Analysis of an intense hail event in the Po Valley using observation and WRF model

An analysis of a large hailstorm that occurred in the Po Valley, Northern Italy, on July 26, 2021 is carried out to highlight the role of Cloud Condensation Nuclei (CCN) in the development of the storm and its evolution. To this aim radar reflectivity, satellite brightness temperature, accumulated rainfall and hail and model simulations are used. Particular attention is paid to the role of the CCNs in the supercell structure. Two simulations are carried out using the WRF model at high resolution in combination with the Hailcast module. A supercell developed in the early afternoon of July 26 along the foothills of the Apennine sustained by the convergence of warm and humid air from the southwest and southeast and the downslope wind from the Alps. The two model simulations highlighted the different impact of the CCNs on the supercell. The continental CCN allows for a well-structured supercell with a BWER and a tilted updraft, whereas the maritime CCN produces a very tight vertical structure and a stronger updraft than the one for continental CCN with a straight axis. This leads to rainfall that begins later, lasts longer, and spreads farther northeastward. The Continental CCN developed an early precipitation more concentrate in the area where the storm occurred. The two simulations highlight the different impact of the CCN environment on the supercell. The continental CCN configuration produces an earlier, more persistent and tilted updraft, with a clearer BWER structure and a stronger mature-phase surface-impact signal. The maritime CCN configuration instead develops a more compact and vertically oriented plume; the storm starts later and at a considerable distance from the observed one, with a later and more pulsed precipitation response that spreads farther northeastward