Quantification of the secondary ice production mechanisms: droplet shattering on freezing vs. droplet-ice collision

对用于天气预报和气候变化预测的云过程进行可靠的建模，需要对混合相云中的冰的形成有一个良好的了解。然而，现场测量的冰晶浓度经常被发现比成核冰粒的浓度高出许多数量级。这种差异促使大气研究界探索二次制冰机制，即通过现有冰粒的碎裂或在液滴冻结事件期间产生额外冰粒的过程。在莱比锡莱布尼茨对流层研究所(TROPOS)和卡尔斯鲁厄理工学院气象和气候研究所(KIT)的合作下，我们计划研究两种拟议的SIP机制：(A)液滴-冰碰撞(Rime-Split)引起的二次冰粒的产生和(B)冰滴的碎裂。该项目的主要目标将是：(1)开发一种新的实验装置--冰滴分裂和碎裂实验(IDEFIX)，用于研究通过(A)和(B)产生的二次冰粒；(2)通过对SIP事件的高速视频观测来确定SIP的物理机制；(3)对于机制(A)和机制(B)，将两种机制产生的二次冰粒的数量量化为温度、液滴大小和撞击速度的函数，以及(4)为(A)和(B)两个SIP机制开发参数。IDEFIX的开发将得益于两个合作伙伴的长期专业知识：在定义良好的热力学控制系统中水滴/冰粒成核、生长和蒸发方面的专业知识，TROPOS团队贡献的对这些水流星的探测专业知识，以及KIT团队贡献的对自由悬浮冰滴的高速视频观测专业知识。IDEFIX的模块化设计将允许双方合作伙伴利用各自的现场能力进行模块开发，然后将实验工作结合在TROPOS现场进行的一系列测量活动中。

Reliable modeling of cloud processes for weather predictions and climate change projections requires a sound understanding of the ice formation in mixed-phase clouds. However, ice crystal concentrations measured in-situ are often found to exceed the concentration of ice nucleating particles by many orders of magnitude. This discrepancy motivates the atmospheric research community to search for Secondary Ice Production (SIP) mechanisms, i.e., processes producing additional ice particles by e.g. fragmentation of existing ice particles or during droplet freezing events. In a collaborative effort between the Leibniz-Institute for Tropospheric Research (TROPOS) in Leipzig and the Institute of Meteorology and Climate Research of Karlsruhe Institute of Technology (KIT), we plan to investigate two proposed SIP mechanisms: production of secondary ice particles caused by (A) droplet-ice collisions (rime-splintering) and (B) splintering of freezing droplets. These two SIP mechanisms have been suggested to be of particular relevance in mixed-phase clouds.The main objectives of the proposed project will be: (1) to develop a new experimental set-up, the Ice Droplet splintEring and Fragmentation eXperiment (IDEFIX), for investigating secondary ice particle production via (A) and (B), (2) to identify the physical mechanisms of SIP via high speed video observations of the SIP events, (3) to quantify the number of secondary ice particles produced by both mechanisms as a function of temperature, droplet size and impact velocity for mechanism (A) and droplet size and droplet composition for mechanism (B), and (4) to develop parameterizations for both SIP mechanisms (A) and (B). These parameterizations will be applied by external collaborators for SIP representation in cloud microphysics resolving models.The development of IDEFIX will benefit from the long-standing expertise of both collaboration partners: expertise in droplet/ice particle nucleation, growth and evaporation in a well-defined thermodynamic controlled system and the detection of those hydrometeors contributed by TROPOS team and expertise in high-speed video observation of freely suspended freezing droplets contributed by KIT team. The modular design of IDEFIX will allow both collaboration partners to use their respective on-site capabilities for module development and then combine the experimental efforts in a series of measuring campaigns conducted at TROPOS site.