Radiative Cooling and Homogeneous Droplet Freezing in Laboratory Clouds

目前太阳辐射和红外辐射之间的全球不平衡继续使地球变暖。在变暖的大气中，水的含量也在继续增加。水的相变——蒸发、凝结和冻结——是形成龙卷风、超级单体雷暴、飓风和其他恶劣天气的重要过程；但目前我们对这些相变过程及其与辐射的相互作用的认识仍然不足。为了提高我们对这些水的相位变化的认识，改进恶劣天气预报和长期气候变化预测，本研究将进行实验室和分析/理论研究。本项目将帮助培养一名在HBCU机构获得硕士学位的女博士研究生。当地将为K-12学生举办有关水性质、相变传热和全球气候变化的讲习班。在这项研究中，辐射诱导的云滴生长、冷却和冻结将在实验室产生的云中进行实验研究，并通过分析和理论模型进行计算。将进行实验室实验，使云滴暴露在辐射冷却下，类似于在热带外、冷季气旋或其他风暴系统的云顶生成细胞中发生的情况。冷却可以使液滴均匀地冻结。将测量由于传质（主要是通过冷凝生长）引起的液滴尺寸变化以及温度变化。建立了辐射冷却和冷凝过程的解析模型。本文将对最近受到质疑的均匀（液滴-非特异性）过饱和和液滴-非特异性辐射的常见建模假设进行研究和改进。实验室测量将用于通过比较测量和分析预测来开发和验证新的建模假设。新的理论将发展液滴特异性，非均匀过饱和的影响。还将获得关于溶质均匀冻结液滴状态的新的基础科学信息。这些结果将影响云微物理学中的一个基本问题：在复杂的、不断变化的流体动力学、热力学和辐射大气环境中对云滴演化进行数值模拟的问题。这项研究的结果可能会导致对基本过程的新的和更深入的理解，这不仅有利于环境和气候，而且有利于其他工程和技术领域，如相变热交换器设计和电子冷却。这项研究的最终广泛影响是为理解云微物理、进行天气预报、全球环流和气候模拟提供了更好的科学基础。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.