Collaborative Research: Impacts of Microphysical, Thermodynamic, and Dynamical Processes on Nocturnal and Oceanic Convective Systems via Analyses from PECAN and HAIC/HIWC

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Mesoscale convective systems (MCSs) are responsible for a large portion of summertime precipitation in the Great Plains and Midwest of the United States and produce a large range of hazards, including strong winds, hail, flash flooding and occasional tornadoes. Thus, accurate understanding and forecasting of these systems is important for public safety. Processes responsible for the growth and evolution of a particular class of MCSs, occurring at night, are especially poorly understood. This project examines mechanisms responsible for the evolution of such nocturnal storms using a unique set of data obtained by an instrumented aircraft flying in clouds generated by and trailing behind nocturnal thunderstorms during the 2015 Plains Elevated Convection at Night (PECAN) field experiment. In particular, the project is assessing how small-scale processes occurring in clouds generate cold air which in turn generates features that act to force new air masses to rise, leading to the development of new thunderstorms that subsequently generate more trailing clouds and rain. The second component of the project examines the properties of clouds generated by thunderstorms occurring in the Tropics. This component was motivated by a problem faced by aviation, namely that aircraft flying at commercial flight altitudes can encounter conditions where there is no noticeable echo on their radar yet encounter large amounts of small ice crystals not detected by the radar; such conditions can be problematic for aircraft engines as evidenced by several engine events over the last 30 years. This project uses data collected by research aircraft flying in such conditions. The research aircraft were instrumented with probes measuring cloud properties during the 2015 and 2016 High Ice Water Content (HIWC) projects conducted off the coast of Darwin, Australia and French Guiana. The project is examining processes that lead to the development of large numbers of small ice crystals by identifying the environmental conditions under which these small crystals form and grow. In both projects, model simulations are also being used to evaluate the role of specific processes in driving and maintaining thunderstorms (PECAN) and for generating small ice crystals (HIWC). Further, both projects acquired next generation polarimetric radar data coincident with the aircraft cloud observations so that cloud properties retrieved from polarimetric radar data can be validated, meaning that data from polarimetric radar in the future can ultimately be used to extend the limited range duration of the field experiment measurements. At a more technical level, the PECAN data are being used to evaluate the hypothesis that microphysical cooling processes in developing and mature stratiform regions of MCSs force downdraft circulations that create mesoscale gravity wave features on the stable nocturnal boundary layer (SNBL) that in turn focus, organize and maintain future convective activity. In context of storm kinematics and dynamics as derived from multiple Doppler radar analyses and modeling investigations, the PECAN data are being used to (1) characterize the microphysical and thermodynamic structure of the transition zone, notch and rear anvil regions in formative, mature and dissipative stages of the PECAN elevated nocturnal MCSs; (2) quantify and understand how the horizontal and vertical distribution of latent cooling evolves across MCSs during their lifecycles and contributes to downdrafts and gravity wave features; and (3), determine how gravity wave features on the SNBL give rise to lifting that then drives convection and maintains the organization and long lifetime of nocturnal MCSs. The HIWC data are being used to (1) characterize the microphysical and thermodynamic properties of high IWC regions and contrasting them against properties derived from observations obtained in regions without high IWCs; (2) determine polarimetric radar signatures of high IWC regions and investigate how these signatures differ depending upon the relative prevalence of small ice crystals; (3) develop representations of size distributions and mass-dimensional relationships for high IWC regions as a volume or surface of equally realizable solutions in the appropriate coefficient phase space; and (4) conduct simulations to identify processes responsible for the numerous small ice crystals found above oceanic convective cores, and compare against HIWC observations. The broad impacts are (1) improved understanding so that operational forecasters and nowcasters can better anticipate the evolution of convective phenomena; (2) enhanced education and training opportunities (several graduate students are earning advanced degrees); (3) incorporation of PECAN and HAIC/HIWC data into courses taught and textbooks published by the PIs; (4) advanced scientific understanding of high IWC conditions that have led to about 150 power loss and/or pitot tube failures in commercial aircraft over the last several years.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.