Mixed-phase clouds are essential elements in Earth’s weather and climate system, exist at temperatures between 0°C and -38°C and are characterized by simultaneously occurring supercooled liquid water droplets and ice particles (Fig. 1). Primary ice particles are formed via nucleation processes. Above approx. -38°C ice nucleation takes place if an ice nucleating particle (INP) catalyzes the process. Aircraft measurements of particular mixed-phase clouds demonstrated a strong discrepancy between the observed ice particle and INP number concentration of one to four orders of magnitude [1-5]. Secondary ice production (SIP) mechanisms such as mechanical fragmentation or sublimation of ice crystals, droplet shattering on freezing and ice splinter production due to droplet-ice collisions and ice-ice collisions have been hypothesized [6-8] which can increase the total ice particle number concentration by multiplication of primary ice particles and hence might explain the observed discrepancy.
Here at TROPOS, we focus on a SIP mechanism which is a consequence of droplet-ice collisions, i.e. when small supercooled droplets freeze upon contact with a larger ice particle (rimer or graupel, Fig. 2) and eject small ice splinters under certain conditions to be determined. Commonly, this SIP mechanisms is known as Hallett-Mossop  or rime-splintering process.
The DFG funded project is a joined project together with the Institute of Meteorology and Climate Research of the Karlsruhe Institute of Technology (KIT) [10, 11].
The main objectives of the project are:
- Development of a new experimental set-up for investigating secondary ice particle production via droplet-ice collision and droplet shattering on freezing
- Identification of the SIP underlying physical mechanisms for both processes
- Quantification of the number of secondary produced ice particles for both processes.
The new experimental set-up IDEFIX (‘Ice Droplets splintEring on FreezIng eXperiment’) is developed, built, characterized and first experiments were conducted successfully in close cooperation with KIT at TROPOS.
This project is funded by the German Research Foundation DFG (grant HA 8322/1-1).
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