Droplet Freezing in the Atmosphere

用于预测降雨量和云对太阳和地面辐射影响的大气模型对云中冰晶类型及其浓度的变化很敏感，但对云中水滴冻结的过程还不够深入，无法对冰含量做出可靠的估计。该项目是对在实验室观察或理论基础上提出的几种液滴冻结机制的大气相关性进行的调查。这包括溶液液滴的均匀冻结和由于蒸发而导致的非均匀冻结，蒸发产生有机表面膜或液滴内的固相包裹体。这些过程的数值模拟将与几个现场方案的现有数据进行比较，以确定模拟结果可以在多大程度上符合观测结果。其中一个目标是研究云边缘的蒸发可以通过非均相成核促进冻结的条件。另一种是试图解释最近的观察结果，即溶液液滴的均匀冻结温度只取决于水的活度，水活度是液滴与纯水相比平衡蒸汽压降低的量度。这项研究的结果将从根本上有助于了解大气中冰的形成，从而能够改进天气和气候模型，在这些模型中，云组成的细节是重要的。

Atmospheric models used to predict precipitation and the effects of clouds on solar and terrestrial radiation are sensitive to variations in the types of ice crystals in clouds and their concentration, yet the processes by which drops freeze in clouds are not understood well enough to make reliable estimates of the ice content. This project is an investigation of the atmospheric relevance of several droplet freezing mechanisms that have been suggested on the basis of laboratory observations or theory. These include homogeneous freezing of solution drops and heterogeneous freezing as a result of evaporation that creates either organic surface films or solid phase inclusions within drops. Numerical modeling of these processes will be compared with existing data from several field programs to determine the extent to which the modeling results can be brought into accord with observations. One of the objectives is to study the conditions under which evaporation at the edges of clouds can promote freezing by heterogeneous nucleation. Another is to attempt to explain recent observations indicating that the homogeneous freezing temperature of solution drops depends only on the water activity, a measure of the reduction in equilibrium vapor pressure of the drop compared to pure water. Results of this research will contribute fundamentally to understanding ice formation in the atmosphere, and thus enable the improvement of weather and climate models in which the details of cloud composition are important.