Secondary ice production mechanisms: rime splintering and droplet shattering on freezing – focus on the cloud microphysics

• Applicants: Dr. Alexei Kiselev; Professorin Dr. Mira Pöhlker
• Subject Area: Atmospheric Science
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 386137169

The climatic effect of clouds depends strongly on the concentration, size and shape of the droplets and ice crystals. Observations have shown that the concentration of ice crystals measured in mixed phase clouds can exceed that of ice crystals formed primarily by ice nucleating particles by orders of magnitude. This indicates the potential significance of secondary ice production (SIP) processes across different cloud types and regions. Several mechanisms for SIP have been proposed, including rime splintering (RS) (also known as the Hallett-Mossop process) and droplet shattering upon freezing (DF). In contrast to primary ice formation, there are hardly any recent SIP laboratory studies, many key experiments are decades old and a re-evaluation with modern technologies is lacking. To carry out detailed laboratory studies to investigate RS and DF mechanisms, we developed two new experimental setups. Regarding RS, in contrast to previous laboratory experiments, we did not observe efficient SIP during riming within the investigated parameter range in the previous project. Rather, we observed a small number of SIP particles due to sublimating and breaking off graupel structures. Two suspected RS mechanisms could be excluded. Based on these results, critical questions remain open, such as the role of droplet sizes smaller than 12 µm, the roughness of the ice surface and the influence of rimer rotation as well as ambient humidity and pressure on the RS mechanism, which are the subject of the current project idea. Regarding the DF SIP mechanism, we found that the frequency of potential SIP events—such as breakup, splitting, jetting, and bubble bursting—increases when droplets freeze in free fall. We also measured the pressure rise inside freezing drizzle-sized droplets and demonstrated that freezing droplets undergo a series of pressure release events (PRE), potentially leading to the release of SIP particles. However, we could not determine the exact number of SIP particles generated per PRE or explore the relationship between droplet size, ambient pressure and the number of PREs so far. Answering these open questions is crucial for developing accurate parameterizations that can be incorporated into cloud models to improve the prediction of ice formation processes and cloud microphysics in weather and climate simulations. In this follow-up proposal, we aim to address these unanswered questions and further deepen our understanding of the RS and DF SIP mechanisms by expanding the range of environmental parameters in our experimental setups, making them more representative of realistic cloud conditions. This will be accomplished by modifying the experimental setups designed and built during the first phase of the project.

• DFG Programme: Research Grants