Simulations of Selective Seeding of Hailstorms—A Summertime Case Study over Switzerland

Abstract Hailstorms can cause a lot of damage to agriculture and property. Therefore, efforts exist to mitigate hail damage by means of seeding a developing hailstorm with ice nucleating particles. Motivated by the Swiss hail mitigation campaign, we examined the impact of silver iodide (AgI) perturbations on a convective storm observed over northern Switzerland on 6 July 2019. We evaluated the effectiveness of an early seeding strategy and investigated the concept of beneficial competition, where an increased number of ice nucleating particles (INPs) leads to the formation of smaller, less damaging hailstones. We used the Consortium for Small-Scale Modeling (COSMO) Regional Weather and Climate Model to simulate this case. AgI particles were added as a prognostic variable to the hailstorm during its cumulus stage and were released in the updraft region near the cloud base with concentrations ranging from 0.2 to 2000 cm −3 in ensemble simulations. While seeding delayed the onset of precipitation, increased the graupel concentration, and reduced supercooled liquid water, especially in the upper part of the convective cloud, no systematic change in the overall hail size has been found. We did, however, observe fewer grid points with mean hail diameters larger than 30 mm in all seeding concentrations across all altitudes, corresponding to a decrease of the largest hail sizes (>30 mm) across all atmospheric levels by up to 11%.