| 16 Jan 2026
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.
Review of: Volcanic aerosol effects on warm and cold cloud microphysics: ICON-ART simulations of Holuhraun and La Soufrière eruptions
This manuscript investigates the cloud response to volcanic aerosol in the ICON-ART model. The investigation includes simulations of the initial stages of two volcanic eruptions, the 2014-2015 Holuhraun eruption and the 2021 La Soufrière eruption. The two eruptions are different in nature as Holuhraun emitted almost exclusively SO2 and the aerosol was injected mainly into the lower trophosome, while the La Soufrière eruption consisted of both SO2 and ash particles and reached the upper troposphere and stratosphere. The simulations of the Holuhraun eruptions are mainly used to investigate changes in liquid cloud and rain properties in response to the increased number of CCN from the eruption. The results indicate a strong increase in the cloud droplet number concentration and a decrease in warm rain processes in the clouds impacted by the volcanic aerosol. In the simulation of the La Soufrière eruption the changes in ice cloud properties in response to both the ash and the sulphate aerosol is investigated. The model simulations show that the ash particles can act at INPs and reduce the ice crystal number concentrations by supressing homogeneous nucleation.
The manuscripts address relevant scientific questions that are in the scope of ACP. The investigation, in particular the parts investigating ice cloud responses to volcanic aerosol in ICON-ART are novel. The scientific methods are valid and mostly described in sufficient detail. The findings of the investigation are interesting there is good interpretation and analysis of the results. The figures are clear, and the structure and language of the manuscript is of good quality. I recommend the manuscript publications after the comments below has been addressed.
Major comments:
The description of the model simulations is generally good but some descriptions of the volcanic emissions are vague and not explained properly. On page 4 line 124 it says “ash particles are given predefined small values”. And again on line 125 it says “source strength of both ash and SO2 is zero, and small predefined background values are considered”. Small and background is also mentioned a few times in the results section. The term “small” is not very quantitative. I would like the authors to explain these predefined and background values in more detail. What levels are these values at, and why were these not set to zero?
From the General Setup section I understand that there is only sea salt aerosol, sulphate and ash considered in the model. This would make the conditions in the model very clean if no anthropogenic or land-based aerosol sources are included. Is there a particular reason to inly include sea-salt aerosol? Why was this compound chosen? There is some mention in the paper that only including this aerosol does impact the results of the study but I think perhaps this could be a bit extended. That no dust is included should means that there are no other INPs available. Nevertheless, on page 12, line 274 you write that “along with the heterogeneous freezing driven by background INPs”. What are these background INPs?
Minor comments:
Page 4, line 115: It would be good to mention the altitudes that the volcanic plume reached in the case description, in particular for the La Soufrière eruption but also for the Holuhraun eruption.
Page 4, line 116: Please add the length of the La Soufrière simulations here in the case descriptions.
Page 5, line 123: What does “Second” refer to here?
Page 6, line 150: Since Figure 7 is the second figure to be referenced in the manuscript, should it be called Figure 2?
Page 9, line 230: “we” should be changed to “We”. How far from the domain boundaries did you exclude data?
Page 11, figure caption for figure 3: It seems a bit strange to explain the sub figures in the order a, b, d, e, c, f. Please consider changing this.
Page 15, line 339: I agree that the ice distributions are rather symmetric but for the snow data (Fig 8b and d) the mean values and median values are rather far away from each other, in particular in fig 8d.
Page 16, figure caption for figure 8: Should there be a space after the comma in “(a,b)”? This is missing in several figure captions.