Laboratory and Field Studies of Cloud-Turbulence Interactions via Digital Holography

云是地球系统的重要组成部分，与太阳和地球发射的电磁辐射相互作用，催化大气中的化学反应，并通过水循环重新分配水和能量。云的物理特性部分取决于组成云滴(或其他云粒)碰撞和合并的速度。认为这一过程强烈地依赖于云粒子的空间分布以及云粒子与其局地湍流环境的相互作用。然而，我们对湍流中惯性沉降粒子的动力学的理解是有限的。尤其麻烦的是，缺乏适合于与理论和计算工作进行比较的测量。我们通过使用数字全息术研究湍流中云滴的详细动力学来解决湍流在云微物理中的这一方面的作用。具体地说，我们将继续发展实验室和野外全息术，研究液滴的空间分布、液滴相对速度的分布，以及液滴的拉格朗日性质，如加速度。研究包括三个部分：1)实验室湍流云室中液滴的空间相关性、相对速度分布和拉格朗日加速度分布的研究。在实验室中，液滴的尺寸分布和湍流得到了很好的表征，并且可以在很大的调节参数空间范围内变化。我们将研究引力和湍流引起的碰撞之间的相互作用。此外，我们还将探索电荷在云滴动力学中的作用。2)从山顶实验室观测大气云中拉格朗日液滴的动力学。一个无法在模拟或实验室中(至少目前不能)检查的参数是湍流雷诺数，因此我们将对随平均风速移动的雪橇的液滴动力学进行平行研究，从而能够进行拉格朗日液滴测量。由此生成的自然云中液滴运动的3D图像将为我们提供有助于碰撞-合并过程的内部工作的详细图像。3)液滴大小和空间分布的观测NSF/NCAR C130上的新HOLODEC2仪器。这台全息仪器将从每个约10厘米的样本体积中提供液滴大小和空间分布的估计。这提供了微物理局部测量，而不是空间平均测量，因此将为微物理变异性提供新的视角。研究活动的一个重要方面将是本科生和研究生的教育。这项工作将为学生提供宝贵的机会熟悉仪器科学，并将允许学生在三个令人兴奋的领域工作：大气科学，物理光学和全息术，以及基础流体力学。与NCAR、对流层研究所和马克斯·普朗克动力学和自组织研究所的合作将提供一个真正跨学科和国际化的独特教育环境。该项目有力地支持和补充了从工业多相流到基本非线性动力学等领域正在进行的研究，我们预计将在这一水平上继续做出贡献。此外，更好地理解云过程与改进天气预报(特别是定量降水预报)以及云在气候系统中所起的作用直接相关。这项工作将有助于更全面和定量地了解云滴相互作用的细节，从而有助于更全面、更定量地了解受降水形成强烈影响的更大尺度上发生的过程。

Clouds are a crucial part of the earth system, interacting with electromagnetic radiation emitted by the sun and the earth, catalyzing chemical reactions in the atmosphere, and redistributing water and energy through the hydrologic cycle. The physical characteristics of clouds are determined in part by the rate at which the constituent cloud droplets (or other cloud particles) collide and coalesce. It is thought that this process depends strongly on the spatial distribution of cloud particles and the interaction of cloud particles with their local turbulent environment. Our understanding of the dynamics of inertial, settling particles in turbulence, however, is limited. Especially troublesome is the dearth of measurements suitable for comparison with theoretical and computational work.Intellectual merits. We address this aspect of the role of turbulence in cloud microphysics by investigating the detailed dynamics of cloud droplets in turbulent flows using digital holography. Specifically, we will continue the development of laboratory and field holography to study the spatial distribution of droplets, the distribution of droplet relative velocities, and the Lagrangian properties of droplets, such as acceleration. The research involves three components:1) Studies of droplet spatial correlations, relative velocity distributions, and Lagrangian acceleration distributions in a laboratory turbulence cloud chamber. In the laboratory the droplet size distribution and the turbulence are well characterized and can be varied over a wide range of the governing parameter space. We will investigate the interplay between gravitational and turbulence induced collisions. Furthermore, we will explore the role of electric charge on the dynamics of cloud droplets.2) Observations of Lagrangian droplet dynamics in atmospheric clouds from a mountain-top laboratory. The one parameter that cannot be checked in simulation or in the laboratory (at least not currently) is the turbulence Reynolds number, so we will carry out a parallel study of droplet dynamics from a sled moving with the mean wind speed, capable of making Lagrangian droplet measurements. The resulting 3D view of droplet motions in natural clouds will give us a detailed picture of the internal workings that contribute to the collision-coalescence process.3) Observations of droplet size and spatial distributions the new HOLODEC2 instrument aboard the NSF/NCAR C130. This holographic instrument will provide estimates of the droplet size and spatial distributions from each sample volume of ~10 cm3. This provides a microphysically-local, rather than spatially-averaged measurement, and therefore will provide a new perspective on microphysical variability.Broader Impacts. An important aspect of the research activities will be the education of undergraduate and graduate students. This work will provide valuable opportunities for students to become familiar with instrumentation science and will allow students to work at the interface of three exciting fields: atmospheric science, physical optics and holography, and fundamental fluid mechanics. The collaborations with NCAR, the Institute for Tropospheric Research, and the Max Planck Institute for Dynamics and Self Organization will provide a unique educational environment that is genuinely interdisciplinary and international. The project strongly supports and complements ongoing research in fields ranging from industrial multiphase flows to fundamental nonlinear dynamics, and we anticipate continuing our contributions at that level. Furthermore, improved understanding of cloud processes is directly relevant to improving weather forecasts (especially quantitative precipitation forecasting) as well as the role that clouds play in the climate system. This work will contribute to a more complete and quantitative understanding of the details of cloud droplet interactions, and therefore the processes occurring at larger scales that are so strongly influenced by precipitation formation.