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

信风积云中的夹带、超巨粒子和暖雨的形成

• 批准号: 0342421
• 负责人: Sonia Lasher-Trapp
• 金额: $ 35.28万
• 依托单位: Purdue University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-09-01 至 2009-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0342421&HistoricalAwards=false
• 关键词: Entrainment; Ultragiant; Particles; Warm; Rain

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.

暖雨过程占热带地区福尔斯降水的很大一部分。 尽管对这一主题的研究已经进行了50年，但对暖雨产生的定量理解仍然难以捉摸。 人们通常认为，传统的凝结和聚结理论不能在20分钟的观测时间内产生降雨。已经提出了几种想法来解释如何更快地形成直径为50微米的液滴，以启动聚结过程，而另一些人则描述了较小的液滴可能比目前认为的更容易聚结的方式。暖雨是由云外的干燥空气夹带而成，然后在云内混合。 实验室结果表明，云中的一些液滴可能不会受到夹带的影响，而另一些则完全蒸发，因此随后的增长可以在更少的液滴中共享，使它们更大。 PI和她的合作者最近的工作表明，夹带和混合可以产生大的液滴。 从这个模型框架的结果将进行测试和约束的观测，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种机制，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提出了相互矛盾的结果，云内的一个较大的热循环，或较小的夹带在云边缘的涡旋占主导地位。 研究的一个目标是通过分析新的观测结果并结合高分辨率数值模拟来深入了解对积云卷吸非常重要的云运动。积云吸入的超巨型粒子是另一种产生降水引发所需的大滴的潜在机制。 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情况下开始合并。 关于这一机制的最大问题是，大气中是否存在足够的这些粒子，以显着影响积云中的暖雨产生。 研究的最终目标是对大气中超巨粒子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由卷吸和混合形成的大滴的相对重要性进行比较，将在海洋上空积云降雨（RICO）实地活动期间收集数据以支持这些目标。 RICO的目的是描述和了解信风积云在所有尺度上的特性，特别强调降水。 飞机和雷达观测的云将进行分析，并与详细的拉格朗日微物理计算进行高分辨率云模拟，以满足项目objectives.The拟议的工作旨在填补目前的积云卷吸，超巨型粒子，以及它们在暖雨形成的作用的理解漏洞。 它的独特之处在于，由夹带和混合形成的超巨粒子和大滴的作用将在类似的建模框架内进行比较，并受到RICO项目期间收集的关键观测结果的约束。 获得关于这些专题的新知识将有助于在天气和气候预报模型中为对流和降水制定新的参数化，从而影响更广泛的科学努力。