Using high-speed holography to quantify secondary ice processes

冰和混合相云在大气中有重要作用。它们与入射的太阳辐射和出射的地面辐射相互作用，产生影响地表的降水。在这些云中，冰可以通过两种主要途径之一形成，i）初级冰形成和ii）次级冰形成。当水从气相直接沉积到固相而形成冰时，或者当液态水冻结时，发生初级冰形成。初级冰的形成是由气溶胶介导的，取决于其化学成分，气溶胶可以促进冷凝，沉积和冻结。许多研究活动已经并将继续集中于了解不同气溶胶类型对这些过程的作用，最终目标是提高预测云粒子特性（数量，大小和形态）的准确性。尽管在理解初级冰形成方面取得了这些重要进展，但研究飞机的现场观测经常突出显示云中冰粒的预期数量密度与测量结果之间的重大差异。30多年来，这些差异一直被报道，并成为许多辩论的主题。已经开发了几种理论，试图使用各种环境数据集和实验室研究来解释这些差异。这些理论通常涉及粒子碰撞或冻结过程，通过碎裂过程增加云中冰粒的数量，被称为冰增殖过程或二次冰过程。不幸的是，由于直接观测的困难，各种次级冰理论都是高度不确定的，难以量化的（如果有的话），并且未经观测证实。尽管存在潜在的不确定性，但试图评估这些过程重要性的各种研究表明，次生冰对云演化、降水形成和闪电产生的影响范围很大。我们提出了一系列工作来解决长期存在的次生冰问题。该项目的基础是基于使用一种新的高速数字全息显微镜。该系统提供了以高时间分辨率为整个3-D体积提供高放大率图像的能力。当与补充设备（例如液滴发生器，冷台，环境室）相结合时，我们将在实验室中重新创建并充分观察感兴趣的关键微物理过程。这些实验室实验的结果将在一系列数值模型中实施，以评估这些过程对云的特性、云的寿命和降水形成的影响。

Ice and mixed phase clouds have important effects in the atmosphere. They interact with incoming solar radiation and outgoing terrestrial radiation, and generate precipitation which impacts on the surface. Within these clouds, ice can form from one of two major pathways, i) primary ice formation and ii) secondary ice formation. Primary ice formation occurs when ice forms due the direct deposition of water from the gas to solid phase, or when liquid water freezes. Primary ice formation is mediated by aerosols which may, depending on their chemical composition, facilitate condensation, deposition, and freezing. Much research activity has and continues to focus on developing an understanding of the role of different aerosol types on these processes, with the ultimate goal of improving the accuracy of predicted cloud particle properties (number, size, and morphology). Despite these crucial advances in understanding primary ice formation, in-situ observations from research aircraft routinely highlight major discrepancies between the anticipated number density of ice particles in clouds versus that measured. These discrepancies have been reported for over 3 decades and have been the subject of much debate. Several theories have been developed to attempt to explain these discrepancies using variety of ambient datasets and laboratory studies. These theories, which typically involve particle collisions or freezing processes, act to increase the number of ice particle in clouds via fragmentation processes, and are termed ice-multiplication processes, or secondary ice processes. Unfortunately, the various secondary ice theories are highly uncertain, poorly quantified (if at all), and unverified by observations due to the difficulty of direct observation. Various studies which have attempted to assess the importance of these processes, despite the underlying uncertainty, show that there is large scope for impact of secondary ice on cloud evolution, precipitation formation, and lightning generation. We propose a collection of work to address the longstanding issue of secondary ice. The foundations for this project are based on the use of a new high-speed digital holographic microscope. This system provides the ability to provide high magnification imagery for entire 3-D volumes at high temporal resolution. When coupled with complementary apparatus (e.g. droplet generators, cold stages, environmental chambers), we will recreate and fully observe key microphysical processes of interest in the laboratory. The results of these laboratory experiments will be implemented in a series of numerical models to assess the impact of these processes on cloud properties, cloud lifetime, and precipitation formation.