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Thomas R. Parish

). It is appropriate to acknowledge the wide variety of local wind maxima described in the literature that have been identified by the term low-level jet (e.g., Stensrud 1996 ). In the discussion that follows, the focus is on the summertime Great Plains nocturnal maximum. Jet profiles linked to transient synoptic disturbances and those tied to the lower branch of a transverse circulation associated with an upper-level jet stream ( Uccellini 1980 ) are not considered. Forcing mechanisms for the

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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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Evgeni Fedorovich, Jeremy A. Gibbs, and Alan Shapiro

). Suggested theories of the LLJ point to the jet being a result of the force imbalance in the atmospheric boundary layer induced by the sudden release of the frictional constraint near sunset ( Blackadar 1957 ). In the case when the synoptic-scale pressure gradient is the dominant forcing, the atmospheric flow response to the force imbalance happens in a form of inertial oscillation. The frictional stress, which was not explicitly considered in the Blackadar analysis, was included in the follow-up study

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

systems do not observe temperature, moisture, or wind conditions at this height with adequate temporal and spatial resolutions. To improve the understanding and forecast skill of NCI, the Plains Elevated Convection at Night (PECAN; Geerts et al. 2017 ) field campaign included NCI as a primary scientific objective. This manuscript brings together past NCI research with data from PECAN to document frequencies and forcing mechanisms of different NCI categories. While the impact of NCI in the U.S. Great

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Thomas R. Parish and Richard D. Clark

after sunset. Maximum wind speeds often exceed 20 m s −1 . Wind directions typically shift from southerly early in the evening to southwesterly later on at night. Observations from soundings and profilers indicate that the LLJ is geographically centered over the Great Plains states (e.g., Bonner 1968 ; Mitchell et al. 1995 ; Whiteman et al. 1997 ). Bonner (1968) depicts the center of LLJ frequency over central Oklahoma. The forcing of the LLJ has received considerable attention during the past

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Sean Stelten and William A. Gallus Jr.

challenging. Although previous studies had shown that quantitative precipitation forecast skill increased as the strength of the large-scale forcing increased ( Jankov and Gallus 2004 ; Szoke et al. 2004 ), Duda and Gallus (2013) found that CI forecast skill in 3-km horizontal grid spacing versions of the Weather Research and Forecasting (WRF) Model did not follow such trends. Wilson and Roberts (2006) , however, found that CI tended to be better predicted in a 10-km version of the Rapid Update Cycle

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Alan Shapiro, Evgeni Fedorovich, and Joshua G. Gebauer

LLJ), or along or north of the intersection of an LLJ with a cold front, are more or less clear, but the mechanisms that force ascent on a lateral flank of an LLJ are still not well understood. In this regard, we believe that the recent Pu and Dickinson (2014) explanation for such a mechanism is not wholly satisfactory. In a study of vertical motions in Great Plains LLJs using a North American Regional Reanalysis (NARR) June–July climatology, Pu and Dickinson (2014) suggest that after midnight

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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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