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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/acp-2018-573
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-573
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 25 Jun 2018

Research article | 25 Jun 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

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

Remo Dietlicher, David Neubauer, and Ulrike Lohmann Remo Dietlicher et al.
  • Institute for Atmospheric and Climate Science, ETH Zürich, Universitätstrasse 16, 8092 Zürich, Switzerland

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.

Remo Dietlicher et al.
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Remo Dietlicher et al.
Remo Dietlicher et al.
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Latest update: 15 Nov 2018
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Short summary
Ice crystals in clouds cover a spectrum of shapes and sizes. We show the first results of a consistent representation of the cloud ice spectrum in the climate model ECHAM6-HAM2. The simulated cloud fields are linked to their sources by new diagnostics. We find that only a small fraction of ice clouds is initiated by freezing of cloud droplets in the mixed-phase temperature regime while most ice forms at temperatures colder than −35 °C.
Ice crystals in clouds cover a spectrum of shapes and sizes. We show the first results of a...
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