Impact of cloud seeding on simulated hailstorms and its dependence on CAPE, wind shear, and tracking thresholds

Hailstorms are a damaging weather phenomenon worldwide. In response, several countries – including Switzerland – have implemented hail mitigation strategies, most notably through cloud seeding with ice-nucleating particles (INPs). In this study, we investigate the impact of silver iodide (AgI) perturbations on eight convective storms observed in Switzerland and southern Germany. Our focus is on evaluating the effectiveness of an early seeding strategy and examining its relationship with two key meteorological parameters: Convective Available Potential Energy (CAPE) and 0–6 km wind shear. We also assess how different storm-tracking thresholds influence the interpretation of seeding effects. Simulations were conducted using the Consortium for Small-Scale Modeling Regional Weather and Climate Model (COSMO). AgI particles were introduced as a prognostic variable during the cumulus stage and released into the updraft region near the cloud base at a concentration of 20 cm−3. The results indicate that early seeding increases both the mass and number concentration of ice and graupel, accompanied by stronger updrafts. In contrast, the response of hail mass is ambiguous and varies with the tracking method. Hail size and hail-covered area also show no systematic dependence on CAPE or wind shear. Despite the variability in the hail response, our results show that early seeding increases the mean hail diameter in 80 % of the cases, with a median increase of 7.6 % – corresponding to a 31.3 % increase in kinetic energy – while simultaneously reducing the spatial extent of the hail-affected area by 39.8 % (median), with 92.4 % of simulations exhibiting a decrease in hail area.