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

Plains, nocturnal CI (NCI) remains challenging to forecast, especially in the absence of surface boundaries ( Wilson and Roberts 2006 ; Reif and Bluestein 2018 ; Weckwerth et al. 2019 ). This is due in part to the paucity of routine thermodynamic profiling in the PBL and the limited resolution of satellite observations within the lowest few kilometers of the atmosphere ( Kahn et al. 2011 ; Steinke et al. 2015 ; Weckwerth et al. 2019 ). Previous studies based on radiosonde and surface data or

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Thomas R. Parish

. Here conditions include a maximum jet speed of 15 m s −1 or greater at the 0900 UTC profile with a concurrent decrease such that the 700-hPa wind vector has a magnitude less than 10 m s −1 . These LLJ criteria limited the composite output to 47% of the June/July cases for the 2-yr period. This percentage is in accord with statistics from the Whiteman et al. (1997) study. The large-scale circulation in the lower atmosphere over the southeastern part of the United States during summer is

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Alan Shapiro, Evgeni Fedorovich, and Joshua G. Gebauer

model for the nocturnal motion. We consider, in turn, lateral variations in the free-atmosphere geostrophic wind and CBL buoyancy. The nocturnal state following the shutdown of mixing is modeled as a two-dimensional (2D) inviscid flow of a stably stratified fluid. In this scenario, flow convergence cannot occur at the terminus of a jet (there is no terminus) but is parallel to the jet axis. The shutdown of mixing that triggers the convergence in our theory is the same mechanism that triggers an

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Thomas R. Parish and Richard D. Clark

at 0000 UTC 20 June 2015 ( Fig. 1b ) differs significantly from that shown at the surface in Fig. 1a . Isotherm orientation at 600 hPa displays only a weak relationship to the underlying terrain. We used the height field at 600 hPa as an estimate of the free atmosphere forcing during PECAN. For this case, weak northwesterly geostrophic winds were present at 600 hPa over central Kansas. The horizontal pressure gradient associated with the synoptic forcing at middle levels of the troposphere thus

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Thomas R. Parish

Great Plains low-level jet have been debated since the earliest observations were made. Blackadar (1957) suggested that the nocturnal wind maximum was the result of an inertial oscillation of the ageostrophic wind. Acceleration occurs as the result of frictional decoupling of the lower atmosphere from the surface owing to radiational cooling near sunset that disrupts the daytime force balance. Blackadar (1957) indicated that wind maxima are not related to diurnal oscillations in the pressure

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Kevin R. Haghi, Bart Geerts, Hristo G. Chipilski, Aaron Johnson, Samuel Degelia, David Imy, David B. Parsons, Rebecca D. Adams-Selin, David D. Turner, and Xuguang Wang

. Observations such as these pose needed challenges to theories and models, thereby improving our ability to forecast their evolution and impacts. PECAN observations are also useful to explore how a bore triggers new convection. As an example, the bore in Fig. 6 produced sufficient lift for CI, but the resulting deep convection was rather benign and scattered because ultimately the middle troposphere was too dry to maintain the convection. Assessing not only the moisture content of the atmosphere, but also

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

observed bore dissipation to the weakening of the wave duct in the near-surface stable layer. Blake et al. (2017) used the simulation to investigate the role of a bore in the maintenance of a nocturnal MCS. The basic theory of bores as hydraulic jumps in the depth of the stable layer, maintained by the ducting of vertically propagating wave energy, has been confirmed by numerous laboratory experiments and by analytically using a simple two-layer model of the atmosphere (e.g., Maxworthy 1980

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Evgeni Fedorovich, Jeremy A. Gibbs, and Alan Shapiro

documented over the Great Plains of the United States (e.g., Blackadar 1957 ; Hoecker 1963 ; Bonner 1968 ; Parish et al. 1988 ; Mitchell et al. 1995 ; Zhong et al. 1996 ; Whiteman et al. 1997 ; Banta et al. 2002 ; Song et al. 2005 ; Banta 2008 ; Walters et al. 2008 ; Klein et al. 2016 ). The LLJ wind speed profile has a pronounced maximum that typically occurs at levels within 500 m above the ground. The maximum wind often exceeds the free-atmosphere geostrophic value by up to 70%. However

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

moisture advection and ascent as the LLJ veered with time ( Fig. 10h ). Fig . 10. West–east vertical cross sections of (top) wind speed (shaded; kt), θ (gray contours; K), and wind barbs (kt) for (a) 0300, (b) 0600, and (c) 0900 UTC; (middle) water vapor mixing ratio (shaded; g kg −1 ), θ (gray contours; K), and zonal winds (vectors; m s −1 ) for (d) 0300, (e) 0600, and (f) 0900 UTC; and (bottom) CAPE (shaded; J kg −1 ) and θ (gray contours; K) for (g) 0300, (h) 0600, and (i) 0900 UTC 6 Oct 2014

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John M. Peters, Erik R. Nielsen, Matthew D. Parker, Stacey M. Hitchcock, and Russ S. Schumacher

and Peters 2017 ). EIL changes have been shown to influence MCS’s location and precipitation characteristics ( Schumacher 2015b ; Schumacher and Peters 2017 ). This is most notable for elevated MCSs, given that gradual (e.g., of order 10–50 cm s −1 ) meso- β -scale (of order 20–200 km) to meso- α -scale (of order 200–1000 km) lift in the lower atmosphere is often the driving mechanism in the initiation and upwind propagation of elevated MCSs (e.g., Trier et al. 2010 ; Peters and Schumacher 2015

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