In-plume and out-of-plume analysis of aerosol-cloud interactions derived from the 2014–15 Holuhraun volcanic eruption

Abstract. Aerosol effective radiative forcing (ERF) has persisted as the most uncertain aspect of anthropogenic forcing over the industrial period, limiting our ability to constrain estimates of climate sensitivity and to confidently predict 21st century climate change. Aerosol-cloud interactions are the most uncertain component of aerosol ERF. The 2014–15 Holuhraun volcanic eruption acted as large source of sulphur dioxide, providing an opportunistic experiment for studying aerosol-cloud interactions at a climatically relevant scale. We evaluate the observed aerosol-induced perturbation to cloud properties inside the volcanic plume in the first month of the eruption and compare the results to those from UKESM1 (UK Earth System Model). In the first two weeks, as expected, we find an in-plume shift to smaller and more numerous cloud droplets in both the observations and the simulations, as well as an observed change in the distribution of liquid water path (LWP) values inside the plume. However, in the third week, the in-plume shift to smaller and more numerous cloud droplets is neither observed nor modelled, and there are discrepancies between the observed and modelled response in the fourth week. Analysis of the model simulations and trajectory modelling reveals that airmass history and background meteorological factors can strongly influence aerosol-cloud interactions between the weeks of our analysis. Overall, our study supports the findings of many previous studies; that the aerosol impact on cloud effective radius is significant, with a less significant effect on in-cloud LWP.