04 May 2022
04 May 2022
Status: this preprint is currently under review for the journal ACP.

Conditions favorable for secondary ice production in Arctic mixed-phase clouds

Julie Thérèse Pasquier1, Jan Henneberger1, Fabiola Ramelli1, Annika Lauber1,a, Robert Oscar David2, Jörg Wieder1, Tim Carlsen2, Rosa Gierens3, Marion Maturilli4, and Ulrike Lohmann1 Julie Thérèse Pasquier et al.
  • 1Institute for Atmospheric and Climate Science, ETH Zürich, Zurich, Switzerland
  • 2Department of Geosciences, University of Oslo, Oslo, Norway
  • 3Institute for Geophysics and Meteorology, University of Cologne, Cologne, Germany
  • 4Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), Potsdam, Germany
  • anow at: Center for Climate Systems Modelling (C2SM), ETH Zürich, Zurich, Switzerland

Abstract. The Arctic is very susceptible to climate change and thus warming much faster than the rest of the world. Clouds influence terrestrial and solar radiative fluxes, and thereby impact the amplified Arctic warming. The partitioning of thermodynamic phases (i.e. ice crystals and water droplets) within mixed-phase clouds (MPCs) especially influences their radiative properties. However, the processes responsible for ice crystal formation remain only partially characterized. In particular, so-called secondary ice production (SIP) processes, which create supplementary ice crystals from primary ice crystals and the environmental conditions that they occur in, are poorly understood. The microphysical properties of Arctic MPCs were measured during the Ny-Ålesund AeroSol Cloud ExperimENT (NASCENT) campaign to obtain a better understanding of the atmospheric conditions favorable for the occurrence of SIP processes. To this aim, the in-situ cloud microphysical properties retrieved by a holographic cloud imager mounted on a tethered balloon system were complemented by ground-based remote sensing and ice nucleating particle measurements. During six days investigated in this study, SIP occurred during 40 % of the in-cloud measurements and high SIP events with number concentrations larger than 10 L-1 of small pristine ice crystals in 3.5 % of the in-cloud measurements. This demonstrates the role of SIP for Arctic MPCs. The highest concentrations of small pristine ice crystals were produced at temperatures between -3 °C and -5 °C and were related to the occurrence of drizzle drops freezing upon collision with ice crystals. This suggests that a large fraction of ice crystals in Arctic MPCs is produced via the droplet shattering mechanism. From evaluating the ice crystal images, we could identify ice-ice collision as a second SIP mechanism that dominated when fragile ice crystals were observed. Moreover, SIP occurred over a large temperature range and was observed in up to 95 % of the measurements down to -24 °C due to the occurrence of ice-ice collisions. This emphasizes the importance of SIP at temperatures below -8 °C, which are currently not accounted for in most numerical weather models.

Julie Thérèse Pasquier et al.

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Julie Thérèse Pasquier et al.

Julie Thérèse Pasquier et al.


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Short summary
It is important to understand how snowflakes and cloud droplets form in clouds, as their concentrations and sizes determine the exact radiative properties of the clouds. Normally, snowflakes form from aerosols, but we found evidence for the formation of additional snowflakes from the original ones over a large temperature range within Arctic clouds. Especially, additional snowflakes were formed during collisions of several snowflakes or during the freezing of large cloud droplets.