Treatment of Key Aerosol and Cloud Processes in Earth System Models – Recommendations from the FORCeS Project

Uncertainty in estimations of the net contribution of anthropogenic aerosol particles and particularly aerosol-cloud -interactions to the Earth’s radiation budget limits our ability to understand past climate change and project the future. Earth System Models (ESMs) are among the key tools for assessing the magnitude and impacts of changes in various forcing agents on the global climate system. Hence, improving aerosol and cloud descriptions in ESMs is an important way forward to increase the confidence in estimates of climate impacts of aerosol perturbations in the past, present and future. In the framework of the FORCeS project, we combined experimental and theoretical approaches to bridge the current key gaps in the fundamental understanding of essential aerosol and cloud processes and their descriptions in selected European ESMs. Regarding aerosol types and processes, we focused on organic aerosol, particulate nitrate, absorbing aerosols, and ultrafine aerosol sources including new particle formation and growth. In terms of cloud processes, we targeted cloud droplet activation, hydrometeor growth and evaporation, ice formation as well as aerosol processing and scavenging by clouds. The selection of the processes was conducted based on the indentified knowledge gaps in terms of the scientific understanding. Here, we review the state-of-the-art, outline our approach for arriving at recommendations for improving the representation of key aerosol and cloud processes within ESMs, and finally provide such recommendations applicable in models operating at the Earth system scale. The limitations of the recommendations, as well as alternative approaches and future research directions are also discussed.