Elucidating ice formation pathways in the aerosol-climate model ECHAM6-HAM2

Abstract. Cloud microphysics schemes in global climate models have long suffered from a lack of reliable satellite observations of cloud ice. At the same time there is a broad consensus that the correct simulation of cloud phase is imperative for a reliable assessment of Earth's climate sensitivity. Combining new satellite products (from CloudSat and CALIPSO) and physically-based ice microphysics parameterizations allows for rapid progress in reducing the inter-model spread in predicting the cloud phase partitioning at sub-zero temperatures. This work introduces a new method to build a sound cause-and-effect relation between the microphysical parameterizations employed in our model and the resulting cloud field through a quantitative cloud formation pathway analysis. We find that heterogeneous freezing in super-cooled liquid clouds only dominates ice formation in roughly 7 % of the simulated cloud volume, a small fraction compared to almost 65 % of the cloud volume governed by homogeneous freezing below −35 °C. Compared to the CALIPSO-GOCCP satellite product, our model overestimates the relative frequency of occurrence of cloud ice in the mixed-phase temperature regime. The ice formation pathway analysis reveals that this is caused by too much cloud ice propagating from the cirrus into the mixed-phase cloud regime, an unexpected result. This suggests that further efforts to improve the cloud phase partitioning must target cloud overlap assumptions for sedimentation and the related below cloud sublimation.