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Evgeni Fedorovich, Jeremy A. Gibbs, and Alan Shapiro

1. Introduction The nocturnal low-level jet (LLJ) is an atmospheric boundary layer wind maximum that typically develops under dry and clear conditions after sunset. The jet reaches a peak magnitude a few hours after midnight, and then decays after sunrise with the onset of convective mixing ( Shapiro and Fedorovich 2010 ). LLJs have been observed in many locations throughout the world (see, e.g., Stensrud 1996 ; Baas et al. 2009 ; Van de Wiel et al. 2010 ) but have been most extensively

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Stacey M. Hitchcock and Russ S. Schumacher

top. The domain was set to translate east at 9 m s −1 and north at 8 m s −1 . Fifth-order positive definite advection, the Klemp–Wilhelmson time-splitting, vertically implicit pressure solver ( Klemp and Wilhelmson 1978 ), and a 2-s time step were used. Open radiative lateral boundary conditions were used to allow fast moving gravity waves created during initiation to escape the domain. Rayleigh damping was applied near the model top, and a free slip condition was applied at the surface. This

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Yun Lin, Jiwen Fan, Jong-Hoon Jeong, Yuwei Zhang, Cameron R. Homeyer, and Jingyu Wang

polluted aerosol condition (i.e., anthropogenic emissions are on). The effect of anthropogenic aerosols is investigated by comparing UlandAero with No_Aero, meaning under the condition of urban land condition. The joint effect of both urban effect and anthropogenic aerosols is obtained by comparing UlandAero with No_UlandAero. Table 1. Major model simulations. The initial and boundary conditions used to drive the real-case simulations are produced from the High-Resolution Rapid Refresh (HRRR) analysis

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

description of RAP is available in Benjamin et al. (2016) . RAP was chosen for this study because it is often used to provide environmental context in atmospheric research (e.g., Peters et al. 2017 ; Stelten and Gallus 2017 ; Wilson et al. 2018 ; Degelia et al. 2019 ; Hitchcock et al. 2019 ), and it provides initial and lateral boundary conditions to the High Resolution Rapid Refresh (HRRR), a 3 km grid spacing convection-allowing modeling system. d. Methodology for lidar and RAP comparisons The

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David B. Parsons, Kevin R. Haghi, Kelton T. Halbert, Blake Elmer, and Junhong Wang

“weakly forced” conditions, with an upscale growth beginning during the evening and reaching a maximum extent after midnight. Significant attention has been paid to the question of how these propagating envelopes of convection are maintained in this nocturnal environment (e.g., Li and Smith 2010 ; Geerts et al. 2017 ). The environment typically includes a nocturnal low-level jet (NLLJ) that transports warm, moist air northward above the stable nocturnal boundary layer (e.g., Means 1952 ; Curtis

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Coltin Grasmick, Bart Geerts, David D. Turner, Zhien Wang, and T. M. Weckwerth

solitary wave, finding that the wave passage reduced convective inhibition (CIN) and lowered the LFC, making CI more likely. However, other studies (e.g., Toms et al. 2017 ) have shown that propagating bores may lift parcels to their lifting condensation level (LCL) without producing CI. Thus, a better understanding of the mechanisms and conditions when these propagating wave structures trigger CI is needed. The present study analyzes nocturnal, elevated CI near an MCS outflow boundary during the

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Dylan W. Reif and Howard B. Bluestein

-stationary front. During the International H 2 O Project (IHOP_2002; Weckwerth et al. 2004 ), approximately 80% of all nocturnal convection initiation (CI) events were elevated ( Wilson and Roberts 2006 ). They suggested that a zone of elevated convergence combined with areas of midlevel instability provided conditions for elevated convection to form with no nearby surface boundary. Even though elevated convective systems typically occur above a nocturnal stable layer, they still produce severe weather

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Thomas R. Parish

ageostrophic wind arising due to the sudden decay of turbulence in the boundary layer after sunset. Airborne measurements ( Parish et al. 1988 ) and numerical experiments (e.g., Zhong et al. 1996 ) leave little doubt that LLJ winds are supergeostrophic and veer throughout the night in a manner prescribed by the Blackadar mechanism. A second theory is that proposed by Holton (1967) and suggests that the oscillation in the wind is tied to the diurnal heating and cooling of the sloping Great Plains terrain

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Elizabeth N. Smith, Joshua G. Gebauer, Petra M. Klein, Evgeni Fedorovich, and Jeremy A. Gibbs

-facing slope like that of the Great Plains, two new buoyancy related drivers of NLLJ evolution can be described. First, differential PBL geostrophic wind , due to gradients of buoyancy along the slope, can be connected to observed and simulated NLLJ features. In the three PECAN cases discussed above, observations and simulations showed deeper boundary layers over the western portion of the slope. Warmer conditions through the boundary layer depth allow buoyancy [defined in Eq. (3) ] to reach larger

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

successfully meet the additional dynamical requirements regarding the object’s propagation speed and direction. Therefore, this example shows how the tracker is able to handle the complex dynamical behavior of convective outflow boundaries by exploiting their statistical and dynamical properties simultaneously. The addition of dynamical constraints in the object tracker is beneficial for several reasons. First, the simultaneous fulfillment of three different conditions relaxes the prescribed threshold

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