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

尽管过去已经做出了努力，但云中初始冰晶形成(称为“成核”)的一些基本方面仍然不确定。以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，所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硅玻璃毛细管的顶端成核和生长。在相同的等温条件下，两个分离良好的晶体的存在将使人们能够精确地观察沉积和升华过程。与以前的方法(如扩散腔、基片腔和电动力学天平)相比，这种方法的优点包括更好地控制热力学条件以及表征潜在的晶体-晶体和晶体-基片相互作用的能力。在新的双毛细管气室中，将在一定的气压和温度范围内进行实验，包括与卷云形成相关的条件，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之前观察到的晶体习性中有多少散射是由成核模式的变化引起的。这项研究的智力优势在于开发更准确、无人工制品的数据来描述晶体形状(习惯)的发展和沉积系数作为环境条件的函数。这些结果将有助于改进对当前晶体生长模型的评估。通过对研究生进行新仪器设计和数据分析技术培训，以及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。其他外展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.