Impacts of the Icelandic Holuhraun volcanic eruption on cloud properties using regional model cloud-aerosol simulations

Abstract. Aerosol-cloud interactions remain a significant uncertainty in climate prediction, largely due to the complexity of measuring and modelling these processes. Volcanic eruptions, such as the Holuhraun event in 2014, offer valuable opportunities to study these interactions by introducing substantial aerosol perturbations. In this study, we investigate the impacts of the Icelandic Holuhraun volcanic eruption on cloud properties using the CASIM cloud microphysics model and the UKCA-GLOMAP aerosol microphysics model within the high-resolution regional model of the UK Met Office Unified Model.

For a four-week simulation, our findings indicate a more than 80 % increase in droplet number concentration during the eruption with reductions in cloud droplet sizes, both of which are statistically significant at 0.05 level in t-tests. In contrast, the effects of the volcanic eruption on liquid water path and cloud fraction are not generally significant. During the third week of September, neither satellite observations nor model simulations show significant impacts of the volcanic plume on cloud properties when comparing in-plume to out-of-plume properties. Our simulations suggest that the volcanic aerosol effect during this period was masked by factors affecting the out-of-plume atmospheric conditions, such as natural meteorological variability or non-volcanic aerosols possibly originating from Europe. When volcano on/off simulations are examined, the droplet number increase and the reduction in droplet size remain evident, indicating that these effects are still active. This highlights the crucial role of realistic models in revealing aerosol-cloud interactions that can be obscured in observations due to environmental/meteorological factors.