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1. Self-Organization and Maintenance of Simulated Nocturnal Convective Systems from PECAN

   https://doi.org/10.1175/MWR-D-20-0263.1  

   Parker, Matthew D. ( April 2021 , Monthly Weather Review)

   null (Ed.)

   Abstract The Plains Elevated Convection at Night (PECAN) field project was designed to explain the evolution and structures of nocturnal mesoscale convective systems (MCSs) and relate them to specific mechanisms and environmental ingredients. The present work examines four of the strongest and best-organized PECAN cases, each numerically simulated at two different levels of complexity. The suite of simulations enables a longitudinal look at how nocturnal MCSs resemble (or differ from) more commonly studied diurnal MCSs. All of the simulations produce at least some surface outflow (“cold pools”), with stronger outflows occurring in environments with more CAPE and weaker near-ground stability. As these surface outflows emerge, the lifting of near-ground air occurs, causing each simulated nocturnal MCS to ultimately become “surface-based.” The end result in each simulation is a quasi-linear convective system (QLCS) that is most intense toward the downshear flank of its cold pool, with the classical appearance of many afternoon squall lines. This pathway of evolution occurs both in fully heterogeneous real-world-like simulations and horizontally homogeneous idealized simulations. One of the studied cases also exhibits a back-building “rearward off-boundary development” stage, and this more complex behavior is also well simulated in both model configurations. As a group, the simulations imply that a wide range of nocturnal MCS behaviors may be self-organized (i.e., not reliant on larger-scale features external to the convection). 

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2. Seven-Doppler Radar and In Situ Analysis of the 25–26 June 2015 Kansas MCS during PECAN

   https://doi.org/10.1175/MWR-D-19-0151.1  

   Miller, Rachel L. ; Ziegler, Conrad L. ; Biggerstaff, Michael I. ( December 2019 , Monthly Weather Review)

   This case study analyzes a nocturnal mesoscale convective system (MCS) that was observed on 25–26 June 2015 in northeastern Kansas during the Plains Elevated Convection At Night (PECAN) project. Over the course of the observational period, a broken line of elevated nocturnal convective cells initiated around 0230 UTC on the cool side of a stationary front and subsequently merged to form a quasi-linear MCS that later developed strong, surface-based outflow and a trailing stratiform region. This study combines radar observations with mobile and fixed mesonet and sounding data taken during PECAN to analyze the kinematics and thermodynamics of the MCS from 0300 to 0630 UTC. This study is unique in that 38 consecutive multi-Doppler wind analyses are examined over the 3.5 h observation period, facilitating a long-duration analysis of the kinematic evolution of the nocturnal MCS. Radar analyses reveal that the initial convective cells and linear MCS are elevated and sustained by an elevated residual layer formed via weak ascent over the stationary front. During upscale growth, individual convective cells develop storm-scale cold pools due to pockets of descending rear-to-front flow that are measured by mobile mesonets. By 0500 UTC, kinematic analysis and mesonet observations show that the MCS has a surface-based cold pool and that convective line updrafts are ingesting parcels from below the stable layer. In this environment, the elevated system has become surface based since the cold pool lifting is sufficient for surface-based parcels to overcome the CIN associated with the frontal stable layer.

    

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3. Wave Disturbances and Their Role in the Maintenance, Structure, and Evolution of a Mesoscale Convection System

   https://doi.org/10.1175/JAS-D-18-0348.1  

   Zhang, Shushi ; Parsons, David B. ; Wang, Yuan ( January 2020 , Journal of the Atmospheric Sciences)

