Radiative Cooling and Homogeneous Droplet Freezing in Laboratory Clouds

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The current global imbalance between incoming solar radiation and outgoing infrared radiation continues to warm our planet. The amount of water in the warming atmosphere also continues to increase. Phase changes of water—evaporation, condensation, and freezing—are the important processes that drive tornadoes, super-cell thunderstorms, hurricanes and other severe weather; but our understanding about these phase-change processes and their interactions with radiation is still insufficient at present. To increase our understanding of these phase changes of water and improve severe weather forecast and long-term climate change prediction, laboratory and analytical/theoretical study will be carried out in this study. This project will help to train a female PhD graduate student who received her master’s degree at an HBCU institution. Local workshops will be conducted for K-12 students on water properties, phase-change heat transfer, and global climate change.In this research, radiatively induced cloud-droplet growth, cooling, and freezing will be studied both experimentally in laboratory-produced clouds and computationally with analytical and theoretical modeling. Laboratory experiments will be conducted in which cloud droplets will be exposed to radiative cooling, similar to what can happen in cloud-top generating cells in extra-tropical, cold-season cyclones or other storm systems. Cooling will be such as to induce homogeneous freezing of droplets. Changes in droplet size due to mass transfer (primarily growth via condensation) will be measured as well as temperature changes. An analytical model of the radiative cooling and condensation process will be developed. The common modeling assumptions of homogeneous (droplet-non-specific) supersaturation and droplet-non-specific radiation, which have recently been questioned, will be investigated and improved upon. Laboratory measurements will be used to develop and validate new modeling assumptions by comparing the measurements with analytical predictions. New theory will be developed for the effect of droplet-specific, non-homogeneous supersaturation. New basic scientific information about the state of homogeneously frozen droplets with solute will also be obtained. These results will impact a fundamental problem in cloud microphysics: the problem of numerically modeling cloud droplet evolution in a complex, constantly changing fluid dynamic, thermodynamic, and radiative atmospheric environment. The results of this study could lead to a new and deeper understanding of basic processes that will not only benefit the environment and climate but other areas of engineering and technology such as phase-change heat exchanger design and electronics cooling. The ultimate broad impact of this study is a better scientific basis for understanding cloud microphysics and for conducting weather prediction and global circulation and climate modeling.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.