Volcanic aerosol effects on warm and cold cloud microphysics: ICON-ART simulations of Holuhraun and La Soufrière eruptions

Abstract. Volcanic eruptions are a rich source of particles, such as sulfate and ash, that can act as cloud condensation nuclei (CCN) or ice nucleating particles (INPs). They offer a unique opportunity to study the effects of aerosols on clouds because they create a natural laboratory with high concentrations of aerosols adjacent to an unperturbed environment. In this study, we used the ICON–ART (ICOsahedral Nonhydrostatic weather and climate model with Aerosols and Reactive Trace gases) modeling system to simulate the initial phases of the 2014–2015 Holuhraun and the 2021 La Soufrière eruptions. Our goal was to improve our understanding of how cloud microphysical processes respond to volcanic aerosols. For the Holuhraun eruption, which was non-explosive and rich in SO₂, we found that the number concentration of cloud droplets increased significantly due to the presence of volcanic aerosols. Warm rain processes, such as autoconversion and accretion, and consequently rain decreased. Additionally, we found that volcanic aerosols weakened the riming process and reduced the graupel concentration. The La Soufrière eruption provides an opportunity to investigate the competition between homogeneous and heterogeneous ice nucleation because it emitted both ash and SO₂. Our results show that ash particles acting as INPs deplete water vapor. This reduces the total number of ice crystals by hindering homogeneous ice nucleation.