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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-2019-401
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-401
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 10 May 2019

Submitted as: research article | 10 May 2019

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

Modelling the effect of condensed-phase diffusion on the homogeneous nucleation of ice in supercooled water

Kathryn Fowler, Paul Connolly, and David Topping Kathryn Fowler et al.
  • Centre for Atmospheric Science, The School of Earth and Environmental Sciences, The University of Manchester

Abstract. In-situ studies of low temperature cirrus clouds have found unexpectedly low ice crystal numbers and consistently high supersaturations, which suggest that our understanding of the freezing mechanisms under these conditions are incomplete. Computational models typically use homogeneous nucleation to predict the ice nucleated in supercooled water. However, the existence of ultra-viscous organic aerosol in the upper troposphere has offered alternative ice nucleation pathways, which have been observed in laboratory studies. The possible effects of aerosol viscosity on cloud micro-physical properties have traditionally been interpreted from simple model simulations of an individual aerosol particle based on equilibration timescales. In this study, to gain insight into the formation of ice in low temperature cirrus clouds, we have developed the first cloud parcel model with bin micro-physics to simulate condensed phase diffusion through each individual aerosol particle. Our findings demonstrate, for the first time, the complex relationship between the rate of ice formation and the viscosity of secondary organic aerosol, driven by two competing effects – which cannot be explained using existing modelling approaches. The first is inhibition of homogeneous ice nucleation below 200 K, due to restricted particle growth and low water volume. The second occurs at temperatures between 200 K and 220 K, where water molecules are slightly more mobile and a layer of water condenses on the outside of the particle, causing an increase in the number of frozen aerosol particles. Our new model provides a basis to better understand and simulate cirrus cloud formation on a larger scale, addressing a major source of uncertainty in climate modelling through the representation of cloud processes.

Kathryn Fowler et al.
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Status: final response (author comments only)
Status: final response (author comments only)
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Kathryn Fowler et al.
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Latest update: 16 Sep 2019
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Short summary
Observations of low temperature cirrus clouds have found unexpectedly low ice crystal numbers and high supersaturations, suggesting an incomplete understanding of the freezing mechanisms under these conditions. The existence of viscous organic aerosol has offered alternative ice nucleation pathways, which have been observed in laboratory studies. We have developed the first cloud parcel model to investigate the effect of viscosity on ice nucleation.
Observations of low temperature cirrus clouds have found unexpectedly low ice crystal numbers...
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