Preprints
https://doi.org/10.5194/acp-2022-714
https://doi.org/10.5194/acp-2022-714
 
24 Oct 2022
24 Oct 2022
Status: this preprint is currently under review for the journal ACP.

A bin-microphysics parcel model investigation of secondary ice formation in an idealised shallow convective cloud

Rachel L. James1, Jonathan Crosier1,2, and Paul J. Connolly1 Rachel L. James et al.
  • 1Department of Earth and Environmental Sciences, The University of Manchester, Manchester, UK
  • 2National Centre for Atmospheric Science (NCAS), University of Manchester, Manchester, UK

Abstract. We provide the first systematic study of ice formation in idealised shallow clouds from collisions of supercooled water drops with ice particles (‘mode 2’). Using the University of Manchester bin-microphysics parcel model, we investigated the sensitivity of ice formation due to mode 2 for a wide range of parameters: aerosol particle size distribution, updraft speed, cloud base temperature, cloud depth, ice-nucleating particle concentration and freezing fraction of mode 2. We provide context to our results with other secondary ice production mechanisms as single mechanisms and combinations (rime-splintering, spherical freezing fragmentation of drops [‘mode 1’] and ice-ice collisions). There was a significant sensitivity to aerosol particle size distribution when updraft speeds were low (0.5 m s−1); secondary ice formation did not occur when the aerosol particle size distribution mimicked polluted environments. Where secondary ice formation did occur in simulated clouds, significant ice formation in the shallower clouds (1.3 km deep) was due to mode 2 or a combination which included mode 2. The deeper clouds (2.4 km deep) also had significant contributions from rime-splintering or ice-ice collisions SIP mechanisms. While simulations with cloud base temperatures of 7 °C were relatively insensitive to ice-nucleating particle concentrations, there was a sensitivity in simulations cloud base temperatures of 0 °C. Increasing the ice-nucleating particle concentration delayed ice formation. Our results suggest that collisions of supercooled water drops with ice particles may be a significant ice formation mechanism within shallow convective clouds where rime-splintering is not active.

Rachel L. James et al.

Status: open (until 06 Dec 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Rachel L. James et al.

Rachel L. James et al.

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Short summary
Secondary ice production (SIP) may significantly enhance the ice particle concentration in mixed-phase clouds. We present a systematic modelling study of secondary ice formation in idealised shallow convective clouds for a range of conditions. Our results suggest that the SIP mechanism, collisions of supercooled water drops with more massive ice particles, might be a significant ice formation mechanism in shallow convective clouds outside the rime-splintering temperature range (-3 °C to -8 °C).
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