Quantification of Clustering in Warm (RICO) Cumuli

虽然地球大气中最常见的降水形成模式依赖于冰晶的存在，但在某些情况下，完全被限制在冰点以下的云--即所谓的“暖云”--经历了令人惊讶的快速(通常很难预料到)大小与降水一样大小的粒子的发展。这种发展的关键是碰撞-合并过程，在这个过程中，云大小的液滴相互作用，形成能够从云中坠落的更大的粒子。传统上，在较小的尺寸范围(延伸到100微米)观测到的小成分液滴是不存在的，或者充其量是不可靠的。在过去的五年中，SPEC公司在海军研究办公室的支持下完成了机载2D-S(二维立体)粒子成像探测器的硬件开发，并与以前由美国国家科学基金会支持的积云降雨(RICO)野外活动一起进行了初步评估。这项最初的工作在阐明这一关键尺寸范围内的水滴相互作用方面显示出相当大的前景。特别令人感兴趣的是“聚集”现象，在这种现象中，原本均匀分布的液滴集中在小的空间区域，在那里碰撞-合并类型的相互作用可能会大大加速，因此产生的降雨率通常仅限于含有冰晶的混合相云。这样的条件伴随着中纬度地区几次引人注目的暖季洪水事件，包括1997年科罗拉多州柯林斯堡的洪水。这一事件造成5人死亡，数十人受伤，造成超过2亿美元的损失。此外，可能发生这种过程的较浅/较暖的云(如海洋平流层积云)在副热带地区非常常见，它们似乎在调节地球气候方面发挥着关键作用。这项工作是对美国国家科学基金会/国家大气研究中心C-130机上RICO期间收集的2D-S探测器数据的继续分析。这一仪器提供了关于相对较小的水滴及其在空间上变化的浓度的新信息，并将被用来比过去的研究更可靠地显示和量化暖积云中水滴“聚集”的情况。调查人员将专注于改进统计/数据处理技术(以及相关的计算机软件，其中大部分将提供给2D-S仪器的未来用户)，以消除不具代表性的人工制品对云滴测量的污染，例如飞机和仪器在云层中飞行时雨滴飞溅造成的污染。将根据相对于云边界的位置、云龄、湍流强度(认为能够将液滴旋转到局部集中区)以及整个RICO数据集的其他云特性来评估液滴聚集的存在和程度，整个RICO数据集包括在长达两个月的实验期内的多个长轨迹C-130飞行。在拟议项目的后期阶段，摘要信息Re：这种集群行为将与犹他大学的合作者共享，以便与寻求准确复制云中小规模过程细节的高分辨率计算机模型的输出进行直接比较。因此，RICO观测将被用来指导模型的设计和校准，以便准确地再现所观察到的液滴聚集的程度和适宜的规模。这项工作的“更广泛的影响”最终可能给社会带来好处，从更准确地预测某些山洪暴发事件，到更好地理解在地球辐射平衡中发挥关键作用的浅海云层，从而提高对全球气候的理解。此外，这项研究将继续通过改善研究生和本科生教育(都是在大学)来促进更广泛的影响。在犹他州和SPEC公司所在的科罗拉多州博尔德地区)，并可能被列入即将播出的为期一年的探索频道系列节目，该系列节目重点介绍与全球气候变化相关的进程。

While the most common modes of precipitation formation in the earth's atmosphere rely on the presence of ice crystals, in some instances clouds confined totally below the freezing level--so-called "warm clouds"--undergo surprisingly rapid (and often poorly anticipated) development of large precipitation-sized particles. Key to such developments is the collision-coalescence process in which cloud-sized droplets interact so as to form these larger particles capable of falling out of the cloud. Traditionally, observations of the small component droplets at smaller size ranges (extending to sizes 100 micrometers) have been non-existent or at best unreliable. During the past ~5 years, hardware development of an aircraft-mounted 2D-S (two-dimensional stereo) particle imaging probe by SPEC Inc. was completed in conjunction with support from the Office of Naval Research, with an initial phase of evaluation in conjunction with the Rain in Cumulus over the Ocean (RICO) field campaign previously supported by NSF. This initial work showed considerable promise in illuminating water droplet interactions in this key size range. Of particular interest is the phenomenon of "clustering," in which otherwise uniformly distributed droplets become concentrated in small spatial zones where collision-coalescence type interactions may be greatly accelerated, thus producing precipitation at rates normally confined to mixed-phase clouds incorporating ice crystals. Such conditions have accompanied several notable warm-season flooding events in the midlatitudes, including the Fort Collins, Colorado flood of 1997. This single event killed five people, injured scores of others, and caused in excess of $200 million in damage. Moreover, comparatively shallow/warm clouds (e.g., marine stratocumulus) in which such processes may occur are exceedingly common at subtropical latitudes, where they appear to play a key role in regulating earth's climate.This work represents a continuation of analysis of 2D-S probe data collected during RICO aboard the NSF/NCAR (National Science Foundation/National Center for Atmospheric Research) C-130 aircraft. This instrument provides novel information on comparatively small water droplets and their spatially-varying concentrations, and will be used to more reliably demonstrate and quantify the occurrence of droplet "clustering" in warm cumulus clouds than has been possible in past studies. The investigators will focus on refinement of statistical/data-processing techniques (and associated computer software, much of which will be made available to benefit future users of the 2D-S instrument) to eliminate contamination of cloud droplet measurements by unrepresentative artifacts such as caused by splashing of raindrops as the aircraft and instruments fly through clouds. The presence and degree of droplet clustering will be evaluated with respect to location relative to cloud boundaries, cloud age, intensity of turbulence (thought capable of swirling droplets into locally-concentrated zones) and other cloud properties across the breadth of the entire RICO dataset, which encompassed multiple long-track C-130 flights over a two-month long experimental period. In later stages of the proposed project, summary information re: this clustering behavior is to be shared with a collaborators at the University of Utah, for direct comparison with output from a high-resolution computer model that seeks to accurately replicate details of small-scale processes within clouds. The RICO observations will thus be used to guide design and calibration of the model so as to accurately reproduce the degree and preferred scale of droplet clustering observed. The model can subsequently be improved and more fully exploited to study the persistence and impacts of such clusters upon relevant cloud microphysical processes that control development of warm clouds and precipitation falling from them.The "Broader Impacts" of this work may ultimately have benefits to society ranging from more accurate anticipation of certain flash-flood events to improved understanding of shallow marine clouds that play a key role in earth's radiative balance and hence global climate. In addition, this research will continue to contribute to broader impacts through improved graduate and undergraduate student education (both at the Univ. of Utah and in the Boulder, Colorado area where SPEC Incorporated is located), and is also slated for potential inclusion in an upcoming year-long Discovery Channel series highlighting processes related to global climate change.