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Bart Geerts, David Parsons, Conrad L. Ziegler, Tammy M. Weckwerth, Michael I. Biggerstaff, Richard D. Clark, Michael C. Coniglio, Belay B. Demoz, Richard A. Ferrare, William A. Gallus Jr., Kevin Haghi, John M. Hanesiak, Petra M. Klein, Kevin R. Knupp, Karen Kosiba, Greg M. McFarquhar, James A. Moore, Amin R. Nehrir, Matthew D. Parker, James O. Pinto, Robert M. Rauber, Russ S. Schumacher, David D. Turner, Qing Wang, Xuguang Wang, Zhien Wang, and Joshua Wurman

nocturnal environment that allows convection to become organized and propagate across the plains even as surface-based CAPE vanishes ( Heideman and Fritsch 1988 ; Tripoli and Cotton 1989a , b ). A multistep process is envisioned in which mesoscale convection over this region has its origin over the Rocky Mountains, which act as an elevated heat source. Convectively induced deep-tropospheric gravity waves emerge over the elevated terrain and propagate eastward, triggering nocturnal convective systems

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

. Rev. , 139 , 247 – 271 , . 10.1175/2010MWR3422.1 Martin , W. J. , and A. Shapiro , 2007 : Discrimination of bird and insect radar echoes in clear air using high-resolution radars . J. Atmos. Oceanic Technol. , 24 , 1215 – 1230 , . 10.1175/JTECH2038.1 Maxworthy , T. , 1980 : On the formation of nonlinear internal waves from the gravitational collapse of mixed regions in two and three dimensions . J. Fluid

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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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Tammy M. Weckwerth, John Hanesiak, James W. Wilson, Stanley B. Trier, Samuel K. Degelia, William A. Gallus Jr., Rita D. Roberts, and Xuguang Wang

array of PECAN Integrated Sounding Arrays (PISAs) to sample the vertical profiles of lower-tropospheric winds, temperature, and water vapor. These PISAs and the scanning radars and lidars of PECAN were used to sample the specific features (e.g., LLJ, bores, and gravity waves) and atmospheric regions (e.g., SBL and lower troposphere) relevant to better understanding of NCI. This manuscript brings together past NCI work and PECAN data to document frequencies of different NCI types, as categorized by

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Hristo G. Chipilski, Xuguang Wang, and David B. Parsons

bore. The objectively determined outflow classification results ( Figs. 3a,c ) verify successfully against the vertical cross sections for the corresponding forecast lead times ( Figs. 3b,d ). More specifically, the vertical cross section from Fig. 3b shows classical density current signatures, such as an enhanced prefrontal updraft and a sudden drop in the virtual potential temperature ( ) following the passage of the boundary. Contrastingly, there is an amplitude-ordered wave train in the wake

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

National Oceanic and Atmospheric Administration . Bull. Amer. Meteor. Soc. , 87 , 327 – 342 , . 10.1175/BAMS-87-3-327 Schwartz , C. S. , and R. A. Sobash , 2017 : Generating probabilistic forecasts from convection-allowing ensembles using neighborhood approaches: A review and recommendations . Mon. Wea. Rev. , 145 , 3397 – 3418 , . 10.1175/MWR-D-16-0400.1 Scorer , R. , 1949 : Theory of waves in the lee of

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J. C. Hubbert

(FRONT) field site near Firestone, Colorado, and gathered data for MASCRAD. For calibration purposes, many solar scans were made over a wide range of temperatures that proved advantageous in detecting a relationship between and temperature. The ambient temperature at S-Pol was obtained from a National Oceanic and Atmospheric Administration (NOAA) Meteorological Assimilation Data Ingest System (MADIS) weather station located relatively close to S-Pol (about 4 km to the west of S-Pol). While this

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Alan Shapiro, Evgeni Fedorovich, and Joshua G. Gebauer

deep-tropospheric gravity waves (e.g., Uccelini 1975 ; Koch et al. 1988 ; Fovell et al. 2006 ; Marsham and Parker 2006 ); cold fronts, density currents, and drylines (e.g., Charba 1974 ; Wilson and Schreiber 1986 ; Mahoney 1988 ; Weckwerth and Wakimoto 1992 ; Hane et al. 1993 ; Ziegler and Rasmussen 1998 ; Weiss and Bluestein 2002 ; Geerts et al. 2006 ; Weckwerth et al. 2008 ); inland or “vegetation” breezes (e.g., Sun and Ogura 1979 ; Mahfouf et al. 1987 ; Segal and Arritt 1992

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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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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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