Moisture Sources and Vorticity Structure of Hailstorms over Switzerland in a Kilometer-Scale Convection-Permitting Model

Abstract

Hail is the costliest phenomenon brought by convective storms in the mid-latitudes. A severe hail event on 28 June 2021 caused 400 million CHF worth of damages to buildings in Switzerland. Hailstorms have a small spatial scale, short lead time, and transient lifetime. Forecasting hailstorms becomes a challenge and timely preventive measures are often not implemented. To understand the development of the hailstorm environment, a case study is conducted using the ensemble hindcast from the COSMO-1E weather forecast prediction model with a convective cell tracking algorithm and a Lagrangian trajectory analysis. Based on the 100 storms tracked, which have a lifespan >2.5 hours and updraft >25 m/s at 400 hPa, the moisture source of the storms is diagnosed, and the role of vorticity in the vicinity of the storms during splitting is examined. The investigation suggests that the moisture uptake along the five-hour backward trajectories can contribute to a mean of 31.18% to the final moisture contents of the storms at their most intense state. The moisture uptake from the boundary layer is the dominating source for hailstorms within these five hours. Storm-centered composites reveal a dipole in vertical vorticity and potential vorticity (PV). Our study confirms that the environmental wind shear determines the orientation of the dipole. The evolution of the storm splitting is studied in detail. We attempt to explore its evolution from the classical meteorological perspective and the idealized vortex dynamics perspective. The results align with the theory that dynamic pressure perturbation is the vital driver in splitting from the meteorological perspective. From the idealized vortex dynamics perspective, the mean aspect ratio of the cyclonic updraft core at 400 hPa reaches beyond 8 at the splitting time. This is believed to be relevant to the instability of Love mode along the periphery of the quasi-two-dimensional vortex patch, eventually leading to the storm splitting. In summary, the convection-permitting high-resolution model has proved to be capable of capturing the internal structure of the storms and it provides valuable insights into the development of hailstorms at the mesoscale.