Simulations of selective seeding of hailstorms over Switzerland: impacts for precipitation and lightning

Hailstorms can cause a lot of damage for 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 July 6, 2019. We evaluated the effectiveness of an early seeding strategy and investigated the concept of beneficial competition, where increased number of INPs lead to the formation of smaller, less damaging hailstones. We used the Consortium for Small-Scale Modeling Regional Weather and Climate Model (COSMO) 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/cm3 to 2000/cm3 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.

The lightning potential index (LPI) depends both on simultaneous occurrence of liquid water and ice species, as well as on the updraft strength. LPI increased in all seeding simulations in terms of intensity and spatial extent, because seeding increased the updraft strength and the graupel weighted ice mixing ratio.