Collaborative Research: The role of microphysical processes and turbulence intermittency in droplet coalescence in warm cumulus clouds

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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.