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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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Coltin Grasmick, Bart Geerts, David D. Turner, Zhien Wang, and T. M. Weckwerth

well-mixed, deep planetary boundary layer (PBL)—to elevated nocturnal convection, which typically organizes at larger scales as the nocturnal stable boundary layer (SBL) deepens, and a low-level jet (LLJ) develops above the SBL ( Corfidi et al. 2008 ; Carbone and Tuttle 2008 ; Reif and Bluestein 2017 ). Convective cells develop when a parcel of air is lofted to its level of free convection (LFC), becoming buoyant with respect to its surrounding environment. Convective cells often initiate in the

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Aaron Johnson, Xuguang Wang, Kevin R. Haghi, and David B. Parsons

conducive for gravity wave ducting ( Crook 1986 ; Koch and Clark 1999 ). The second main ingredient needed for successful NWP forecasts of bores is an accurate forecast of the parent convection that generates the density current, which in turn triggers the bore. This ingredient is satisfied in this study by initializing the simulations from analyses that include the parent convection. This is done using data assimilation that incorporates radar, surface, and upper-air observations. However, it will

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Aaron Johnson and Xuguang Wang

1. Introduction Atmospheric bores are commonly observed in the nocturnal convective environment in the Great Plains ( Haghi et al. 2017 ). This is because the stable boundary layer and low-level jet often provide a suitable wave duct, while convectively generated cold pools frequently provide an obstacle to this stable and ducted low-level flow ( Rottman and Simpson 1989 ; Johnson et al. 2018 ). Several studies have demonstrated the importance of bores in both the initiation and maintenance of

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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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David J. Bodine and Kristen L. Rasmussen

estimates were further refined in Corfidi (2003) to incorporate the effects of the cold pool. In some cases, the MCS may propagate in the direction of new convection initiation and merge with the new convection, leading to enhanced forward propagation through discrete propagation (e.g., Zipser 1977 ; Crook and Moncrieff 1988 ; Fovell et al. 2006 ). Fovell et al. (2006) explored discrete propagation using a numerical model and found that new convection was initiated by gravity waves ahead of the

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

levels on a stretched grid with a 50-hPa model top. The vertical grid spacing is approximately 200 m in the planetary boundary layer increasing to 450 m at 500 hPa. The physical parameterization schemes are fixed for each member following Degelia et al. (2018) and are listed in Table 2 . Table 2. List of the physical parameterization schemes used for all simulations. A different microphysical parameterization scheme is used for the DA and forecast periods. Note that a cumulus parameterization

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Rachel L. Miller, Conrad L. Ziegler, and Michael I. Biggerstaff

( Smull and Houze 1987 ). Two of the main objectives of PECAN are to determine whether nocturnal MCSs are elevated or surface based and to document how these systems interact with the stable nocturnal boundary layer (NBL) to sustain themselves long after sunset ( Geerts 2013 ). Due to surface radiative cooling, it has previously been hypothesized that nocturnal convection is typically elevated and would propagate via gravity currents, undular bores, turbulent bores, and solitary waves ( Carbone et al

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