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 in 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 to −10 °C. The riming process was observed with high-speed video microscopy and infrared thermography, while potential secondary ice (SI) particles in the super-micron size range were detected by a custom-built ice counter. Contrary to earlier HM experiments, where up to several hundreds of SI particles per milligram of 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 to be the explanation for rime splintering: the freezing of droplets upon glancing contact with the rimer and the 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, which is 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 the deformation of drizzle droplets upon freezing, we also observed the formation of spikes on the rimer surface, which might be a source of SIP.

中文翻译：

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二次冰生产——没有证据表明有效的冰霜分裂机制

摘要。混合相云对于地球的天气和气候系统至关重要。通过二次产冰（SIP）进行的冰增殖被认为是观察到的混合相云中冰粒数量浓度大幅增加的原因。在这项研究中，我们重点关注雾凇分裂，也称为哈雷特-莫索普（HM）过程，该过程仍然缺乏物理和定量的理解。我们报告了在代表对流混合相云的温度范围为 -4 至 -10 °C 的条件下，在新开发的装置中进行的雾凇分裂的实验研究。通过高速视频显微镜和红外热成像技术观察了沸腾过程，同时通过定制的冰计数器检测到超微米尺寸范围内潜在的二次冰（SI）颗粒。与早期的 HM 实验相反，在 -5 °C 下每毫克雾凇中发现多达数百个 SI 颗粒，我们没有发现有效 SIP 的证据，这从根本上质疑了雾凇分裂的重要性。此外，我们可以排除两种可能解释雾凇分裂的潜在机制：与雾凇接触时液滴的冻结以及雾凇表面上球形冷冻液滴的破碎。据观察，升华的脆弱雾凇尖顶的断裂只产生很少的 SI 颗粒，这不足以解释早期研究中报告的大量冰颗粒。在湿生长和干生长之间的过渡状态中，类似于毛毛雨滴在冻结时变形的现象，我们还观察到在边缘表面形成尖峰，这可能是 SIP 的来源。