An object-based and Lagrangian view on an intense hailstorm day in Switzerland as represented in COSMO-1E ensemble hindcast simulations

Abstract. On 28 June 2021, Switzerland experienced the passage of several formidable hailstorms, navigating its complex terrain. They unleashed severe hailstones measuring up to 9 cm in diameter. We present a comprehensive case study to unravel the complex processes involved in the genesis, intensification, and dissipation of this impactful weather event. To this end, we investigate ensemble hindcast simulations using the COSMO-1E numerical weather prediction model that includes the HAILCAST hail growth parameterization. A tracking algorithm is introduced that facilitates the object-based analysis of the simulated hailstorms, addressing the inherent challenges of tracking hailstorms within numerical simulation outputs. By scrutinizing the storm's evolution across various phases, particularly during intensification, the study conducts a storm-relative analysis of 100 hailstorms simulated on this day by the 11-member ensemble with lifetimes of >2.5 h. Furthermore, the investigation utilizes Lagrangian air parcel trajectories initiated along the hail track to analyze the inflow of air sustaining the storm updraft. This exploration provides fresh insights into the low-level flow patterns and moisture sources contributing to the storm's vigor, and it reveals the importance of topography for the various stages of the storms. The most important findings from this detailed hailstorm investigation are (i) COSMO-1E with HAILCAST produces realistic storm tracks and lifespans that are in good agreement with radar observations, (ii) intricate storm structures are resolved in the simulations and reveal hail fall followed by precipitation, and (iii) Lagrangian trajectories show that inflowing air can be drawn across the main Alpine crest and experiences rain before reaching saturation conditions in the storm.