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Stacey M. Hitchcock
,
Russ S. Schumacher
,
Gregory R. Herman
,
Michael C. Coniglio
,
Matthew D. Parker
, and
Conrad L. Ziegler

Abstract

During the Plains Elevated Convection at Night (PECAN) field campaign, 15 mesoscale convective system (MCS) environments were sampled by an array of instruments including radiosondes launched by three mobile sounding teams. Additional soundings were collected by fixed and mobile PECAN integrated sounding array (PISA) groups for a number of cases. Cluster analysis of observed vertical profiles established three primary preconvective categories: 1) those with an elevated maximum in equivalent potential temperature below a layer of potential instability; 2) those that maintain a daytime-like planetary boundary layer (PBL) and nearly potentially neutral low levels, sometimes even well after sunset despite the existence of a southerly low-level wind maximum; and 3) those that are potentially neutral at low levels, but have very weak or no southerly low-level winds. Profiles of equivalent potential temperature in elevated instability cases tend to evolve rapidly in time, while cases in the potentially neutral categories do not. Analysis of composite Rapid Refresh (RAP) environments indicate greater moisture content and moisture advection in an elevated layer in the elevated instability cases than in their potentially neutral counterparts. Postconvective soundings demonstrate significantly more variability, but cold pools were observed in nearly every PECAN MCS case. Following convection, perturbations range between −1.9 and −9.1 K over depths between 150 m and 4.35 km, but stronger, deeper stable layers lead to structures where the largest cold pool temperature perturbation is observed above the surface.

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

Abstract

During the 2015 Plains Elevated Convection at Night (PECAN) field campaign, several nocturnal low-level jets (NLLJs) were observed with integrated boundary layer profiling systems at multiple sites. This paper gives an overview of selected PECAN NLLJ cases and presents a comparison of high-resolution observations with numerical simulations using the Weather Research and Forecasting (WRF) Model. Analyses suggest that simulated NLLJs typically form earlier than the observed NLLJs. They are stronger than the observed counterparts early in the event, but weaker than the observed NLLJs later in the night. However, sudden variations in the boundary layer winds, height of the NLLJ maximum and core region, and potential temperature fields are well captured by the WRF Model. Simulated three-dimensional fields are used for a more focused analysis of PECAN NLLJ cases. While previous studies often related changes in the thermal structure of the nocturnal boundary layer and sudden mixing events to local features, we hypothesize that NLLJ spatial evolution plays an important role in such events. The NLLJ is shown to have heterogeneous depth, wind speed, and wind direction. This study offers detailed documentation of the heterogeneous NLLJ moving down the slope of the Great Plains overnight. As the NLLJ evolves, westerly advection becomes significant. Buoyancy-related mechanisms are proposed to explain NLLJ heterogeneity and down-slope motion. Spatial and temporal heterogeneity of the NLLJ is suggested as a source of the often observed and simulated updrafts during PECAN cases and as a possible mechanism for nocturnal convection initiation. The spatial and temporal characteristics of the NLLJ are interconnected and should not be treated independently.

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Aaron Johnson
and
Xuguang Wang

Abstract

Four case studies from the Plains Elevated Convection at Night (PECAN) field experiment are used to investigate the impacts of horizontal and vertical resolution, and vertical mixing parameterization, on predictions of bore structure and upscale impacts of bores on their mesoscale environment. The reduction of environmental convective inhibition (CIN) created by the bore is particularly emphasized. Simulations are run with horizontal grid spacings ranging from 250 to 1000 m, as well as 50 m for one case study, different vertical level configurations, and different closure models for the vertical turbulent mixing at 250-m horizontal resolution. The 11 July case study was evaluated in greatest detail because it was the best observed case and has been the focus of a previous study. For this case, it is found that 250-m grid spacing improves upon 1-km grid spacing, LES configuration provides further improvement, and enhanced low-level vertical resolution also provides further improvement in terms of qualitative agreement between simulated and observed bore structure. Reducing LES grid spacing further to 50 m provided very little additional advantage. Only the LES experiments properly resolved the upscale influence of reduced low-level CIN. Expanding on the 11 July case study, three other cases from PECAN with diverse observed bore structures were also evaluated. Similar to the 11 July case, enhancing the horizontal and vertical grid spacings, and using the LES closure model for vertical turbulent mixing, all contributed to improved simulations of both the bores themselves and the larger-scale modification of CIN to varying degrees on different cases.

