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 model-based 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 that facilitates object-based analysis of the simulated hailstorms is introduced. We show that COSMO-1E with HAILCAST produces realistic storm tracks and lifespans that are in good agreement with radar observations. 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 air parcel trajectories initiated along the hail track to analyze the inflow of air sustaining the storm updraft. This exploration identifies specific low-level flow patterns that contribute to the storm's vigor. The most important findings from this detailed and combined Eulerian and Lagrangian hailstorm analyses are the following: (i) the hailstorms propagate toward local convective available potential energy (CAPE) maxima; (ii) hailfall is directly followed by intense precipitation; (iii) the inflow detaches from the lowest 800 m up to 30 min before storm dissipation; and (iv) Lagrangian analysis reveals a distinct time lag between rain, cloud condensate, and graupel concentrations along the inflowing trajectories.