Entrainment, Ultragiant Particles, and Warm Rain Formation in Trade Wind Cumulus

暖雨过程占热带地区降水的很大一部分。尽管对这一主题进行了50年的研究，但对暖雨产生的定量理解仍然难以捉摸。通常认为，传统的凝结和聚结理论不能在20分钟的观测时间内产生雨。已经提出了几个想法来解释为什么直径50微米的液滴可以更快地形成以启动聚结过程，而另一些人则描述了小液滴可能比目前认为的更容易聚结的方法。产生引发暖雨所需的大雨滴的一个潜在机制是来自云外的干燥空气的夹带及其随后在云内的混合。实验室结果表明，云中的一些液滴可能不受夹带的影响，而另一些则完全蒸发，因此随后的增长可以由更少的液滴共享，使它们变得更大。PI和她的合作者最近的工作表明，由于夹带和混合，可以产生大的液滴。该模型框架的结果将根据观测结果进行测试和约束，然后扩展到合并过程，以了解夹带对暖雨形成的影响。此外，为了充分理解和测试这一机制，还需要更多关于夹带过程本身的信息。过去的观测和数值研究提出了相互矛盾的结果，即云内较大的热环流或云边缘较小的夹带涡流占主导地位。这项研究的一个目标是通过分析新的观测结果，结合高分辨率的数值模拟，深入了解云的运动，这对积云的夹带很重要。被积云吸收的超大颗粒是产生降水起始所需的大雨滴的另一种潜在机制。这些粒子比传统的云凝结核大得多，因此能够在没有任何凝结的前提下开始合并。关于这一机制的最大问题是，大气中是否存在足够多的这些颗粒，以显著影响积云中暖雨的产生。这项研究的最终目标是对大气中超大颗粒的局部浓度进行可靠的测量，并将它们对暖雨形成的相对重要性与夹带和混合形成的大雨滴的相对重要性进行比较。在海洋积云降雨（RICO）野外活动期间，将收集数据以支持这些目标。RICO的目标是表征和了解信风积云在所有尺度上的特性，特别强调降水。飞机和雷达对云的观测将进行分析，并与高分辨率云模拟中进行的详细拉格朗日微物理计算一起使用，以满足项目目标。提出的工作旨在填补目前对积云夹带，超大粒子及其在暖雨形成中的作用的理解中的漏洞。它的独特之处在于，超大颗粒和由夹带和混合形成的大液滴的作用将在类似的建模框架内进行比较，受RICO项目期间收集的关键观测结果的限制。获得这些主题的新知识将有助于在天气和气候预报模式中开发对流和降水的新参数化，从而影响更广泛的科学努力。

Warm rain processes account for a significant fraction of precipitation that falls in the tropics. Despite fifty years of research on this topic, a quantitative understanding of the production of warm rain has remained elusive. It is commonly stated that traditional condensation and coalescence theory cannot produce rain in the observed time of 20 min. Several ideas have been proposed to explain ways in which drops 50 micron diameter can be formed more quickly to initiate the coalescence process, while others describe ways that smaller drops might coalescence more readily than currently thought.One potential mechanism for producing the large drops needed to initiate warm rain is the entrainment of dry air from outside the cloud and its subsequent mixing within the cloud. Laboratory results have suggested that some droplets in the cloud may not be affected by entrainment, while others completely evaporate, so subsequent growth can be shared among fewer droplets, making them larger. Recent work by the PI and her collaborators has demonstrated that large drops can be produced as a result of entrainment and mixing. Results from this modeling framework will be tested against and constrained by observations, and then be extended to coalescence processes in order to understand the influence of entrainment on warm rain formation. Moreover, to fully understand and test this mechanism, more information is required about the process of entrainment itself. Past observational and numerical studies have presented conflicting results about the dominance of a larger thermal circulation within the cloud, or smaller entraining eddies at the cloud edges. A goal of the research is to gain insight into the cloud motions that are important for cumulus entrainment by analyzing new observations in conjunction with high-resolution numerical modeling.Ultragiant particles ingested by cumulus are another potential mechanism to produce the large drops needed for precipitation initiation. These particles are much larger than traditional cloud condensation nuclei, and are thus capable of initiating coalescence without any prior growth by condensation. The greatest question about this mechanism is whether or not enough of these particles exist in the atmosphere to significantly influence warm rain production in a cumulus cloud. A final goal of the research is to make reliable measurements of local concentrations of ultragiant particles in the atmosphere, and compare their relative importance to warm rain formation with that from the large drops formed by entrainment and mixing.Data will be collected in support of these objectives during the Rain in Cumulus over the Ocean (RICO) field campaign. The objective of RICO is to characterize and understand the properties of trade wind cumulus at all scales, with particular emphasis on precipitation. Aircraft and radar observations of the clouds will be analyzed and used with detailed Lagrangian microphysical calculations performed within high-resolution cloud simulations to meet the project objectives.The proposed work seeks to fill holes in current understanding of cumulus entrainment, ultragiant particles, and their roles in warm rain formation. It is unique in that the roles of ultragiant particles and large drops formed by entrainment and mixing will be compared within similar modeling frameworks, constrained by crucial observations collected during the RICO project. Acquiring new knowledge on these topics will be beneficial for developing new parameterizations for convection and precipitation in weather and climate forecast models, thus affecting broader scientific endeavors.