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

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Title: What Is Triggering Ice in Mixed‐Phase Clouds: A Process Analysis With ECHAM6.1‐HAM2.3 Using the Factorial Method.
Authors: Ickes, L.1,2 (AUTHOR) luisa.ickes@chalmers.se, Neubauer, D.2,3 (AUTHOR), Proske, U.2,4 (AUTHOR), Villanueva, D.2 (AUTHOR), Lohmann, U.2 (AUTHOR)
Source: Journal of Geophysical Research. Atmospheres. 6/16/2026, Vol. 131 Issue 11, p1-19. 19p.
Subject Terms: *Vertical mixing (Earth sciences), *Meteorological precipitation, Ice formation & growth, Atmospheric models, Cloud physics, Ice nuclei, Stratus clouds
Abstract: Mixed‐phase clouds can be found at temperatures between 0 and −38° ${}^{\circ}$C 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. Plain Language Summary: Clouds are made of tiny water droplets or ice crystals, and some clouds—called mixed‐phase clouds—have both. Clouds are important because they affect Earth's climate by influencing how much sunlight is reflected or trapped. But scientists still struggle to understand which mechanisms in these clouds turn water into ice under different meteorological and dynamical conditions. This study used a climate model to test three ways ice might form in this model and the importance of each process: (a) Freezing aided by particles, (b) Falling ice crystals from higher clouds or higher cloud layers (called sedimentation), and (c) Air movements that carry ice crystals up and down (vertical transport). It was shown that sedimentation and vertical transport are important to distribute ice crystals throughout clouds, especially in warmer parts of the mixed‐phase range (above −25° ${}^{\circ}$C). Freezing only becomes important at colder temperatures (below −30° ${}^{\circ}$C). In thicker clouds, ice moves around more, making sedimentation and vertical transport even more important. It is suggested to investigate the matter with other models and complement with observations to derive more conclusions for further model development. Key Points: Sedimentation of ice crystals triggers most of the water‐to‐ice phase transition in warm mixed‐phase clouds (>−25° ${}^{\circ}$C)Freezing is the least important ice‐triggering process in warm mixed‐phase clouds (>−25° ${}^{\circ}$C) with increasing importance for colder cloudsThe factorial analysis is a suitable framework to analyze the effect of ice‐triggering processes and facilitates model intercomparison [ABSTRACT FROM AUTHOR]
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Abstract:Mixed‐phase clouds can be found at temperatures between 0 and −38° ${}^{\circ}$C 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. Plain Language Summary: Clouds are made of tiny water droplets or ice crystals, and some clouds—called mixed‐phase clouds—have both. Clouds are important because they affect Earth's climate by influencing how much sunlight is reflected or trapped. But scientists still struggle to understand which mechanisms in these clouds turn water into ice under different meteorological and dynamical conditions. This study used a climate model to test three ways ice might form in this model and the importance of each process: (a) Freezing aided by particles, (b) Falling ice crystals from higher clouds or higher cloud layers (called sedimentation), and (c) Air movements that carry ice crystals up and down (vertical transport). It was shown that sedimentation and vertical transport are important to distribute ice crystals throughout clouds, especially in warmer parts of the mixed‐phase range (above −25° ${}^{\circ}$C). Freezing only becomes important at colder temperatures (below −30° ${}^{\circ}$C). In thicker clouds, ice moves around more, making sedimentation and vertical transport even more important. It is suggested to investigate the matter with other models and complement with observations to derive more conclusions for further model development. Key Points: Sedimentation of ice crystals triggers most of the water‐to‐ice phase transition in warm mixed‐phase clouds (>−25° ${}^{\circ}$C)Freezing is the least important ice‐triggering process in warm mixed‐phase clouds (>−25° ${}^{\circ}$C) with increasing importance for colder cloudsThe factorial analysis is a suitable framework to analyze the effect of ice‐triggering processes and facilitates model intercomparison [ABSTRACT FROM AUTHOR]
ISSN:2169897X
DOI:10.1029/2025JD045160