Preprints
https://doi.org/10.5194/acp-2021-760
https://doi.org/10.5194/acp-2021-760

  22 Sep 2021

22 Sep 2021

Review status: this preprint is currently under review for the journal ACP.

Secondary ice production processes in wintertime alpine mixed-phase clouds

Paraskevi Georgakaki1, Georgia Sotiropoulou1,2, Étienne Vignon3, Anne-Claire Billault-Roux4, Alexis Berne4, and Athanasios Nenes1,5 Paraskevi Georgakaki et al.
  • 1Laboratory of Atmospheric Processes and their Impacts, School of Architecture, Civil & Environmental Engineering, École Polytechnique Fédérale de Lausanne, Lausanne, CH-1015, Switzerland
  • 2Department of Meteorology, Stockholm University & Bolin Center for Climate Research, Stockholm, Sweden
  • 3Laboratoire de Météorologie Dynamique/IPSL/Sorbone Université/CNRS, UMR 8539, Paris, France
  • 4Environmental Remote Sensing Laboratory, School of Architecture, Civil & Environmental Engineering, École Polytechnique Fédérale de Lausanne, Lausanne, CH-1015, Switzerland
  • 5Center for Studies of Air Quality and Climate Change, Institute of Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Patras, GR-26504, Greece

Abstract. Observations of orographic mixed-phase clouds (MPCs) have long shown that measured ice crystal number concentrations (ICNCs) can exceed the concentration of ice nucleating particles by orders of magnitude. Additionally, model simulations of alpine clouds are frequently found to underestimate the amount of ice compared with observations. Surface-based blowing snow, hoar frost and secondary ice production processes have been suggested as potential causes, but their relative importance and persistence remains highly uncertain. Here we study ice production mechanisms in wintertime orographic MPCs observed during the Cloud and Aerosol Characterization Experiment (CLACE) 2014 campaign at the Jungfraujoch site in the Swiss Alps with the Weather Research and Forecasting model (WRF). Simulations suggest that droplet shattering is not a significant source of ice crystals at this specific location – but break-up upon collisions between ice particles is quite active, elevating the predicted ICNCs by up to 3 orders of magnitude, which is consistent with observations. The initiation of the ice-ice collisional break-up mechanism is primarily associated with the occurrence of seeder-feeder events from higher precipitating cloud layers. The enhanced aggregation of snowflakes is found to drive secondary ice formation in the simulated clouds, the role of which is strengthened when the large hydrometeors interact with the primary ice crystals formed in the feeder cloud. Including a constant source of cloud ice crystals from blowing snow, through the action of the break-up mechanism, can episodically enhance ICNCs. Increases in secondary ice fragment generation can be counterbalanced by enhanced orographic precipitation, which seems to prevent explosive multiplication and cloud dissipation. These findings highlight the importance of secondary ice and "seeding" mechanisms – primarily falling ice from above and to a lesser degree blowing ice from the surface – which frequently enhance primary ice and determine the phase state and properties of MPCs.

Paraskevi Georgakaki et al.

Status: open (until 18 Nov 2021)

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  • RC1: 'Comment on acp-2021-760', Anonymous Referee #1, 20 Oct 2021 reply

Paraskevi Georgakaki et al.

Paraskevi Georgakaki et al.

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Short summary
The modelling study focuses on the importance of ice multiplication processes in orographic mixed-phase clouds, which is one of the least understood cloud types in the climate system. We show that the consideration of ice seeding and secondary ice production through ice-ice collisional break-up is essential for correct predictions of precipitation in mountainous terrain, with important implications for radiation processes.
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