Recommendations for Treating Key Aerosol and Cloud Microphysics and Chemistry in Earth System Models

Improving aerosol and cloud descriptions in ESMs can increase the confidence in estimates of climate impacts of changing anthropogenic aerosol emissions. In the FORCeS project funded by the Horizon 2020 framework programme, 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 aerosols, we focused on organic aerosol, particulate nitrate, absorbing aerosols, and ultrafine aerosol sources including new particle formation and growth. In terms of cloud microphysics, we targeted cloud droplet activation, hydrometeor growth and evaporation, ice formation and multiplication as well as aerosol processing and scavenging by clouds. The selection was made combining identified knowledge gaps in scientific understanding of these processes and/or their current representation in ESMs with a perturbed parameter ensemble approach to detecting potential structural deficiencies in an ESM. In this presentation, I will summarize a recently published overview article (Riipinen et al., Tellus B, 2026) where we provide recommendations applicable in models operating at the Earth system scale. Overall, the findings highlight the need for continuous efforts towards smart ways for representing the aerosol number size distribution as well as consistent representations of key parameters (e.g. liquid water content and cloud droplet number concentration). Furthermore, we provide guidance for future ESM evaluation emphasizing, in particular, the need for exploring the consistency of key parameters, process-based (as opposed to parameter-based), and the complementarity of in-situ and remote-sensed measurements for model evaluation.