   Abstract This study investigates a nocturnal mesoscale convective system (MCS) observed during the Plains Elevated Convection At Night (PECAN) field campaign. A series of wavelike features were observed ahead of this MCS with extensive convective initiation (CI) taking place in the wake of one of these disturbances. Simulations with the WRF-ARW Model were utilized to understand the dynamics of these disturbances and their impact on the MCS. In these simulations, an “elevated bore” formed within an inversion layer aloft in response to the layer being lifted by air flowing up and over the cold pool. As the bore propagated ahead of the MCS, the lifting created an environment more conducive to deep convection allowing the MCS to discretely propagate due to CI in the bore’s wake. The Scorer parameter was somewhat favorable for trapping of this wave energy, although aspects of the environment evolved to be consistent with the expectations for an n = 2 mode deep tropospheric gravity wave. A bore within an inversion layer aloft is reminiscent of disturbances predicted by two-layer hydraulic theory, contrasting with recent studies that suggest bores are frequently initiated by the interaction between the flow within stable nocturnal boundary layer and convectively generated cold pools. Idealized simulations that expand upon this two-layer approach with orography and a well-mixed layer below the inversion suggest that elevated bores provide a possible mechanism for daytime squall lines to remove the capping inversion often found over the Great Plains, particularly in synoptically disturbed environments where vertical shear could create a favorable trapping of wave energy. 

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4. Interactions between a Nocturnal MCS and the Stable Boundary Layer as Observed by an Airborne Compact Raman Lidar during PECAN

   https://doi.org/10.1175/MWR-D-18-0388.1  

   Lin, Guo ; Geerts, Bart ; Wang, Zhien ; Grasmick, Coltin ; Jing, Xiaoqin ; Yang, Jing ( August 2019 , Monthly Weather Review)

   Small-scale variations within the low-level outflow and inflow of an MCS can either support or deter the upscale growth and maintenance of the MCS. However, these small-scale variations, in particular in the thermodynamics (temperature and humidity), remain poorly understood, due to a lack of detailed measurements. The compact Raman lidar (CRL) deployed on the University of Wyoming King Air aircraft directly sampled temperature and water vapor profiles at unprecedented vertical and along-track resolutions along the southern margin of a series of mature nocturnal MCSs traveling along a frontal boundary on 1 July 2015 during the Plains Elevated Convection at Night (PECAN) campaign. Here, the capability of the airborne CRL to document interactions between the MCS inflow and outflow currents is illustrated. The CRL reveals the well-defined boundary of a cooler current. This is interpreted as the frontal boundary sharpened by convectively induced cold pools, in particular by the outflow boundary of the downstream MCS. In one CRL transect, the frontal/outflow boundary appeared as a distinct two-layer structure of moisture and aerosols formed by moist stable boundary layer air advected above the boundary. The second transect, one hour later, reveals a single sloping boundary. In both cases, the lofting of the moist stably stratified air over the boundary favors MCS maintenance, through enhanced elevated CAPE and reduced CIN. The CRL data are sufficiently resolved to reveal Kelvin–Helmholtz (KH) billows and the vertical structure of the outflow boundary, which in this case behaved as a density current rather than an undular bore.

    

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5. Simulated Evolution and Severe Wind Production by the 25–26 June 2015 Nocturnal MCS from PECAN

   https://doi.org/10.1175/MWR-D-19-0072.1  

   Parker, Matthew D. ; Borchardt, Brett S. ; Miller, Rachel L. ; Ziegler, Conrad L. ( December 2019 , Monthly Weather Review)

   The 25–26 June 2015 nocturnal mesoscale convective system (MCS) from the Plains Elevated Convection at Night (PECAN) field project produced severe winds within an environment that might customarily be associated with elevated convection. This work incorporates both a full-physics real-world simulation and an idealized single-sounding simulation to explore the MCS’s evolution. Initially, the simulated convective systems were elevated, being maintained by wavelike disturbances and lacking surface cold pools. As the systems matured, surface outflows began to appear, particularly where heavy precipitation was occurring, with air in the surface cold pools originating from up to 4–5 km AGL. Via this progression, the MCSs exhibited a degree of self-organization (i.e., structures that are dependent upon an MCS’s particular history). The cold pools eventually became 1.5–3.5 km deep, by which point passive tracers revealed that the convection was at least partly surface based. Soon after becoming surface based, both simulations produced severe surface winds, the strongest of which were associated with embedded low-level mesovortices and their attendant outflow surges and bowing segments. The origin of the simulated mesovortices was likely the downward tilting of system-generated horizontal vorticity (from baroclinity, but also possibly friction) within the simulated MCSs’ outflow, as has been argued in a number of previous studies. Taken altogether, it appears that severe nocturnal MCSs may often resemble their cold pool-driven, surface-based afternoon counterparts.

    

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