Multi-sensor analysis of thunderstorm microphysics and electrical response to hailfall in the metropolitan area of Bologna

This work presents a multi-sensor diagnostic analysis characterizing the microphysical structure and electrical response of intense convective storms to hailfall in the Bologna metropolitan area. The study investigates whether solid hydrometeors act as physical vectors for downward charge transport, leading to the electrical discharge of convective cores. The observational framework combines ground-based laser disdrometer measurements, 3-D volumetric polarimetric Doppler radar data, and a real-time lightning tracking network. The analysis evaluates 17 hailfall events extracted from a disdrometer archive spanning December 2018 to August 2025. Hydrometeor spectra disaggregation reveals solid particle contamination within the liquid phase, highlighting the limitations of the disdrometer's classification algorithm in mixed-phase conditions where melting occurs. The analysis of the lightning dataset reveals a major anomaly: contrary to the Mediterranean climatology, cloud-to-ground discharges strongly dominate over intra-cloud pulses in these selected events. Temporal evolution of the most intense cases shows that peak electrical activity precedes ground hailfall. This activity progressively declines during the convective core grounding, leading to a near-complete electrical collapse once hailfall ceases, confirming that solid hydrometeors drive the thundercloud's discharge. The integration of radar data overcomes the point-source limitations of ground instruments, offering an expanded view of the systems' spatial evolution and internal dynamics. Cross-referencing 3-D volumetric radar data with lightning and disdrometer observations explains the anomalous signatures observed in the temporal analysis. Ultimately, this integrated framework demonstrates the necessity of volumetric multi-parameter coupling to fully resolve the lifecycle, charge geometries, and structural transitions of localized thunderstorms.