Droplet Activation in Cloud Updrafts

云和气候的计算机模型建立在假设的基础上，这些假设将气溶胶粒子与其上形成的云滴联系起来。观察揭示了这些关系中的不确定性。例如，根据测量的气溶胶颗粒的大小和化学成分计算的云滴浓度通常高估了观测到的浓度。能够作为水滴形成中心的气溶胶是那些对水有亲和力(吸湿性)的气溶胶。在这些粒子中，对大气中的自然云很重要的粒子是那些在环境过饱和度相对较低的情况下形成水滴的粒子。气溶胶种群的云成核能力是由称为云凝结核计数器(CCN计数器)的仪器测量的，该计数器记录单位体积空气中形成的云滴的数量，作为所施加的过饱和度的函数。这个项目是基于使用CCN计数器和其他设备进行的空中观测，研究不同类型云中液滴的形成。这项工作将作为NSF独立支持的两个实地项目的一部分进行：DYCOMS-II，2001年7月在加利福尼亚州海岸外对海洋层积云的研究；以及2002年夏天在怀俄明州对大陆积云的研究。DYCOMS-II(海洋层积动力学和化学)的主要目标是(使用NCARC C-130)获得关于温度、湿度、垂直空气速度和云特性的高分辨率飞机数据，这些数据可用于验证海洋边界层混合过程的大涡模拟。该项目通过提供CCN测量和气溶胶-液滴关系的研究，为DYCOMS做出贡献。同样，它也是基于怀俄明州国王航空公司飞机观测的大陆云研究的重要组成部分。在这两个实地项目中，将使用机载怀俄明州云雷达(毫米波长多普勒雷达)测量云底附近的垂直空气运动，这是测试CCN活动谱与其产生的水滴之间的理论关系所需的参数。这一信息在以前的气溶胶研究中是没有的，可能有助于解决所报告的气溶胶特性和液滴数量之间的一些差异。这项研究的最终目的是改进大气环流和气候的大尺度模式中的云的参数化。

Computer models of clouds and climate rest on assumptions that relate aerosol particles to the cloud droplets that form on them. Observations have revealed uncertainties in these relationships. For example, the concentration of cloud droplets computed from the measured sizes and chemical composition of aerosol particles usually overestimates the observed concentration. The aerosols that are capable of serving as centers for drop formation are the ones with an affinity for water (hygroscopic). Of these, the particles important for natural clouds in the atmosphere are those on which droplets form at relatively low values of ambient supersaturation. The cloud-nucleating ability of an aerosol population is measured by instruments called cloud condensation nucleus counters (CCN counters), which record the number of cloud droplets formed per unit volume of air as a function of the applied supersaturation. This project is an investigation of droplet formation in different types of clouds based on airborne observations with a CCN counter and other equipment. The work will be conducted as part of two field projects that are independently supported by NSF: DYCOMS-II, a study of marine stratocumulus clouds off the coast of California in July 2001; and a Wyoming-based study of continental cumulus clouds in the summer of 2002. The main objective of DYCOMS-II (Dynamics and Chemistry of Marine Stratocumulus) is to obtain high-resolution aircraft data (using the NCAR C-130) on temperature, humidity, vertical air velocity, and cloud properties, that can be used for validating large-eddy simulations of mixing processes in the marine boundary layer. This project contributes to DYCOMS by providing CCN measurements and the study of aerosol-droplet relationships. Likewise, it is an important component of the continental cloud study, based on observations with the Wyoming King Air aircraft. In both of these field projects, the airborne Wyoming Cloud Radar (a mm-wavelength Doppler radar) will be employed for measurements of vertical air motion near cloud base, a parameter that is needed to test the theoretical relationship between the activity spectra of CCN and the drops they produce. This information has not been available in previous aerosol studies, and may help to resolve some of the reported discrepancies between aerosol properties and droplet counts. The ultimate goal of the research is to improve the parameterization of clouds in large-scale models of atmospheric circulation and climate.