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.
