A modeling case study of a severe hail storm in complex topography

On the 28^th of June 2021, a supercell traversed the complex topography of Switzerland. While propagating over a distance of 350 km and lasting 8 hours, the cell produced severe hail with diameters up to 9 cm. A case study is performed, aimed at understanding the development of the supercell environment, and the physical processes associated with the cell’s initiation, intensification, propagation, and dissipation. To this end, ensemble hindcast simulations are conducted with a convection-resolving numerical weather prediction model (COSMO-1E) coupled to a one-dimensional hail growth parameterization (HAILCAST). The simulation setup is identical to the operational ensemble forecasts performed at MeteoSwiss. Object-based analysis of the simulated hail cell is facilitated by a tracking algorithm, developed specifically to address the many challenges associated with tracking hail storms in output from numerical simulations. The hail cell tracks enable a storm-relative analysis of the storm environment and the temporal evolution of the storm structure during its phases of development. The inflow of air feeding the storm updraft is investigated with the aid of Lagrangian air parcel trajectories initiated along the hail track, giving novel insights into the low-level flow and moisture sources of the storm. All ensemble members produce thunderstorms comparable to the observed storm in terms of track position and intensity. However, the observations and the simulations differ with respect to initiation location and the lifetime of the cell. The analysis focuses on the intensification and dissipation stages of the storm, and especially how and why the cell tracks in the ensemble members differ from each other.