Laboratory Study of the Influence of Nucleation and Chemistry on Ice Particle Habit and Growth Rates

尽管过去的努力，一些基本方面的初始冰晶形成（称为“成核”）在云中仍然不确定。 先前的实验产生了分散的结果，部分原因是成核模式没有得到很好的控制，并且所使用的方法引入了伪影。 这些研究人员计划通过开发一种新的双毛细管仪器来消除这些问题，在这种仪器中，单晶在一个可以与蒸汽源隔离的小生长室中的超薄纯二氧化硅玻璃毛细管的尖端处成核和生长。 在相同的等温条件下保持的两个分离良好的晶体的存在将允许精确观察沉积和升华过程。 这种方法相对于以前方法的优势（例如，扩散室、衬底室和电动天平）包括对热力学条件的更大控制以及表征潜在的晶体-晶体和晶体-衬底相互作用的能力。 在新的双毛细管室的实验将在一系列的空气压力和温度，包括有关卷云形成的条件，以及多种成核模式。 通过以不同的方式成核初始晶体，这些研究人员希望确定有多少以前观察到的分散在晶体的习惯引起的变化，在成核mode.The智力这项研究的优点在于发展更精确的，无人为因素的数据描述晶体形状（“习惯”）的发展和沉积系数函数作为环境条件的函数。 这些结果将允许改进当前晶体生长模型的评估。 通过培训一名研究生掌握新仪器设计和数据分析技术，并通过多学科会议和科学期刊传播成果，将产生更广泛的影响。 最终，这些发现将有助于改善卷云在地球气候系统中的表现。 额外的推广活动将包括在网上分发描绘晶体生长和升华的高质量视频显微镜。

Despite past efforts, a number of fundamental aspects of initial ice crystal formation (termed "nucleation") in clouds remain uncertain. Previous experiments have produced scattered results partly because the nucleation mode was not well controlled and the methods used introduced artifacts. These investigators plan to eliminate such problems by developing a new two-capillary instrument in which a single crystal nucleates and grows at the tip of an ultra-thin pure-silica glass capillary within a small growth chamber that can be isolated from the vapor source. The presence of two well-separated crystals held under identical isothermal conditions will allow precise observation of the deposition and sublimation processes. Advantages of this approach over previous methods (e.g., diffusion chambers, substrate chambers, and electrodynamic balances) include greater control over the thermodynamic conditions and the ability to characterize potential crystal-crystal and crystal-substrate interactions. Experiments in the new twin-capillary chamber will be made for a range of air pressures and temperatures, including conditions relevant to cirrus formation, as well as for multiple nucleation modes. By nucleating the initial crystal in different ways, these investigators expect to determine how much of the previously observed scatter in crystal habits arose from variations in nucleation mode.The intellectual merit of this research rests on development of more precise, artifact-free data describing crystal shape ("habit") development and deposition coefficient functions as a function of environmental conditions. These results will allow improved evaluation of current crystal growth models. Broader impacts will accrue through training of a graduate student in techniques of new-instrument design and data analysis and through dissemination of results via multidisciplinary conferences and scientific journals. Ultimately these findings will contribute to improved representation of cirrus clouds in the earth's climate system. Additional outreach will include web-based distribution of high-quality video microscopy depicting crystal growth and sublimation.