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

Pi：Rani，Sarma L./Koch，Donald L.提案编号：1436100/1435953拟议研究的目标是探索积云中水滴的形成和降雨的产生。这是一个与预测降雨以及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，与来自太阳和地球的电磁辐射相互作用，通过水文循环重新分配地球的水和能量，从而对地球气候系统产生重大影响。这项研究解决了目前对暖雨产生的两个中心悬而未决的问题，即导致快速增长的“统计幸运”水滴的形成的机制，以及云滴的增长，这两者都加速了云的“胶体不稳定”，从而导致降水。为了了解湍流的影响，必须深入了解湍流卷吸、混合和间歇等大尺度过程与粒子聚集、液滴相对速度和碰撞效率等微观尺度过程之间的相互作用。在外展和教育方面，建议让研究生和本科生参与湍流、云和粒子物理交汇处的跨学科研究。PIS计划联系阿拉巴马农工大学和亨茨维尔高中的学生，鼓励他们攻读STEM相关领域的本科和/或研究生学位。合作的PIS将进行一项多尺度调查，其中将使用大型涡流模拟(LES)来模拟整个云层，并将能量提供给直接数值模拟(DNS)模型，以解释非常小的水滴如何在云中出现和长大。假设是湍流切变和流动加速的间歇性推动了液滴的形成和合并。所提出的工作是针对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，惯性范围大涡湍流的间歇性信息将包含在数值模拟中。理论研究将集中于发展微观和宏观尺度的云物理随机模型，包括：1)具有云雷诺数统计特性的双分散相互作用液滴附近局部流场的详细微观模型；2)颗粒惯性和差异沉降对随机流场的影响。考虑了液滴的非连续、范德华和静电相互作用对碰撞效率的影响，以及考虑了湍流间歇性和涡动混合对液滴生长随机动力学的影响的大尺度模型。这项拟议的研究预计将揭示气溶胶-云相互作用，这些相互作用在产生降水和影响气候模式方面非常重要。

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.