Collaborative Research: Understanding Observations of High Ice Water Contents in Convective Cloud Systems over Tropical Oceans

近年来，商用飞机在热带海洋上空多次经历了令人惊讶的（至少一次是灾难性的）发动机故障和/或结冰，影响了关键的滑流探测器（例如，用于空速指示的皮托管），同时穿透了其他不寻常的云层，伴随着微弱的（20 dBZ）雷达反射率值。没有大的、高反射的冰粒或混合相条件下的过冷液体特征，它们在飞机结冰中的作用已经被很好地理解，这表明这些区域有高冰水含量（iwc），以小冰晶为主。这种遭遇所固有的安全隐患促使多机构、多国协同开展研究工作，即高冰水含量研究（HIWC），旨在描述和了解这种情况。HIWC的主要现场阶段将在夏季季风期间以澳大利亚的达尔文为基地，并将涉及由HIWC原始组织者提供的仪器精良的湾流- ii飞机穿透0至-50摄氏度之间的强对流海洋云。这里描述的努力将使这些额外的美国调查人员能够以补充HIWC原始组织者计划的方式访问和分析独特的HIWC数据集。这些nsf支持的研究人员将专注于模式模拟的实施和分析，以及适合这些模式在海洋对流云中使用的参数化的发展，以检查特定假设的有效性，即在只有小晶体存在的情况下高iwc的原因。要评估的这些机制包括以下可能性：(1)存在几乎未稀释的非常强的上升气流，夹带最小，当过冷液滴迅速上升到高水平（即温度范围从-35℃到-40℃）时，会发生均匀的冻结，从而在绝热水含量超过7克/立方米的区域产生高浓度的小冰晶；(2)在-10到10摄氏度之间的弱上升气流（0- 10m /s）中，相当一部分云水通过“暖雨”过程形成降水大小的颗粒，但随后冻结并通过哈莱特-莫索普过程支持冰的繁殖，导致大量小冰晶随后上升到商用飞机运行的高空；(3)上升气流强大到足以将雨滴抬升到太冷的水平，使哈莱-莫索普过程无法运行，提供冷凝水负载，减缓上升气流，使雨冻结，释放潜热，随后激活上层深层对流，有利于霰/冰产生高浓度的小颗粒；(4)狭窄、不稳定的“多炮塔”上升气流表现出高度的小尺度变异性，使得上述任何一种机制都可能在特定的时间和/或地点起作用；(5)被认为与高IWC值有关的接触中发生的飞机故障，实际上是由于长时间飞行穿过IWC值仅为每立方米1-2克的较中等范围的铁砧造成的。这些研究人员将重点分析来自云微物理探测器的现场观测结果，以量化假设的伽玛型云和降水粒径分布（通常用于中尺度模型）的斜率、形状和y截距如何随关键预测变量（如IWC、温度和垂直空气速度）而变化，并关注对流核心、附近云顶或其他更遥远区域遇到的不同条件。他们还将进行模式模拟，并根据原位和遥感数据进行评估，以探索上述假设在解释由大量小颗粒组成的高IWC区域的存在方面的有效性，并进一步发展和评估这些模型中使用的参数化。NSF的支持还将允许该团队直接参与G-II的试飞（计划于2013年夏末在佛罗里达州进行），并在达尔文的主要野外活动之前进行后续的初步分析。这项工作的智力价值在于尽早获得一个独特的数据集（国家科学基金会没有直接成本），并结合观测和基于模型的微物理条件分析，这些条件被认为是在一些热带海洋雷暴的核心内部和周围独特发生的，以便更好地表征产生高冰水含量条件的机制，这些条件被认为会影响商用飞机。更广泛的影响将来自于对高空飞行的商业客机造成航空危害的过程和条件的更好理解，包括那些可能由NSF的湾流级研究飞机经历的过程和条件，以及通过改进的热带云参数化的发展，这些参数化适用于与全球气候模型评估相关的数值模型和遥感检索方案。进一步的影响将通过亲自参与实地数据收集和相关的研究生培训和指导参与这项研究的博士后研究员。

In recent years commercial aircraft have on multiple occasions experienced surprising (and in at least one case, catastrophic) engine failures and/or ice accumulations impacting key slipstream probes (e.g., pitot tubes used for airspeed indication) while penetrating otherwise unremarkable clouds accompanied by only weak (20 dBZ) radar reflectivity values over the tropical oceans. The absence of large, highly-reflective ice particles or supercooled liquid characteristic of mixed-phase conditions, whose role in aircraft icing is fairly well understood, suggests these regions have high ice water contents (IWCs) dominated by small ice crystals. Safety hazards inherent in such encounters have motivated a concerted multi-agency, multi-national research effort, the High Ice Water Content Study (HIWC), aimed at characterizing and understanding such conditions. The primary HIWC field phase will be based out of Darwin, Australia during austral summer monsoon, and will involve penetrations of strong convective oceanic clouds between 0 and -50 degC by a well-instrumented Gulfstream-II aircraft supplied by original HIWC organizers. The effort described here will allow these additional U.S.-based investigators to access and analyze the unique HIWC dataset in ways complementary those planned by the original organizers of HIWC.These NSF-supported investigators will focus on the conduct and analysis of model simulations, and on the development of parameterizations suitable for use by such models in oceanic convective clouds to examine the validity of specific hypotheses re: causes of high IWCs in the presence of only small crystals. These mechanisms to be evaluated include the possibility that: (1) The presence of near-undilute and very strong updrafts with minimal entrainment, in which homogeneous freezing takes place when supercooled droplets are