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液滴聚集和生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上取得革命性的进展。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，对地球气候产生重要影响。目前的气候模式高估了观测到的降水起始时间，这种差异主要是由于对微尺度云过程的描述不足。目前的提案旨在通过回答云微物理学中一个悬而未决的核心问题来解决这个问题：是什么机制导致了快速增长的“统计上幸运的”雨滴的形成，加速了云的“胶体不稳定性”，从而导致降水？本研究的主要假设是湍流剪切和加速度在使液滴跨越凝聚-聚结瓶颈，从而缩短降雨形成时间方面起着至关重要的作用。为了理解湍流的作用，有必要深入了解大尺度云过程（如夹带、混合和间歇）和微尺度云过程（如液滴聚集、碰撞和聚并）之间的相互作用。离散颗粒在紊流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破裂和聚并，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，以及气溶胶对云性质的影响。首席研究员积极参与向代表性不足的群体伸出援助之手，并旨在通过一个名为“让我们与粒子一起聚会”的项目加强这些活动。该项目将促进首席研究员与初高中教师和学生的直接互动，还将包括开发一个网站，让学生探索微粒流及其应用。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计算费用。为了解决这一挑战，提出了一种新的多尺度计算方法，其中使用LES和DNS分别捕获大规模和微尺度云过程，同时通过从LES到DNS的惯性范围动能传递来包含这些过程之间的相互作用。多尺度的方法是必要的，因为液滴通过聚结生长可能对湍流剪切中的间歇性非常敏感，而这种间歇性虽然发生在惯性尺度上，但与湍流能量级联的小尺度湍流密切相关。LES- dns方法将包括：(1)在单个云中执行从能量包含尺度到惯性尺度的湍流的LES，以便捕获惯性范围的间歇性。还将为LES开发代表云-大气相互作用的新边界条件；(2)对云湍流惯性到耗散范围内的湍流-液滴相互作用进行DNS，从而解析驱动液滴相对运动的微尺度过程。在惯性范围内，LES和DNS分辨尺度之间的重叠将有助于采用一种新的LES到DNS能量传递方法来包含液滴动力学的间歇性效应。

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.