New insights on Secondary Ice Production during Riming

The radiative budget, atmospheric charging and the ability of mixed-phase clouds to form precipitation strongly depends on the presence of ice crystals. Observations indicate that secondary ice formation processes play an important role in increasing ice crystal number concentration in mixed-phase clouds. We focus on secondary ice formation during riming, which is also knows as rime-splintering (RS) or Hallett-Mossop process. Most knowledge about RS is based on old laboratory experiments with quantitatively inconsistent results and lacks a fundamental mechanistic understanding.

To overcome this knowledge gap, we developed an experimental set-up IDEFIX (Ice Droplets splintering on FreezIng eXperiment) to study RS using high-speed video microscopy, thermography system and custom-built ice counter to detect secondary ice particles. In contrast to earlier studies - no efficient secondary ice formation was observed under near-atmospheric conditions. This fundamentally questions the RS process and motivates further laboratory investigations in a broader parameter range to elucidate under which conditions RS is existing. Recent insights on the dependence of RS efficiency on ice surface roughness and the role of small riming droplets will be presented.

Our study will contribute to the development of new parameterizations of secondary ice formation for cloud microphysics-resolving models.