Integrated Laboratory and Modeling Studies of Early Cold-Cloud Development

在温度低于0摄氏度的大气云中发生的微物理过程链中，一个令人困惑的环节是冰粒从蒸汽相中生长出来。最近的实验室测量表明，对于小冰晶来说，质量调节系数（一种衡量生长效率的指标）非常小。数值模拟研究表明，卷云中模拟的冰粒浓度和过饱和度，以及混合相云的冰川速率，都敏感地依赖于假设的质量调节系数。如果质量调节系数较低，则生长的冰粒不会像系数较高时那样有效地消耗水蒸气的过饱和。然后，在上升的云包中，过饱和度达到更高的水平，并且由于较高的过饱和度，更多的冰粒在合适的气溶胶颗粒上成核。生长中的小冰粒浓度的增加最终耗尽了过饱和蒸汽，云达到成熟状态，小冰粒浓度高于质量调节系数较高时的情况。为了减少过去测量和模拟中的不确定性，并测试有关气相沉积分子机制的假设，这项综合实验室模型研究的重点是冰从气相开始的早期生长。近年来，实验室技术和数值模拟能力都已经非常成熟，现在可以取得重大的新进展。实验室方法利用电动悬浮将单个冰粒从系统壁上分离出来，并允许在精确控制的环境条件下无限期地跟踪颗粒的生长。关于冰生长速率作为大小和过饱和度函数的新数据将有助于限制云模式中冰生长的数学表示。一套数值模型将与实验室数据结合使用，以指导实验室工作，从机制方面解释实验结果，并提供将我们的实验室结果外推到云尺度系统的方法。这项实验室模拟研究提供的协同作用将使人们对目前限制准确模拟冷云演化能力的冰过程有新的了解。这项研究对大气科学和社会具有潜在的广泛影响。对冷云微物理演化的进一步了解将加强对云在天气和气候过程中所起作用的认识。该研究将培养研究生，并使高级本科生接触到现代建模和实验研究方法。过去向不同观众展示云过程的成功将与学院博物馆合作，帮助将这些过程与新一代的K-12学生联系起来。利用高校资源，将包裹微物理模型发展成为基于网络的高校课堂教学工具。

One of the confounding links in the chain of microphysical processes occurring in atmospheric clouds at temperatures below 0 degress C is the growth of ice particles from the vapor phase. Recent laboratory measurements suggest that the mass accommodation coefficient, a measure of growth efficiency, is very small for small ice crystals. Numerical modeling studies have shown that the simulated concentrations of ice particles and supersaturations in cirrus clouds, as well as the rates of glaciation of mixed-phase clouds, all depend sensitively on the assumed mass accommodation coefficient. If the mass accommodation coefficient is low, then growing ice particles do not deplete the supersaturation of water vapor as effectively as they would if the coefficient was higher. Then supersaturations reach higher levels in ascending cloud parcels, and more ice particles are nucleated on suitable aerosol particles due to the higher supersaturations. The increase in concentration of smaller growing ice particles eventually depletes the supersaturated vapor and the cloud reaches a mature state with higher concentrations of smaller particles than would be the case if the mass accommodation coefficient were higher.This integrated laboratory-modeling study is focused on the early growth of ice from the vapor phase in order to reduce uncertainties in past measurements and simulations, and to test hypotheses regarding the molecular mechanisms of vapor deposition. Both laboratory techniques and numerical modeling capabilities have matured greatly in recent years, now permitting significant new progress to be made. The laboratory methods make use of electrodynamic levitation to isolate individual ice particles from system walls and permit particle growth to be followed indefinitely under precisely controlled environmental conditions. New data on ice growth rates as functions of size and supersaturation will help constrain the mathematical representation of ice growth in cloud models. A suite of numerical models will be used in conjunction with the laboratory data to guide the laboratory work, interpret the experimental findings in terms of mechanisms, and provide a means for extrapolating our laboratory results to cloud-scale systems. The synergism afforded by this laboratory-modeling study will allow new light to be shed on ice processes that currently are limiting capability to simulate cold-cloud evolution accurately.This research has potentially broad impacts on the atmospheric sciences and society. Improved understanding of microphysical evolution of cold clouds will enhance understanding of the roles played by clouds in weather and climate processes. The research will train graduate students and give advanced undergraduate students exposure to modern research methods in modeling and experimentation. Past successes in demonstrating cloud processes to diverse audiences will be continued in collaboration with the College museum, helping to relate these processes to new generations of K-12 students. Moreover, parcel microphysical models will be developed into web-based college classroom teaching tools using College resources.