quickly lofted to high levels (viz. temperatures ranging from-35 to -40 degC), thus resulting large concentrations of small ice crystals in zones where adiabatic water contents exceed 7 grams per cubic meter; (2) A considerable fraction of cloud water in weak (0-10 m/s) updrafts between -10 and 10 degC forms precipitation-sized particles via a "warm rain" process, but then freezes and supports ice-multiplication via the Hallett-Mossop process leading to large numbers of small ice crystals that are subsequently lofted to high levels where commercial aircraft operate; (3) Updrafts strong enough to loft raindrops to levels too cold for the Hallett-Mossop process to operate provide condensate loading that slows updrafts so that rain freezes, releasing latent heat that subsequently invigorates deep upper-level convection favoring graupel/ice generating high concentrations of small particles; (4) Narrow, unsteady "multi-turreted" updrafts exhibiting a high degree of small-scale variability exist such that any/all of the above mechanisms might operate at specific times and/or locations; and (5) Aircraft malfunctions occurring in exposures thought to be associated with high IWC values are actually the result of extended flight through anvils possessing IWCs only in the more moderate range of 1-2 grams per cubic meter. These investigators will focus on analysis of in-situ observations from cloud microphysics probes to quantify how the slope, shape and y-intercept of assumed gamma-type cloud and precipitation particle size distributions (as commonly employed in mesoscale models) vary with key prognostic variables such as IWC, temperature, and vertical air velocity with attention to differing conditions encountered in convective cores, nearby cloud anvils, or other more distant regions. They will also conduct model simulations, to be evaluated against HIWC in-situ and remote sensing data, to explore the validity of the above hypotheses for explaining the existence of high IWC regions consisting of large number of small particles and to further develop and evaluate parameterizations used in such models. NSF support will also allow this team to directly participate in G-II test flights (planned for Florida during late-summer of 2013) and follow-on preliminary analyses to be conducted in advance of the primary field activities to be based out of Darwin.The intellectual merit of this effort rests upon early access to a unique dataset (collected at no direct cost to NSF) and combined observational and model-based analysis of microphysical conditions thought to uniquely occur in and around the cores of some tropical-oceanic thunderstorms in order to better characterize mechanisms involved in generation of high ice water content conditions thought to impact commercial aircraft. Broader Impacts will derive from improved understanding of processes and conditions contributing to aviation hazards for high flying commercial airliners and including those that may be experienced by NSF's Gulfstream-class research aircraft, and through development of improved parameterizations for tropical clouds as applicable to both numerical models and remote sensing retrieval schemes relevant to evaluation of global climate models. Further impacts will come through hands-on involvement in field data collection and associated training of graduate students and mentoring of postdoctoral fellows involved in this research.