Numerical simulations of two giant hail events in northeastern Italy with WRF-HAILCAST

The northeastern Italian plains are particularly prone to severe weather, due to the combined influence of moisture from the nearby Adriatic Sea and the orographic features of this region. These factors contribute to significant hailstorms with large to giant hailstones. This study investigates two of such events that occurred in this region.

In the early morning of 1 August 2021 a supercell developed, locally producing hailstones with diameters up to 9 cm. The second event, on 24 July 2023, involved two supercells, one of which produced a hailstone with the European record-breaking diameter of 19 cm. Notably, both events resulted in the largest hailstones being observed in the same location—the village of Azzano Decimo.

The main focus of this work is the best-possible simulation of these two events by means of the Weather Research and Forecasting (WRF) model at 1 km resolution, coupled with the HAILCAST hail growth parameterization, which provides estimates of the maximum hail size at the ground. The 1 August 2021 event is first examined through several simulations performed using different setups. The results highlight a significant sensitivity to the forcing meteorological model and the initialization time. In particular, WRF gives better results with ECMWF-IFS initial and boundary conditions with respect to GFS, especially when simulations are initialized more than 24 hours before the event. Moreover, results are significantly affected by the microphysical scheme and the land surface model, whereas the planetary boundary layer parameterization seems to have a minor influence. However, the development of the supercell is properly simulated, with hailstone diameters comparable to observations, only when data from radiosoundings of Udine Rivolto are nudged into the model.

To validate the model setup, the 24 July 2023 event was also simulated, incorporating all available radiosonde data. The model was able to reasonably simulate two supercells with hailstone diameters up to 10 cm. This study shows the significant impact that radiosonde data nudging can have on convective simulations, recommends a WRF setup that produces reliable numerical predictions, and demonstrates the effectiveness of HAILCAST in accurately simulating giant hail events.

The upcoming physical analysis, taking advantage of the best simulations and of the observations available for the two events (including polarimetric radar variables and ground-based weather stations), will investigate common patterns to identify the dynamic and thermodynamic conditions that contribute to large hail production in this peculiar region of the northeastern Italian plains.