Investigating the Role of Turbulence in Hastening Warm-Cloud Precipitation

Sarma Rani, University of Alabama, Huntsville

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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.