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David M. Loveless
,
Timothy J. Wagner
,
David D. Turner
,
Steven A. Ackerman
, and
Wayne F. Feltz

Abstract

Atmospheric bores have been shown to have a role in the initiation and maintenance of elevated convection. Previous observational studies of bores have been case studies of more notable events. However, this creates a selection bias toward extraordinary cases, while discussions of the differences between bores that favor convective initiation and maintenance and bores that do not are lacking from the literature. This study attempts to fill that gap by analyzing a high-temporal-resolution thermodynamic profile composite of eight bores observed by multiple platforms during the Plains Elevated Convection at Night (PECAN) campaign in order to assess the impact of bores on the environment. The time–height cross section of the potential temperature composite displays quasi-permanent parcel displacements up to 900 m with the bore passage. Low-level lifting is shown to weaken the capping inversion and reduce convective inhibition (CIN) and the level of free convection (LFC). Additionally, low-level water vapor increases by about 1 g kg−1 in the composite mean. By assessing variability across the eight cases, it is shown that increases in low-level water vapor result in increases to convective available potential energy (CAPE), while drying results in decreased CAPE. Most cases resulted in decreased CIN and LFC height with the bore passage, but only some cases resulted in increased CAPE. This suggests that bores will increase the potential for convective initiation, but future research should be directed toward better understanding cases that result in increased CAPE as those are the types of bores that will increase severity of convection.

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

Abstract

Nocturnal mesoscale convective systems (MCSs) frequently develop over the Great Plains in the presence of a nocturnal low-level jet (LLJ), which contributes to convective maintenance by providing a source of instability, convergence, and low-level vertical wind shear. Although these nocturnal MCSs often dissipate during the morning, many persist into the following afternoon despite the cessation of the LLJ with the onset of solar heating. The environmental factors enabling the postsunrise persistence of nocturnal convection are currently not well understood. A thorough investigation into the processes supporting the longevity and daytime persistence of an MCS was conducted using routine observations, RAP analyses, and a WRF-ARW simulation. Elevated nocturnal convection developed in response to enhanced frontogenesis, which quickly grew upscale into a severe quasi-linear convective system (QLCS). The western portion of this QLCS reorganized into a bow echo with a pronounced cold pool and ultimately an organized leading-line, trailing-stratiform MCS as it moved into an increasingly unstable environment. Differential advection resulting from the interaction of the nocturnal LLJ with the topography of west Texas established considerable heterogeneity in moisture, CAPE, and CIN, which influenced the structure and evolution of the MCS. An inland-advected moisture plume significantly increased near-surface CAPE during the nighttime over central Texas, while the environment over southeastern Texas abruptly destabilized following the commencement of surface heating and downward moisture transport. The unique topography of the southern plains and the close proximity to the Gulf of Mexico provided an environment conducive to the postsunrise persistence of the organized MCS.

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Matthew D. Flournoy
and
Michael C. Coniglio

Abstract

To better understand and forecast nocturnal thunderstorms and their hazards, an expansive network of fixed and mobile observing systems was deployed in the summer of 2015 for the Plains Elevated Convection at Night (PECAN) field experiment to observe low-level jets, convection initiation, bores, and mesoscale convective systems. On 5–6 July 2015, mobile radars and ground-based surface and upper-air profiling systems sampled a nocturnal, quasi-linear convective system (QLCS) over South Dakota. The QLCS produced several severe wind reports and an EF-0 tornado. The QLCS and its environment leading up to the mesovortex that produced this tornado were well observed by the PECAN observing network. In this study, observations from radiosondes, Doppler radars, and aircraft are assimilated into an ensemble analysis and forecasting system to analyze this event with a focus on the development of the observed tornadic mesovortex. All ensemble members simulated low-level mesovortices with one member in particular generating two mesovortices in a manner very similar to that observed. Forecasts from this member were analyzed to examine the processes increasing vertical vorticity during the development of the tornadic mesovortex. Cyclonic vertical vorticity was traced to three separate airstreams: the first from southerly inflow that was characterized by tilting of predominantly crosswise horizontal vorticity along the gust front, the second from the north that imported streamwise horizontal vorticity directly into the low-level updraft, and the third from a localized downdraft/rear-inflow jet in which the horizontal vorticity became streamwise during descent. The cyclonic vertical vorticity then intensified rapidly through intense stretching as the parcels entered the low-level updraft of the developing mesovortex.

