What Is Triggering Ice in Mixed-Phase Clouds: A Process Analysis With ECHAM6.1-HAM2.3 Using the Factorial Method

Journal article, 2026

Mixed-phase clouds can be found at temperatures between 0 and -38C and consist of supercooled cloud droplets and ice crystals. The phase of mixed-phase clouds is crucial for the radiation budget, which determines the atmosphere's energy balance. The transition of a supercooled cloud to a mixed-phase or ice cloud is triggered by different processes that form or introduce ice crystals into a supercooled cloud. Once ice crystals are present, they grow at the expense of the cloud droplets due to the Wegener-Bergeron-Findeisen process. This causes a partial or complete glaciation of the mixed-phase cloud. In the global climate model ECHAM6.1-HAM2.3, three trigger processes introduce initial ice crystals into a supercooled stratiform cloud: heterogeneous freezing, sedimentation of ice crystals from upper cloud layers (in-cloud seeding or seeder-feeder process), and vertical transport (vertical diffusion and vertical advection) of ice crystals. The role of these three ice-triggering processes was investigated by conducting a set of simulations and analyzing them using a statistical framework (factorial method). For that framework, the supercooled liquid fraction of a mixed-phase cloud was used as an indicator of the microphysical structure and phase of the cloud. It was found that the sedimentation of the ice crystals is crucial for triggering the ice in mixed-phase stratiform clouds in ECHAM6.1-HAM2.3. Our results affect the model's sensitivity to freezing parameterizations and variations in aerosol-cloud interactions.