How well are aerosol–cloud interactions represented in climate models? – Part 2: Isolating the aerosol impact on clouds following the 2014–15 Holuhraun eruption

Abstract. Aerosols significantly influence Earth’s radiative balance, yet considerable uncertainty exists in the underpinning mechanisms, particularly those involving clouds. These aerosol-cloud interactions (ACIs) are the most uncertain element in anthropogenic radiative forcing, hampering our ability to constrain Earth’s climate sensitivity and understand future climate change. The 2014–2015 Holuhraun volcanic eruption in Iceland released sulphur dioxide (SO₂) into the lower troposphere on a level comparable to continental-scale emissions. The resultant volcanic plume across a near-pristine North Atlantic Ocean presents an ideal opportunistic experiment to explore the representation of ACIs within general circulation models (GCMs). We present Part 2 of a two-part inter-model comparison study that utilises satellite remote sensing observations to assess modelled cloud responses to the volcanic aerosol within 8 state-of-the-art GCMs during September and October 2014. We isolate the aerosol effect from meteorological variability and find that the GCMs adeptly capture the observed cloud microphysical changes associated with the ACI first indirect effect (i.e., Twomey effect). Meanwhile, a clear divergence exists in the GCM responses of large-scale cloud properties, namely cloud liquid water content, that are expected from the precipitation suppression mechanism of the ACI second indirect effect (i.e., rapid adjustments). We propose that this is due to limitations and differences in the autoconversion schemes under high aerosol loading. Despite the individual GCM differences, the collective large-scale responses of the multi-model ensemble agree well with observations. Finally, our multi-model ensemble estimates that Holuhraun had a global radiative forcing of -0.018 ± 0.007 Wm⁻² across September and October 2014.