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

近年来，商用飞机已经多次经历了令人惊讶的（并且在至少一种情况下是灾难性的）发动机故障和/或影响关键滑流探针（例如，用于空速指示的皮托管），同时穿透热带海洋上空雷达反射率值仅为弱（20 dBZ）的普通云层。 大的，高反射性的冰粒或过冷液体的混合相条件下，其在飞机结冰的作用是相当好的理解的特征的情况下，表明这些地区有高的冰水含量（IWC）占主导地位的小冰晶。 这种遭遇中固有的安全危险促使多机构、多国进行协调一致的研究工作，即高冰水含量研究，目的是描述和了解这种情况。 主要HIWC现场阶段将在澳大利亚夏季季风期间在澳大利亚达尔文之外进行，并将涉及由原始HIWC组织者提供的仪器齐全的湾流-II飞机在0和-50摄氏度之间穿透强对流海洋云。 这里所描述的努力将使这些额外的美国-这些由国家科学基金会资助的研究人员将侧重于模式模拟的进行和分析，以及开发适用于海洋对流云中此类模式的参数化，以检查特定假设的有效性，这些假设涉及：仅存在小晶体时高IWC的原因。 这些有待评估的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，其中当过冷液滴迅速上升到高水平时发生均匀冻结（即温度范围为-35至-40 ℃），从而在绝热水含量超过7克/立方米的区域中产生大量的小冰晶;（2）相当一部分云水在弱（0-10 m/s）-10至10 ℃的上升气流通过“暖雨”过程形成降水大小的颗粒，但随后冻结并通过Hallett-Mossop过程支持冰增殖，导致大量的小冰晶，随后在商用飞机运行时上升到高水平;（3）上升气流足够强，使雨滴上升到对Hallett-Mossop过程来说太冷的水平，从而提供冷凝物加载，减缓上升气流，使雨水冻结，释放潜热，随后使深层上层对流变热，有利于产生高浓度的小颗粒的<$子/冰;（4）存在狭窄、不稳定的“多塔楼”上升气流，表现出高度的小尺度变化，因此上述机制中的任何/所有机制都可能在特定时间和/或地点起作用;以及（5）在被认为与高IWC值相关的暴露中发生的飞机故障实际上是通过具有IWC的砧座进行长时间飞行的结果仅在1-2克/立方米的更适度的范围内。 这些研究人员将集中分析云微物理探测器的现场观测结果，以量化假设的伽马型云和降水粒子大小分布的斜率、形状和y截距（如中尺度模式中常用的）随关键预报变量（如IWC、温度和垂直空气速度）而变化，并注意对流核心、附近云砧或其他更远的地区。 他们还将进行模型模拟，并将对照重武器公约的现场和遥感数据进行评估，以探讨上述假设是否能够解释由大量小颗粒组成的高重武器公约区域的存在，并进一步开发和评估此类模型中使用的参数化。 NSF的支持也将使这个团队直接参与G-II试飞（计划于2013年夏末在佛罗里达进行），并在基于达尔文的主要实地活动之前进行后续初步分析。这项工作的智力价值在于抢先体验独特的数据集（收集时没有直接费用给NSF），并结合观测和基于模型的微物理条件分析认为，独特的发生在核心和周围的一些热带-海洋雷暴，以便更好地表征被认为影响商用飞机的高冰水含量条件的产生所涉及的机制。 更广泛的影响将来自对高空飞行商业客机造成航空危害的过程和条件的更好理解，包括NSF湾流级研究飞机可能经历的危险，以及通过改进热带云的参数化，适用于与全球气候模型评估相关的数值模型和遥感检索计划。 进一步的影响将通过实地数据收集和相关的研究生培训以及对参与这项研究的博士后研究员的指导来实现。

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.