climatological framework that complements existing ingredient-based and event-based analyses of severe convection in the Mediterranean region.
• Highlights the role of Atlantic cut-off lows in driving Mediterranean extremes, specifically for severe convective storms in the Alpine region and north-western Italy.
• Establishes conceptual models for forecasters and a background climatological framework for anticipating hail risk under different synoptic scenarios, paving the way for regionally tailored predictive tools.
• Identifies a recent shift in circulation types favoring Alpine hail, contributing to understanding the evolving hail risk in the context of climate variability.

Funding

• Project “Hail Hazard in the Mediterranean” (H2Med)
• Progetti di Rilevante Interesse Nazionale (PRIN)
• Italian Government Ministero dell'Università e della Ricerca (MUR) - National Recovery and Resilience Plan (PNRR)

Citation

@article{Capozzi2026Identification,
  author = {Capozzi, Vincenzo and Fucci, Alberto and Budillon, Giorgio and Fusco, Giannetta and Arnone, Enrico and Cortesi, Nicola and Monte, Giulio and Laviola, Sante},
  title = {Identification of atmospheric circulation schemes that promote summer hail events in the Northern Italy},
  journal = {Atmospheric Research},
  year = {2026},
  doi = {10.1016/j.atmosres.2026.108990},
  url = {https://doi.org/10.1016/j.atmosres.2026.108990}
}