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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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Aaron Johnson and Xuguang Wang

in analyses valid at 1300 UTC, which are used to initialize a deterministic forecast from the ensemble mean analysis. The quality of the analyses is evaluated by verification of the forecasts, which all use the same physics configuration and differ only in their initial conditions. The forecast physics configuration is Thompson’s microphysics scheme, the Mellor–Yamada–Nakanishi–Niino (MYNN; Nakanishi and Niino 2009 ) planetary boundary layer (PBL) scheme, the Goddard shortwave ( Tao et al. 2003

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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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Sean Stelten and William A. Gallus Jr.

as several runs of a 4-km WRF-ARW model using four different planetary boundary layer (PBL) schemes, are verified to study the predictability of these initiation events. The goal of the present study is to identify the basic characteristics of and prediction deficiencies associated with nocturnal elevated CI. 2. Data and methodology a. Classification of CI events This study examines likely elevated pristine nocturnal CI (PNCI) cases during May–August of 2015. Several dates were omitted from 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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