Laboratory and Field Studies of Cloud-Turbulence Interactions via Digital Holography

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Clouds are a crucial part of the earth system, interacting with electromagnetic radiation emitted by the sun and the earth, catalyzing chemical reactions in the atmosphere, and redistributing water and energy through the hydrologic cycle. The physical characteristics of clouds are determined in part by the rate at which the constituent cloud droplets (or other cloud particles) collide and coalesce. It is thought that this process depends strongly on the spatial distribution of cloud particles and the interaction of cloud particles with their local turbulent environment. Our understanding of the dynamics of inertial, settling particles in turbulence, however, is limited. Especially troublesome is the dearth of measurements suitable for comparison with theoretical and computational work.Intellectual merits. We address this aspect of the role of turbulence in cloud microphysics by investigating the detailed dynamics of cloud droplets in turbulent flows using digital holography. Specifically, we will continue the development of laboratory and field holography to study the spatial distribution of droplets, the distribution of droplet relative velocities, and the Lagrangian properties of droplets, such as acceleration. The research involves three components:1) Studies of droplet spatial correlations, relative velocity distributions, and Lagrangian acceleration distributions in a laboratory turbulence cloud chamber. In the laboratory the droplet size distribution and the turbulence are well characterized and can be varied over a wide range of the governing parameter space. We will investigate the interplay between gravitational and turbulence induced collisions. Furthermore, we will explore the role of electric charge on the dynamics of cloud droplets.2) Observations of Lagrangian droplet dynamics in atmospheric clouds from a mountain-top laboratory. The one parameter that cannot be checked in simulation or in the laboratory (at least not currently) is the turbulence Reynolds number, so we will carry out a parallel study of droplet dynamics from a sled moving with the mean wind speed, capable of making Lagrangian droplet measurements. The resulting 3D view of droplet motions in natural clouds will give us a detailed picture of the internal workings that contribute to the collision-coalescence process.3) Observations of droplet size and spatial distributions the new HOLODEC2 instrument aboard the NSF/NCAR C130. This holographic instrument will provide estimates of the droplet size and spatial distributions from each sample volume of ~10 cm3. This provides a microphysically-local, rather than spatially-averaged measurement, and therefore will provide a new perspective on microphysical variability.Broader Impacts. An important aspect of the research activities will be the education of undergraduate and graduate students. This work will provide valuable opportunities for students to become familiar with instrumentation science and will allow students to work at the interface of three exciting fields: atmospheric science, physical optics and holography, and fundamental fluid mechanics. The collaborations with NCAR, the Institute for Tropospheric Research, and the Max Planck Institute for Dynamics and Self Organization will provide a unique educational environment that is genuinely interdisciplinary and international. The project strongly supports and complements ongoing research in fields ranging from industrial multiphase flows to fundamental nonlinear dynamics, and we anticipate continuing our contributions at that level. Furthermore, improved understanding of cloud processes is directly relevant to improving weather forecasts (especially quantitative precipitation forecasting) as well as the role that clouds play in the climate system. This work will contribute to a more complete and quantitative understanding of the details of cloud droplet interactions, and therefore the processes occurring at larger scales that are so strongly influenced by precipitation formation.