Quantification of the secondary ice production mechanisms: droplet shattering on freezing vs. droplet-ice collision

• Applicants: Dr. Susan Hartmann; Dr. Alexei Kiselev
• Subject Area: Atmospheric Science
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 386137169

Reliable modeling of cloud processes for weather predictions and climate change projections requires a sound understanding of the ice formation in mixed-phase clouds. However, ice crystal concentrations measured in-situ are often found to exceed the concentration of ice nucleating particles by many orders of magnitude. This discrepancy motivates the atmospheric research community to search for Secondary Ice Production (SIP) mechanisms, i.e., processes producing additional ice particles by e.g. fragmentation of existing ice particles or during droplet freezing events. In a collaborative effort between the Leibniz-Institute for Tropospheric Research (TROPOS) in Leipzig and the Institute of Meteorology and Climate Research of Karlsruhe Institute of Technology (KIT), we plan to investigate two proposed SIP mechanisms: production of secondary ice particles caused by (A) droplet-ice collisions (rime-splintering) and (B) splintering of freezing droplets. These two SIP mechanisms have been suggested to be of particular relevance in mixed-phase clouds.The main objectives of the proposed project will be: (1) to develop a new experimental set-up, the Ice Droplet splintEring and Fragmentation eXperiment (IDEFIX), for investigating secondary ice particle production via (A) and (B), (2) to identify the physical mechanisms of SIP via high speed video observations of the SIP events, (3) to quantify the number of secondary ice particles produced by both mechanisms as a function of temperature, droplet size and impact velocity for mechanism (A) and droplet size and droplet composition for mechanism (B), and (4) to develop parameterizations for both SIP mechanisms (A) and (B). These parameterizations will be applied by external collaborators for SIP representation in cloud microphysics resolving models.The development of IDEFIX will benefit from the long-standing expertise of both collaboration partners: expertise in droplet/ice particle nucleation, growth and evaporation in a well-defined thermodynamic controlled system and the detection of those hydrometeors contributed by TROPOS team and expertise in high-speed video observation of freely suspended freezing droplets contributed by KIT team. The modular design of IDEFIX will allow both collaboration partners to use their respective on-site capabilities for module development and then combine the experimental efforts in a series of measuring campaigns conducted at TROPOS site.

• DFG Programme: Research Grants