Investigating the sign of stratocumulus adjustments to aerosols in the global storm-resolving model ICON

Abstract

Abstract. Since pre-industrial times, aerosol emissions have caused brightening of stratocumulus clouds, thereby cooling the climate. However, observational studies and climate models disagree on the magnitude of this cooling, in particular because of the liquid water path (LWP) response of stratocumulus clouds to increasing aerosols, with climate models predicting an increase in LWP, and satellites observing a weak decrease. With higher-resolution global climate models, there is hope to bridge this gap. In this study, we present simulations conducted with the ICOsahedral Non-hydrostatic climate model (ICON) used as a global storm-resolving model (GSRM) with 5 km horizontal resolution. We compare the model outputs with geostationary satellite data, and we observe that, while ICON produces realistic low-cloud cover in the stratocumulus regions, these clouds look cumuliform and the sign of LWP adjustments to aerosols disagrees with satellite data. We evaluate this disagreement with a causal approach, which combines time series with knowledge of cloud processes in the form of a causal graph, allowing us to diagnose the sources of discrepancies between satellite and model studies. We find that the positive LWP adjustment to increasing aerosols in ICON results from a superposition of processes, with an overestimated positive response due to precipitation suppression and cloud deepening under a weak inversion, despite small negative influences from cloud-top entrainment enhancement. Such analyses constitute a methodology that can guide modelers on how to modify model parameterizations and set-ups to reconcile conflicting studies concerning the sign and magnitude of LWP adjustments across different data sources.