Hidden Large Aerosol‐Driven Cloud Cover Effect Over High‐Latitude Ocean

Abstract Understanding how aerosols affect cloud cover is critical for reducing the large uncertainty of the aerosol‐cloud interaction (ACI). The 2014 Holuhraun effusive eruption in Iceland resulted in a significant increase in cloud drop number concentration ( N d ) relative to the climatological N d observed during periods of relatively infrequent volcanic activity. Previous studies show a significant “Twomey” effect during this eruption; however, aerosol‐induced changes in cloud fraction (Cf) appeared negligible. This leads to the question of why changes in aerosols do not cause Cf changes. To address this question, prediction models were derived to predict Cf based on N d and meteorological parameters. These validated models allow us to investigate aerosol perturbations on Cf in various N d scenarios by controlled meteorological conditions. Here our analysis unveiled that the increase in N d was primarily observed under polluted conditions where N d surpassing the threshold of 60 cm −3 . After this point, cloud cover stops increasing even as N d increases. On the contrary, the cloud cover did increase by 9.0% under conditions of clean backgrounds ( N d < 60 cm −3 ). Accordingly, the aerosol‐driven cloud adjustment is hidden behind the seemingly insignificant cloud cover effect in areas with large background N d . These findings provide insights into the importance of considering background N d and the saturation status of cloud covers in ACI studies.