Aerosol Background Concentrations Influence Aerosol-Cloud Interactions as Much as the Choice of Aerosol-Cloud Parameterization

We use an independent observational estimate of aerosol-cloud interactions (ACI) during the 2014 Holuhraun volcanic eruption in Iceland to evaluate four ACI parameterizations in a regional model. All parameterizations reproduce the observed pattern of liquid cloud droplet size reduction during the eruption, but strongly differ on its magnitude and on the resulting effective radiative forcing (ERF). Our results contradict earlier findings that this eruption could be used to constrain liquid water path (LWP) adjustments in models, except to exclude extremely high LWP adjustments of more than 20 g m-2 . The modeled ERF is very sensitive to the non-volcanic background aerosol concentration: doubling the non-volcanic aerosol background weakens the ACI ERF by 30%. Since aerosol biases in climate models can be an order of magnitude or more, these results suggest that aerosol background concentrations could be a major and under-examined source of uncertainty for modeling ACI.