Investigating the Role of Turbulence in Hastening Warm-Cloud Precipitation

该项目的目标是通过对湍流在积云中液滴聚合和增长中的作用进行高分辨率、多尺度的计算研究，在目前对暖云降水的理解方面取得革命性的进展。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，并通过水文循环重新分配地球的水和能量，对地球气候产生重大影响。目前的气候模型高估了观测到的开始降水的时间，这种差异主要是由于对微尺度云过程的描述不足所致。目前的提案旨在通过回答云微物理中的核心悬而未决的问题来解决这一问题：是什么机制导致快速增长的“统计幸运”水滴的形成，这些水滴加速了云的“胶体不稳定”，从而导致降水？这项研究的主要假设是，湍流切变和加速在使液滴跨越凝结-合并瓶颈方面发挥了关键作用，从而减少了雨形成的时间。为了了解湍流的作用，必须深入了解大尺度云过程(如卷吸、混合和间歇)与微尺度过程(如液滴聚集、碰撞和合并)之间的相互作用。离散粒子在湍流流体中的运动在广泛的应用中具有重要意义，例如了解燃烧系统中燃料液滴的分解和合并，量化湍流在海洋中浮游植物或云中液滴的运输和生长中的作用，以及气溶胶对云特性的影响。首席调查员积极参与与代表性不足的群体的接触，并旨在通过一项名为“让我们与粒子一起狂欢”的计划来加强这些活动。这项计划将促进首席研究人员与初中和高中教师和学生的直接互动，并将包括开发一个网站，允许学生探索充满粒子的流动及其应用。模拟积云中湍流-液滴相互作用的主要障碍是与解析云中整个湍流尺度谱相关的高昂的计算费用。为了应对这一挑战，提出了一种新的多尺度计算方法，其中大尺度和微尺度云过程分别使用大涡模拟和数值模拟来捕捉，而这些过程之间的相互作用则通过惯性量程动能从大涡模拟到模拟模拟来考虑。多尺度方法是必要的，因为聚结的液滴生长可能对湍流切变中的间歇性非常敏感，虽然这种切变发生在惯性尺度上，但通过湍流能量级联与小尺度湍流密切相关。大涡模拟方法包括：(1)在单个云中进行从能量尺度到惯性尺度的大涡模拟，从而捕捉到惯性范围的间歇性。还将为大涡模拟发展代表云-大气相互作用的新的边界条件；以及(2)在云湍流的惯性到耗散范围内进行湍流-液滴相互作用的数值模拟，从而解决驱动液滴相对运动的微尺度过程。LES和DNS分辨尺度在惯性范围内的重叠将有助于使用一种新的LES到DNS能量转移方法来包含对液滴动力学的间歇性影响。

The goal of this project is to make transformative advances in the current understanding of warm-cloud precipitation by performing a high-resolution, multiscale computational investigation of the role of turbulence in droplet coalescence and growth in cumulus clouds. Warm cumulus clouds exert significant influence on earth's climate by processing atmospheric aerosols, interacting with electromagnetic radiation from the sun and earth, and redistributing earth's water and energy through the hydrologic cycle. Current climate models overestimate the observed times for precipitation initiation, the discrepancy arising primarily due to an insufficient representation of microscale cloud processes. The current proposal aims to address this problem by answering the central outstanding question in cloud microphysics: What are the mechanisms leading to the formation of fast-growing "statistically fortunate" drops that hasten the "colloidal instability" of the cloud, resulting in precipitation? The primary hypothesis of this study is that turbulent shear and acceleration play a crucial role in enabling droplets bridge the condensation-coalescence bottleneck, thereby reducing the time for rain formation. To understand the role of turbulence, it is essential to gain insights into the interactions between large-scale cloud processes such as entrainment, mixing and intermittency, and microscale processes such as droplet clustering, collisions and coalescence. The motion of discrete particles in a turbulent fluid is of great significance in a broad range of applications, such as understanding the breakup and coalescence of fuel droplets in combustion systems, quantifying the role of turbulence in the transport and growth of phytoplankton in oceans or droplets in clouds, and the effects of aerosols on cloud properties. The principal investigator is heavily involved in outreach to underrepresented groups, and aims to intensify these activities through a program called "Let's Party with Particles". This program will promote direct interactions of the principal investigator with middle and high school teachers and students, and will also involve developing a website that allows students to explore particle-laden flows and their applications.The principal obstacle to simulating turbulence-droplet interactions in a cumulus cloud is the prohibitive computational expense associated with resolving the entire spectrum of turbulent scales in a cloud. To address this challenge, a novel multiscale computational approach is proposed wherein the large-scale and microscale cloud processes are captured using LES and DNS respectively, while the interactions between these processes are included through a transfer of inertial-range kinetic energy from LES to DNS. A multiscale approach is necessary since droplet growth by coalescence may be extremely sensitive to intermittency in turbulent shear, which, although occurring on inertial scales, is intimately tied to small-scale turbulence through the turbulent energy cascade. The LES-DNS approach will consist of: (1) Performing LES of turbulence from the energy-containing to the inertial scales in a single cloud, such that inertial-range intermittency is captured. New boundary conditions representative of cloud-atmosphere interactions will also be developed for LES; and (2) Performing DNS of turbulence-droplet interactions in the inertial to dissipative range of cloud turbulence, such that the microscale processes driving droplet relative motion are resolved. The overlap between the LES and DNS-resolved scales in the inertial range will facilitate the inclusion of intermittency effects on droplet dynamics using a novel LES to DNS energy transfer method.