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

• Main Authors: Pasquier, Julie Thérèse, Henneberger, Jan, Ramelli, Fabiola, Lauber, Annika, David, Robert Oscar, Wieder, Jörg, Carlsen, Tim, Gierens, Rosa, Maturilli, Marion, Lohmann, Ulrike
• Format: Text
• Language: English
• Published: 2022
• Subjects: Arctic; Ny-Ålesund; Climate change; Ny Ålesund
• Online Access: https://doi.org/10.5194/acp-2022-314; https://acp.copernicus.org/preprints/acp-2022-314/

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.