Efficient ice multiplication from freezingraindrop fragmentation

The number of ice particles in mixed-phase clouds often exceeds the concentration of ice-nucleating
particles by several orders of magnitude. This discrepancy can be explained by secondary ice
production, which is a set of physical processes that can multiply the number of ice particles in the
atmosphere. Due to their transient and microscopic nature, observations and quantifications of these
processes are scarce. One such process is the fragmentation of drops upon freezing, whereby ice
splinters are produced when a drop undergoes a phase transition from liquid to ice. Recent
laboratory studies suggest that this process could significantly contribute to ice crystal number
concentrations. However, the number of ice fragments that can be produced under realistic
atmospheric conditions remains highly uncertain.
In this talk, we present cases of droplet fragmentation occurring during refreezing rain episodes in
Hyytiälä, Finland. These cases were identified using a combination of ground-based in situ
observations and cloud radar. Based on the classification of in situ image data, we evaluate the
effectiveness of the process from an event-based perspective. Additionally, we identify the different
modes of deformation that occur during refreezing and demonstrate how their frequencies change
over time.
These results provide novel insights into the effectiveness of drop fragmentation upon freezing,
addressing a long-standing knowledge gap in cloud microphysics.