Investigation of cloud response to the 2014 Holuhraun and the 2021 La Soufrière eruptions using the ICON-ART model

Aerosols act as cloud condensation nuclei (CCN) and ice nuclei (IN) within cloud droplets, therefore they influence the microphysical features of clouds. Although many numerical and observational studies have investigated aerosol-cloud interaction, the extent and quality of aerosol impact on cloud formation and precipitation processes are not clear yet. Volcanic eruptions, which are rich sources of various chemical compounds in the atmosphere, can help to improve the understanding of aerosol effects on clouds by providing natural laboratories with locally high aerosol conditions adjacent to an unperturbed environment.

In the present study, we selected two volcanoes that emitted different aerosols and trace gases into the atmosphere to investigate their impact on the cloud microphysical processes: the 2014 Holuhraun and the 2021 La Soufrière eruption. The first one is an Icelandic volcano that mostly emitted sulfur dioxide (SO2), which forms sulfate particles serving as CCN. The second one is located on the Caribbean island of Saint Vincent and is an ash-rich eruption, so it is an appropriate case to study heterogeneous ice nucleation since ash particles serve as IN.

We simulated the initial phases of these eruptions using the ICOsahedral Nonhydrostatic model with Aerosols and Reactive Trace gases (ICON-ART) and performed different sensitivity experiments. For each case, we conducted two different simulations, in one of which the volcanic emission is considered, while in the other one, it is not (termed ‘Plume’ and ‘No-plume’).  For more detailed analysis, we divided the simulated area into two areas inside and outside of the plume using these criteria, column abundance of SO2>1 DU for Holuhraun and mass concentration of ash>10^-4 gm^-3 for La Soufrière.

For the Holuhraun case, the results showed a pronounced effect of volcanic aerosols on the different hydrometeors and process rates. Furthermore, the range and distribution of the liquid water path (LWP) have a good agreement with MODIS-Aqua satellite retrievals. In the Plume simulation, an increase in the number of cloud droplets but with smaller sizes was observed while we saw a reduction of graupel mass concentration in this simulation. These results confirmed our expectations, since in the Plume case the aerosol number concentration increases, resulting in an enhancement of the cloud droplet number concentration but with a smaller droplet size. In addition, the riming rate which highly depends on the cloud droplet sizes was reduced in the Plume simulation and led to the reduction of graupel mass that is mostly created by riming.

For the La Soufrière case, we mostly concentrated on heterogeneous ice nucleation as this case was an ash-rich eruption. Our preliminary results showed that the enhancement of ash particles reduced the ice crystal number concentration although the number of heterogeneously nucleated particles increased. To explain this behavior, we refer to the fact that additional heterogeneous freezing suppresses homogeneous freezing so that the total number of ice crystals is reduced. 

Keywords: Aerosol, Cloud, ICON-ART Model, volcanic aerosols, aerosol-cloud interactions, Holuhraun eruption, La Soufrière eruption