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

transect of θ υ shows the MCS outflow as a nearly 600-m-deep surface-based cold pool, with slightly (~2 K) lower θ υ than the postfrontal air: the average CRL-retrieved θ υ below 1.10 km MSL was 307 K in the cold-frontal density current (0215–0219 UTC, Fig. 7c ), compared to 305 K in the MCS outflow (0221–0225 UTC). This gradient provides sufficient solenoidal forcing for the lofting of the less dense air mass (e.g., Miao and Geerts 2007 ). The preexisting BL air between the cold front and MCS

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Thomas R. Parish

results from model simulations (e.g., Zhong et al. 1996 ) show that the summertime LLJ is centered geographically over the southern Great Plains from Texas northward to Nebraska with a maximum over northern Oklahoma and southern Kansas. Two paradigms continue to be espoused in describing the forcing of the LLJ (e.g., Jiang et al. 2007 ; Du and Rotunno 2014 ). The first theory, proposed by Blackadar (1957) , considered the LLJ to be supergeostrophic, the result of an inertial oscillation of the

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Stanley B. Trier, James W. Wilson, David A. Ahijevych, and Ryan A. Sobash

categories: 1) strong synoptic forcing associated with a cold front and a midtropospheric short wave ( Fig. 1b ), 2) interaction of a nocturnal LLJ with a quasi-stationary lower-tropospheric front ( Figs. 1a,d,e ), and 3) a nocturnal LLJ located immediately downstream from a midtropospheric ridge axis in the absence of a well-defined surface front ( Fig. 1c ). Fig . 1. Mean 500-hPa geopotential height and horizontal winds with 850-hPa wind speed (shaded) from DART ensemble analyses (see section 3c ) at

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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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Dylan W. Reif and Howard B. Bluestein

). It is surprising that the NB CI mode events have a distinct initiation location (south-central Nebraska and north-central Kansas). Without any strong surface forcing, why is there a clear initiation location? This question is addressed in the following sections. Fig . 5. Initiation location (each box is 1° latitude by 1° longitude) for (a) areal storm type, (b) linear storm type, (c) single-cell storm type, (d) AB CI mode, (e) CS CI mode, and (f) NB CI mode. The color scales are different to

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

wind maxima that occur in the lowest levels of the atmosphere, typically within a kilometer of the ground. Blackadar (1957) showed that nocturnal LLJs can arise from an inertial oscillation that is triggered by the reduction of the friction force in the atmospheric boundary layer at sunset as thermally generated turbulence decays. The resulting force imbalance causes the ageostrophic wind vector to rotate clockwise around the geostrophic wind vector with time. The maximum LLJ wind speed occurs

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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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James N. Marquis, Adam C. Varble, Paul Robinson, T. Connor Nelson, and Katja Friedrich

horizontal flow convergence frequently aids convection initiation processes by forcing low-level air parcels upward, locally reducing CIN, deepening boundary layer moisture below cloud base, and providing a focal area for moist updrafts to detrain into the overlying free troposphere, reducing the negative entrainment effect ( Ziegler et al. 1997 ; Markowski and Richardson 2010 ; Moser and Lasher-Trapp 2017 ). Common mesoscale convergence features that trigger deep convection initiation (hereafter CI

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

June event featured forcing mechanisms across a spectrum of scales (e.g., shortwave trough, LLJ, outflow boundary), a multiscale DA approach is used like that described in Degelia et al. (2018) . Sensitivity tests are performed to determine the best covariance localization radii ( Table 3 ) for each PECAN observation type described in the previous section. These settings are tuned to produce the highest fractions skill score (FSS; discussed in section 4a ) for the nocturnal CI event of interest

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