How well are aerosol–cloud interactions represented in climate models? – Part 2: Isolating the aerosol impact on clouds following the 2014–2015 Holuhraun eruption

Abstract. Aerosols significantly influence Earth's radiative balance, yet considerable uncertainty exists in the underpinning mechanisms, particularly those involving clouds. 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 sulfur dioxide (SO2) into the lower troposphere on a level comparable to continental-scale emissions. The resultant volcanic plume across an often near-pristine region of the northern North Atlantic Ocean presents an ideal opportunistic experiment to explore ACI representation within general circulation models (GCMs). We present Part 2 of a two-part AeroCom (Aerosol Comparisons between Observations and Models) Phase III inter-model comparison study that utilises satellite remote sensing observations to assess modelled cloud responses to the Holuhraun attributed volcanic aerosol within eight state-of-the-art GCMs during September and October 2014. We isolate the aerosol effect from meteorological variability and find that the GCMs – particularly their multi-model ensemble response – 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, expected from the precipitation suppression mechanism of the ACI second indirect effect (i.e. rapid adjustments). We attribute this to limitations and differences in their autoconversion schemes under high aerosol loading, specifically in sub-grid-variability representations. Finally, our multi-model ensemble estimates that Holuhraun had a global radiative forcing of −0.11 ± 0.04 Wm−2 across September and October 2014.