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

暖雨过程占热带降雨量的很大一部分。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，传统的凝结理论不能在20分钟的观测时间内产生雨。已经提出了几种方法来解释如何更快地形成直径50微米的水滴来启动合并过程，而另一些人则描述了更小的水滴可能比目前认为的更容易合并的方式。产生引发暖雨所需的大水滴的一个潜在机制是来自云外的干燥空气的夹带和随后在云中的混合。实验室结果表明，云中的一些液滴可能不受卷吸影响，而另一些液滴则完全蒸发，因此随后的增长可以在较少的液滴之间分享，从而使它们变大。PI和她的合作者最近的工作表明，由于夹带和混合，可以产生大液滴。这个模拟框架的结果将与观测结果进行对比和约束，然后扩展到合并过程，以了解卷吸对暖雨形成的影响。此外，为了充分理解和测试这一机制，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给出了相互矛盾的结果，关于云中较大的热循环或云边缘较小的卷吸涡流的主导地位。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，深入了解对积云夹带很重要的云运动。积云吞噬的超大粒子是产生降水所需大水滴的另一种潜在机制。这些粒子比传统的云凝聚核大得多，因此能够在不通过凝结而事先增长的情况下启动合并。关于这一机制的最大问题是，大气中是否存在足够多的这种粒子，从而显著影响积云中的暖雨产生。这项研究的最终目标是可靠地测量大气中超巨粒子的局部浓度，并将它们与由夹带和混合形成的大水滴在形成暖雨方面的相对重要性进行比较。将在海洋积云中的雨(RICO)野外活动中收集数据来支持这些目标。RICO的目标是在所有尺度上描述和了解信风积云的特性，特别是对降水的重视。飞机和雷达对云层的观测将被分析和使用，并在高分辨率云模拟中执行详细的拉格朗日微物理计算，以满足项目目标。拟议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的空白。它的独特之处在于，超大粒子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，并受到在RICO项目期间收集的关键观测的限制。在这些主题上获得新的知识将有助于在天气和气候预报模式中开发新的对流和降水参数化，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.