Clouds are not only fascinating to observe for their myriad of shapes, but are also scientifically challenging because their formation requires both knowledge about the large-scale meteorological environment as well as knowledge about the details of cloud droplet and ice crystal formation on the micro-scale. The ice phase in clouds remains enigmatic because ice crystal number concentrations can exceed the number concentrations of those aerosol particles acting as centers for ice crystals (so-called ice nucleating particles, INPs) by orders of magnitude. Thus, other sources of ice crystals must exist as well. Two commonly discussed mechanisms are secondary ice formation and the seeder-feeder process.

Secondary ice formation refers to the production of ice splinters caused by break-up of cloud droplets during freezing, by splinter ejections when riming occurs or mechanical break-up of ice crystals. The seeder-feeder process refers to an external source of cloud particles. A typical setting is an overlying cloud from which sedimenting ice crystals enter a lower-lying cloud and cause an enhancement of precipitation.

We conducted field experiments in the Swiss Alps and in the Arctic and obtained in-situ and remote sensing observations of mixed-phase clouds. We investigated the importance of secondary ice formation and the seeder-feeder process in these clouds from observations and modelling as I will discuss in my talk.