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

Abstract

A novel object-based algorithm capable of identifying and tracking convective outflow boundaries in convection-allowing numerical models is presented in this study. The most distinct feature of the proposed algorithm is its ability to seamlessly analyze numerically simulated density currents and bores, both of which play an important role in the dynamics of nocturnal convective systems. The unified identification and classification of these morphologically different phenomena is achieved through a multivariate approach combined with appropriate image processing techniques. The tracking component of the algorithm utilizes two dynamical constraints, which improve the object association results in comparison to methods based on statistical assumptions alone. Special attention is placed on some of the outstanding challenges regarding the formulation of the algorithm and possible ways to address those in future research. Apart from describing the technical details behind the algorithm, this study also introduces specific algorithm applications relevant to the analysis and prediction of bores. These applications are illustrated for a retrospective case study simulated with a convection-allowing ensemble prediction system. The paper highlights how the newly developed algorithm tools naturally form a foundation for understanding the initiation, structure, and evolution of bores and convective systems in the nocturnal environment.

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

Abstract

The vertical structures of a leading outflow boundary ahead of a continental nocturnal MCS and of the upstream environment are examined in order to answer the question of whether this vertical structure affects new cell formation and thus MCS maintenance. The MCS in question, observed on 15 July 2015 as part of the Plains Elevated Convection at Night (PECAN) experiment, formed near sunset as a surface-based, density current–driven system. As the night progressed and a stable boundary layer developed, convection became elevated, multiple fine lines became apparent (indicative of an undular bore), and convection increasingly lagged the outflow boundary. Bore-like boundaries became most apparent where the outflow boundary was oriented more perpendicular to the low-level jet, and the lower troposphere was more susceptible to wave trapping. This case study uses a rich array of radiosonde data, as well as airborne Raman lidar and ground-based interferometer data, to profile the temperature and humidity in the lower troposphere. In all soundings, the lifting of air in the residual mixed layer over a depth corresponding to the Raman lidar observed vertical displacement reduced CIN to near zero and enabled deep convection, even though most unstable CAPE steadily decreased during the evolution of this MCS. Both types of outflow boundaries (density currents and bores) initiated convection that helped maintain the MCS. In the case of density currents, cold pool depth and wind shear determined new cell formation and thus MCS maintenance. For bore-like boundaries, bore transformation and propagation were additional factors that determined whether convection initiated and whether it contributed to the MCS or remained separated.

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

Abstract

This paper presents a case study from an intensive observing period (IOP) during the Plains Elevated Convection at Night (PECAN) field experiment that was focused on a bore generated by nocturnal convection. Observations from PECAN IOP 25 on 11 July 2015 are used to evaluate the performance of high-resolution Weather Research and Forecasting Model forecasts, initialized using the Gridpoint Statistical Interpolation (GSI)-based ensemble Kalman filter. The focus is on understanding model errors and sensitivities in order to guide forecast improvements for bores associated with nocturnal convection. Model simulations of the bore amplitude are compared against eight retrieved vertical cross sections through the bore during the IOP. Sensitivities of forecasts to microphysics and planetary boundary layer (PBL) parameterizations are also investigated. Forecasts initialized before the bore pulls away from the convection show a more realistic bore than forecasts initialized later from analyses of the bore itself, in part due to the smoothing of the existing bore in the ensemble mean. Experiments show that the different microphysics schemes impact the quality of the simulations with unrealistically weak cold pools and bores with the Thompson and Morrison microphysics schemes, cold pools too strong with the WDM6 and more accurate with the WSM6 schemes. Most PBL schemes produced a realistic bore response to the cold pool, with the exception of the Mellor–Yamada–Nakanishi–Niino (MYNN) scheme, which creates too much turbulent mixing atop the bore. A new method of objectively estimating the depth of the near-surface stable layer corresponding to a simple two-layer model is also introduced, and the impacts of turbulent mixing on this estimate are discussed.

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

Abstract

Observations from three nights of the Plains Elevated Convection at Night (PECAN) field campaign were used in conjunction with Rapid Refresh model forecasts to find the cause of north–south lines of convection, which initiated away from obvious surface boundaries. Such pristine convection initiation (CI) is relatively common during the warm season over the Great Plains of the United States. The observations and model forecasts revealed that all three nights had horizontally heterogeneous and veering-with-height low-level jets (LLJs) of nonuniform depth. The veering and heterogeneity were associated with convergence at the top-eastern edge of the LLJ, where moisture advection was also occurring. As time progressed, this upper region became saturated and, due to its placement above the capping inversion, formed moist absolutely unstable layers, from which the convergence helped initiate elevated convection. The structure of the LLJs on the CI nights was likely influenced by nonuniform heating across the sloped terrain, which led to the uneven LLJ depth and contributed toward the wind veering with height through the creation of horizontal buoyancy gradients. These three CI events highlight the importance of assessing the full three-dimensional structure of the LLJ when forecasting nocturnal convection over the Great Plains.

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