PNNL

Abstract

Aerosol-cloud interactions (aci) are the leading source of uncertainty in inferring climate sensitivity from the historical record. Earth system models (ESMs) struggle to represent aci because the processes responsible for this phenomena occur at a much finer time and space scale than can be resolved by any ESM for the foreseeable future. Observational constraints provide a ground truth, but leave us to infer aci processes except in very specific cases where causality can be controlled for. Ultimately, some degree of modelling is required to infer aci and provide an estimate of radiative forcing from observations. Here, we generate and characterize a perturbed parameter ensemble (PPE) hosted in version 3 of the Energy Exascale ESM (E3SMv3) focused on perturbing parameterizations governing aci processes. We perturb 25 parameters over 236 members and integrate the model over present-day and preindustrial emissions. We find that the process representation in E3SMv3 is flexible and is able to generate effective radiative forcings due to aci (ERFaci) ranging from -2.6 to 0.7 W/m2. While global-mean cloud droplet number always increases in response to anthropogenic aerosol, cloud liquid water path can both increase and decrease suggesting that, while not explicitly parameterized, size-dependent entrainment processes are reproduced on the global scale by E3SMv3. Overall, the E3SMv3 process representation is a flexible platform that provides both a robust baseline ESM and the ability to interrogate and incorporate observations into future versions of E3SM.