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Joshua G. Gebauer, Alan Shapiro, Evgeni Fedorovich, and Petra Klein

the region. The thunderstorms are often elevated, in the sense that storm updrafts develop in an elevated region separated from the surface by a nocturnal stable boundary layer ( Colman 1990 ; Wilson and Roberts 2006 ). One feature associated with the initiation and development of these nocturnal thunderstorms is the Great Plains low-level jet (LLJ) ( Pitchford and London 1962 ; Maddox 1983 ; Astling et al. 1985 ; Trier and Parsons 1993 ; Trier et al. 2006 ; Tuttle and Davis 2006 ). LLJs are

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Elizabeth N. Smith, Joshua G. Gebauer, Petra M. Klein, Evgeni Fedorovich, and Jeremy A. Gibbs

1. Introduction Wind maxima called nocturnal low-level jets (NLLJs) often occur during the night in the lowest kilometer of the atmosphere. In the most general sense, the NLLJ is the result of the disruption of the daytime force balance between the Coriolis, pressure gradient, and frictional forces. Once the sun sets, thermally generated turbulence decays, and the stable boundary layer (SBL) forms. The frictional force weakens above the surface, which eliminates the force balance and leads to

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Thomas R. Parish

1. Introduction Nocturnal low-level wind maxima have received considerable attention during the past few decades. In particular, the Great Plains low-level jet (LLJ) has been the topic of extensive study (e.g., Bonner 1968 ; Mitchell et al. 1995 ; Whiteman et al. 1997 ). Wind profiles in the lowest kilometer at Great Plains sites often show profound day-to-night differences. Weak southerly winds in the lowest several hundred meters often persist throughout the daylight hours only to be

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Brian J. Carroll, Belay B. Demoz, David D. Turner, and Ruben Delgado

1. Introduction The Great Plains low-level jet (LLJ) is a primarily nocturnal phenomenon of strong southwesterly winds within the planetary boundary layer (PBL) spanning hundreds of kilometers in width and length, and is most frequent and impactful during the warm-season. LLJs provide major contributions to nocturnal convection in the region, such as mesoscale convective systems (MCSs), via convergence of the wind field and advection of moisture and temperature ( Byerle and Paegle 2003 ; Trier

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Guo Lin, Coltin Grasmick, Bart Geerts, Zhien Wang, and Min Deng

5 . Conclusions are presented in section 6 . 2. Data and methodology a. The 2015 PECAN campaign The 2015 PECAN field campaign aimed to understand the driving mechanisms of nocturnal MCSs, bores, and CI in the presence of a SBL and a low-level jet ( Geerts et al. 2017 ). Cold-pool-generated bores and related solitary waves were found to be relatively common during PECAN. Among other objectives, the PECAN campaign aimed to advance the understanding of the generation and evolution of bores, and

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Stacey M. Hitchcock and Russ S. Schumacher

several different methods of classification, but two distinct categories consistently emerge for events over the central Great Plains. In synoptic-type events, a strong midtropospheric trough and slow moving surface front lead to strong forcing for ascent in a region with southerly flow and associated moisture transport. During the warm season, isentropic ascent of warm, moist air transported by the nocturnal low-level jet (LLJ) can lift an air to saturation on the cool side of a stationary or warm

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Matthew D. Flournoy and Michael C. Coniglio

for the generation of mesovortices can be augmented by the presence of a rear-inflow jet (RIJ), a relatively common feature of mature QLCSs ( Weisman 1992 ) that is frequently associated with severe winds at the surface. Severe winds may or may not be associated with low-level mesovortices, but when they are, the strongest winds are typically found within the equatorward portion of the mesovortex ( Atkins and St. Laurent 2009a ), where the motion of the system, enhanced winds in the RIJ, and

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Samuel K. Degelia, Xuguang Wang, David J. Stensrud, and Aaron Johnson

lee of the Rocky Mountains ( Carbone et al. 2002 ; Li and Smith 2010 ), convective feedbacks such as gravity waves and bores ( Carbone et al. 2002 ; Marsham et al. 2011 ), and the Great Plains low-level jet (LLJ; Pitchford and London 1962 ; Trier and Parsons 1993 ; Higgins et al. 1997 ). The LLJ is a particularly important phenomenon that provides a source of buoyancy ( Trier and Parsons 1993 ; Helfand and Schubert 1995 ; Higgins et al. 1997 ) and forcing ( Pitchford and London 1962

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Tammy M. Weckwerth and Ulrike Romatschke

maximum in the central United States. These include, but are not limited to, deep tropospheric gravity waves generated by the Rocky Mountains elevated heat source (e.g., Tripoli and Cotton 1989a , b ; Mapes et al. 2003a , b ; Warner et al. 2003 ), potential vorticity anomalies (e.g., Raymond and Jiang 1990 ; Li and Smith 2010 ), gravity waves generated by convection (e.g., Fovell et al. 2006 ), influence by the low-level jet (LLJ; Trier and Parsons 1993 ; Fritsch and Forbes 2001 ; Keene and

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Manda B. Chasteen, Steven E. Koch, and David B. Parsons

generally supported by the poleward advection of high equivalent potential temperature ( ) air by a nocturnal low-level jet (LLJ; Fritsch and Maddox 1981 ; Maddox 1983 ). The LLJ may interact with a surface front to provide a focus for convection initiation (CI; Maddox 1983 ; Trier and Parsons 1993 ; Moore et al. 2003 ) or support CI in the absence of a surface boundary ( Wilson and Roberts 2006 ; Pu and Dickinson 2014 ; Reif and Bluestein 2017 ; Gebauer et al. 2018 ). Daytime MCSs are typically

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