Secondary Ice Production – No Evidence of Efficient Rime-Splintering Mechanism

Abstract. Mixed-phase clouds are essential for Earth’s weather and climate system. Ice multiplication via secondary ice production (SIP) is thought to be responsible for the observed strong increase of ice particle number concentration in mixed-phase clouds. In this study, we focus on the rime-splintering also known as the Hallett-Mossop (HM) process, which still lacks physical and quantitative understanding. We report on an experimental study of rime-splintering conducted in a newly developed setup under conditions representing convective mixed-phase clouds in the temperature range of −4 °C to −10 °C. The riming process was observed with high-speed video microscopy and infrared thermography, while potential secondary ice particles (SI) in the super-micron size range were detected by a custom-build ice counter. Contrary to earlier HM experiments, where up to several hundreds of SI particles per mg rime were found at −5 °C, we found no evidence of productive SIP, which fundamentally questions the importance of rime-splintering. Further, we could exclude two potential mechanisms suggested as explanation for rime-splintering: freezing of droplets upon glancing contact with the rimer and fragmentation of spherically freezing droplets on the rimer surface. The break-off of sublimating fragile rime spires was observed to produce very few SI particles, insufficient to explain the large numbers of ice particles reported in earlier studies. In the transition regime between wet and dry growth, in analogy to phenomena of deformation of drizzle droplets upon freezing, we also observed formation of spikes on the rimer surface, which might be a source of SIP.