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

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author Dietlicher Remo, Neubauer David, Lohmann Ulrike,
Project A new parameterization scheme for ice and snow in climate models
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Original article (peer-reviewed)

Journal Atmospheric Chemistry and Physics Discussions
Page(s) 1 - 32
Title of proceedings Atmospheric Chemistry and Physics Discussions
DOI 10.5194/acp-2018-573

Open Access

URL http://doi.org/10.5194/acp-19-9061-2019
Type of Open Access Publisher (Gold Open Access)

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 im- perative for a reliable assessment of Earth’s climate sensitivity. At the core of this problem is understanding the causes for the inter-model spread of the predicted cloud phase partitioning. 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 by analysing ice formation pathways. We find that freezing processes in supercooled liquid clouds only dominate ice formation in roughly 6 % of the simulated clouds, a small fraction compared to roughly 63 % of the clouds governed by freezing in the cirrus temperature regime below − 35 ◦C. This pathway analysis further reveals that even in the mixed- phase temperature regime between −35 and 0 ◦C, the dominant source of ice is the sedimentation of ice crystals that originated in the cirrus regime. The simulated fraction of ice cloud to total cloud amount in our model is lower than that reported by the CALIPSO-GOCCP satellite product. This is most likely caused by structural differences of the cloud and aerosol fields in our model rather than the microphysical parametrizations employed.
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