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，完全限制在冰点以下的云-所谓的“暖云”-经历了令人惊讶的快速（而且往往预测不良）发展的大降水颗粒。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，形成能够从云中落下的较大颗粒。传统上，在较小尺寸范围（延伸到100微米的尺寸）的小组分液滴的观察是不存在的或充其量是不可靠的。在过去的5年中，SPEC公司的机载2D-S（二维立体）粒子成像探头的硬件开发。在海军研究办公室的支持下完成，最初阶段的评估与先前由NSF支持的海洋上空积云降雨（RICO）实地活动相结合。这项初步的工作显示了在这个关键尺寸范围内阐明水滴相互作用的相当大的希望。特别令人感兴趣的是“成簇”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中，在这些区域中碰撞-聚结型相互作用可能会大大加速，从而以通常限于包含冰晶的混合相云的速率产生降水。这种情况伴随着中纬度地区几次显著的暖季洪水事件，包括1997年科罗拉多的柯林斯堡洪水。这一事件造成5人死亡，数十人受伤，并造成超过2亿美元的损失。此外，相对较浅/温暖的云（例如，海洋层积云）在亚热带地区非常常见，它们在调节地球气候方面起着关键作用。这项工作是对NSF/NCAR（国家科学基金会/国家大气研究中心）C-130飞机上RICO期间收集的2D-S探测数据分析的继续。该仪器提供了关于相对较小的水滴及其空间变化浓度的新信息，并将用于比过去的研究更可靠地证明和量化暖积云中水滴“聚集”的发生。研究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，其中大部分将用于2D-S仪器的未来用户），以消除不具有代表性的人为因素对云滴测量的污染，例如飞机和仪器飞过云层时雨滴飞溅造成的污染。水滴聚集的存在和程度将根据相对于云边界的位置，云年龄，湍流强度（认为能够将水滴旋转到局部集中的区域）以及整个RICO数据集的其他云属性进行评估，该数据集包括多个长轨道C-130飞行超过两个月的实验期。在拟议项目的后期阶段，将与犹他州大学的合作者分享这种聚类行为的摘要信息，以便与高分辨率计算机模型的输出进行直接比较，该模型旨在准确复制云中小规模过程的细节。因此，RICO观测结果将用于指导模型的设计和校准，以便准确地再现所观测到的液滴聚集的程度和首选规模。该模式随后可以得到改进和更充分的利用，以研究这些云团的持续性和对控制暖云发展和降水的相关云微物理过程的影响。这项工作的“更广泛影响”最终可能对社会产生好处，包括更准确地预测某些闪光，洪水事件，以提高对浅海云的理解，在地球的辐射平衡，从而全球气候中发挥关键作用。此外，这项研究将继续通过改善研究生和本科生教育（无论是在犹他州大学和博尔德，科罗拉多地区的SPEC公司所在地），以促进更广泛的影响，也是预定在即将到来的为期一年的探索频道系列突出与全球气候变化有关的过程的潜在列入。

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.