Collaborative Research: The Role of Microphysical Processes and Turbulence Intermittency in Droplet Coalescence in Warm Cumulus Clouds

PI: Rani, Sarma L. /Koch, Donald L.提案号：1436100 / 1435953拟议研究的目标是探索积云中液滴的形成和雨的产生。这是一个与预测降雨和龙卷风和山洪暴发等灾害有关的主题。暖积云通过处理大气气溶胶，与太阳和地球的电磁辐射相互作用，并通过水文循环重新分配地球的水和能量，对地球气候系统产生重要影响。这项研究解决了目前对暖雨产生的理解中的两个核心突出问题，即导致快速增长的“统计幸运”雨滴形成的机制，以及云滴的增长，两者都加速了云的“胶体不稳定性”，从而导致降水。为了理解湍流的影响，有必要深入了解大尺度过程（如湍流卷入、混合和间歇性）和微尺度过程（如粒子聚集、液滴相对速度和碰撞效率）之间的相互作用。在推广和教育方面，建议让研究生和本科生参与湍流、云和粒子物理融合的跨学科研究。pi计划接触阿拉巴马a&m大学和亨茨维尔高中的学生，鼓励学生在stem相关领域攻读本科和/或研究生学位。合作的pi将进行多尺度调查，其中将使用大涡模拟（LES）对整个云进行建模，并向直接数值模拟（DNS）模型提供能量，以解释云中非常小的液滴是如何出现和变大的。假设是湍流剪切的间歇性和流动加速驱动了液滴的形成和聚并。提出的工作是在实际尺度上的高雷诺数云湍流。利用所提出的能量传递方法，将惯性范围的LES湍流间歇性信息包含在DNS中。理论研究将侧重于建立微观尺度和宏观尺度云物理的随机模型，包括：1)具有云雷诺数统计特征的双分散相互作用液滴附近局部流场的详细微观模型；2)颗粒惯性和微分沉降对随机流场的影响？通过水滴；3)液滴间的非连续、范德华和静电相互作用对其碰撞效率的影响；4)考虑湍流间歇性和涡流混合对液滴生长随机动力学影响的宏观尺度模型。这项拟议的研究有望揭示气溶胶与云之间的相互作用，这种相互作用在产生降水和影响气候模式方面很重要。

PI: Rani, Sarma L. /Koch, Donald L. Proposal Number: 1436100 / 1435953The goal of the proposed study is to explore the formation of droplets and the production of rain from a cumulus cloud. This is a topic relevant for the prediction of rainfall and of hazards such as tornados and flash flooding. Warm cumulus clouds exert significant influence on the climate system of the earth by processing atmospheric aerosols, interacting with electromagnetic radiation from the sun and earth, and redistributing earth's water and energy through the hydrologic cycle. This study addresses two central outstanding questions in the current understanding of warm rain production, namely, the mechanisms leading to the formation of fast-growing 'statistically fortunate' drops, and the growth of the cloud droplets, both of which hasten the 'colloidal instability' of the cloud, resulting in precipitation. To understand the effects of turbulence, it is essential to gain insights into the interactions between large-scale processes such as turbulent entrainment, mixing and intermittency, and microscale processes such as particle clustering, droplet relative velocities and collision efficiency. In terms of outreach and education, it is proposed to involve graduate and undergraduate students in interdisciplinary research at the confluence of turbulence, cloud and particle physics. The PIs plan to reach out to students at Alabama A&M University and Huntsville High School to encourage the students to pursue undergraduate and/or graduate degrees in STEM-related fields.The collaborating PIs will conduct a multiscale investigation, in which a Large Eddy Simulation (LES) will be used to model the whole cloud and to feed energy to a Direct Numerical Simulation (DNS) model, in order to explain how very small droplets appear and grow in size within a cloud. The hypothesis is that intermittency in turbulent shear and flow acceleration are driving the formation and the coalescence of the droplets. The proposed work is for high Reynolds number cloud turbulence at practical scales. Using the proposed energy transfer method, inertial-range LES turbulence intermittency information will be included in the DNS. The theoretical research will focus on developing stochastic models for both microscale and macroscale cloud physics including: 1) Detailed microscale models for the local flow field in the vicinity of bidisperse interacting drops with statistical properties characteristic of cloud Reynolds numbers; 2) Effects of particle inertia and differential sedimentation on the stochastic flow field ?seen? by the drops; 3) Effects of non-continuum, van der Waals and electrostatic interactions between drops on their collision efficiency; and 4) Macroscale model that accounts for the effects of turbulence intermittency and eddy mixing on the stochastic kinetics of droplet growth. The proposed research is expected to shed light on aerosol-cloud interactions that are important in generating precipitation and in affecting climate patterns.