Precipitation and Coalescence Scavenging in Shallow Southern Ocean Clouds

海洋边界层中的云（比如海洋表面上方一公里的大气层）具有全球冷却效应，因为它们将阳光反射回太空，但太低而无法有效地捕获向外的红外辐射。这些云的气候效应已成为气候变化研究中的一个重要问题，因为温度升高可能会改变云的反射率或其典型寿命，这些因素决定了其冷却效应的强度。 海洋边界层云的反射率在很大程度上取决于云凝结核（CCN）的丰度，云凝结核是一种微小的气溶胶颗粒，它从大气中吸收水分，为云滴的生长提供种子。 当云凝结核更丰富时，可用的云水分布在大量的小水滴上，导致云的反射率比云凝结核更低，因此更少的大水滴。 由较小的水滴组成的云也会持续更长的时间，因为云的水滴会在碰撞和合并的过程中结合在一起当水滴较小时，形成一个雨滴需要更长的时间，因此需要更多的雨滴，这意味着一滴足够大的雨滴可以从云中落下。PI之前的工作为CCN开发了一个简化的预算方程，将CCN丰度的变化与来源联系起来，包括海洋表面生物过程产生的气溶胶，以及包括降水在内的汇。 降水在从液态云中去除云凝结核方面非常有效，因为雨滴中大约一百万个水滴中的每一个都含有一个云凝结核。 PI使用其预算分析表明，CCN通过降水去除，称为降水清除或聚结清除，是CCN去除的主要机制，并解释了海洋CCN丰度的地理变异性。 他们还利用他们的预算推导出液滴数浓度（Nd）的公式。在假定云凝结核的源和汇是平衡的，并且云中所有的云凝结核都有种子液滴的情况下，海洋边界层云中的云凝结核。这里所做的工作使用2018年南大洋期间收集的观测数据，将收支方程和Nd公式应用于南大洋风暴轴上的边界层云中。云、辐射、气溶胶、传输实验研究（SOCRATES），这是一项实地活动，使用研究飞机在塔斯马尼亚南部海洋上空的飞行中对云进行采样（见AGS-1660609）。该飞机测量了云内、云上和云下的云凝结核浓度，并使用雷达和激光雷达观测云滴和雨滴，这些信息可与卫星数据和气象分析相结合，以确定云凝结核浓度并测量云凝结核的源和汇。 预算研究的结果进行了比较，从大涡模拟（LES）模型，产生详细的云，云凝结核，降水模拟的基础上大规模的气象输入和测量云凝结核浓度的输出。 该项目还处理和分析来自飞机雷达和激光雷达的数据，以检查SO云产生的降水，观察产生的降水量，雨滴的大小分布，以及冰相降水产生的程度。这项工作具有社会和科学意义，因为低云提供的冷却程度的变化是估计中最大的不确定性之一全球温度对温室气体的敏感性增加。 SOCRATES运动的主要动机是担心SO云在用于预测未来气候变化的气候模型中表现不佳。 该项目还生成了云、降水和云凝结核属性的数据集，可供全球气候研究人员使用。 此外，该奖项还为研究生提供支持和培训，从而促进该研究领域未来的劳动力。该奖项反映了NSF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。

Clouds in the marine boundary layer (say the kilometer of the atmosphere just above the ocean surface) have a global cooling effect as they reflect sunlight back to space but are too low to effectively trap outgoing infrared radiation. The climatic effect of these clouds has become an important issue in climate change research, as warmer temperatures may alter either the reflectivity of the clouds or their typical lifetimes, the factors which determine the strength of their cooling effect. The reflectivity of marine boundary layer clouds is determined in large part by the abundance of cloud condensation nuclei (CCN), the tiny aerosol particles which absorb moisture from the atmosphere to seed the growth of cloud droplets. When CCN are more abundant the available cloud water is spread over a larger number of smaller droplets, leading to a more reflective cloud than would occur with fewer CCN and hence a smaller number of larger droplets. Clouds made of smaller droplets also last longer, as the process of collision and coalescence that combines cloud droplets (perhaps a million or so) to form a raindrop takes longer when the droplets are smaller and thus more are needed to make a raindrop, meaning a drop big enough to fall from the cloud.Previous work by the PIs developed a simplified budget equation for CCN relating changes in CCN abundance to sources, including the generation of aerosols by biological processes in the surface ocean, and sinks, including precipitation. Precipitation is quite effective in removing CCN from liquid clouds as each one of the million or so droplets in a raindrop contains a CCN. The PIs used their budget analysis to show that CCN removal by precipitation, referred to as precipitation scavenging or coalescence scavenging, is the dominant mechanism for CCN removal and accounts for much of the geographic variability of CCN abundance over the oceans. They also used their budget to derive a formula for the droplet number concentration (Nd) in marine boundary layer clouds under the assumption that the CCN sources and sinks are balanced and all the CCN in a cloud have seeded droplets.Work performed here applies the budget equation and the Nd formula to the boundary layer clouds in the storm track over the Southern Ocean using observations collected during the 2018 Southern Ocean Clouds, Radiation, Aerosol, Transport Experimental Study (SOCRATES), a field campaign that used a research aircraft to sample clouds on flights over the ocean south of Tasmania (see AGS-1660609). The aircraft measured CCN concentrations in, above, and below the clouds, and used radar and lidar to observe cloud droplets and raindrops, information which can be combined with satellite data and meteorological analysis to determine CCN concentrations and measure CCN sources and sinks. Results of the budget study are compared with output from a Large Eddy Simulation (LES) model which generates detailed clouds, CCN, and precipitation simulations based on large-scale meteorological inputs and measurements of above-cloud CCN concentrations. The project also processes and analyzes data from the aircraft radar and lidar to examine the precipitation produced by the SO clouds, looking at the amount of precipitation produced, the size distribution of raindrops, and the extent to which ice phase precipitation is also produced.The work is of societal as well as scientific interest as change in the extent of cooling provided by low clouds is among the largest uncertainties in estimates of the sensitivity of global temperature to greenhouse gas increases. The SOCRATES campaign was largely motivated by concern that the SO clouds are poorly represented in climate models used to make projections of future climate change. The project also produces datasets on cloud, precipitation, and CCN properties that can be used by the worldwide community of climate researchers. In addition, the award provides support and training for a graduate student, thereby promoting the future workforce in this research area.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.