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Dana Mueller, Bart Geerts, Zhien Wang, Min Deng, and Coltin Grasmick

Abstract

This study documents the evolution of an impressive, largely undular bore triggered by an MCS-generated density current on 20 June 2015, observed as part of the Plains Elevated Convection at Night (PECAN) experiment. The University of Wyoming King Air with profiling nadir- and zenith-viewing lidars sampled the south-bound bore from the time the first bore wave emerged from the nocturnal convective cold pool and where updrafts over 10 m s−1 and turbulence in the wave’s wake were encountered, through the early dissipative stage in which the leading wave began to lose amplitude and speed. Through most of the bore’s life cycle, its second wave had a higher or equal amplitude relative to the leading wave. Striking roll clouds formed in wave crests and wave energy was detected to about 5 km AGL. The upstream environment indicates a negative Scorer parameter region due to flow reversal at midlevels, providing a wave trapping mechanism. The observed bore strength of 2.4–2.9 and speed of 15–16 m s−1 agree well with values predicted from hydraulic theory. Surface and profiling measurements collected later in the bore’s life cycle, just after sunrise, indicate a transition to a soliton.

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David J. Bodine and Kristen L. Rasmussen

Abstract

This study examines organizational changes and periods of rapid forward propagation in an MCS on 6 July 2015 in South Dakota. The MCS case was the focus of a Plains Elevated Convection at Night (PECAN) IOP. Data from the Sioux Falls WSR-88D and a high-resolution WRF simulation are analyzed to examine two periods of rapid forward propagation (or surges) and organizational changes. During the first surge (surge A), the northern portion of the convective line propagates eastward faster than the southern portion, and the northern portion of the leading line transitions from a single convective core to a multicellular structure as it merges with convection initiation. Radar reflectivity factor Z and graupel concentrations decrease above the melting layer, while at lower altitudes Z increases. The MCS cold pool also intensifies and deepens beneath an expanded region of high rainwater content and subsaturated air. Throughout surge A, a mesoscale circulation with strong rear-to-front near-surface flow and front-to-rear midlevel flow is also evident. By the end of surge A, the leading edge of the MCS cold pool is beneath developing convection initiation ahead of the original convective line while the original convective updraft weakened and moved rearward. This MCS evolution is similar to discrete propagation events discussed in past studies, except with new convection developing along an intersecting convective band. During surge B, the MCS transitions from a multicellular structure to a single, intense updraft. Smaller microphysical and thermodynamic changes are observed within the MCS during surge B compared to surge A, and the mesoscale circulation continues to develop.

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John M. Peters, Erik R. Nielsen, Matthew D. Parker, Stacey M. Hitchcock, and Russ S. Schumacher

Abstract

This article investigates errors in forecasts of the environment near an elevated mesoscale convective system (MCS) in Iowa on 24–25 June 2015 during the Plains Elevated Convection at Night (PECAN) field campaign. The eastern flank of this MCS produced an outflow boundary (OFB) and moved southeastward along this OFB as a squall line. The western flank of the MCS remained quasi stationary approximately 100 km north of the system’s OFB and produced localized flooding. A total of 16 radiosondes were launched near the MCS’s eastern flank and 4 were launched near the MCS’s western flank.

Convective available potential energy (CAPE) increased and convective inhibition (CIN) decreased substantially in observations during the 4 h prior to the arrival of the squall line. In contrast, the model analyses and forecasts substantially underpredicted CAPE and overpredicted CIN owing to their underrepresentation of moisture. Numerical simulations that placed the MCS at varying distances too far to the northeast were analyzed. MCS displacement error was strongly correlated with models’ underrepresentation of low-level moisture and their associated overrepresentation of the vertical distance between a parcel’s initial height and its level of free convection (, which is correlated with CIN). The overpredicted in models resulted in air parcels requiring unrealistically far northeastward travel in a region of gradual meso-α-scale lift before these parcels initiated convection. These results suggest that erroneous MCS predictions by NWP models may sometimes result from poorly analyzed low-level moisture fields.

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Evgeni Fedorovich, Jeremy A. Gibbs, and Alan Shapiro

Abstract

Nocturnal low-level jets (LLJs) over gently sloping terrain typical of the U.S. Great Plains are investigated by means of direct numerical simulation. Such LLJs develop in a tilted atmospheric boundary layer as a result of inertia–gravity oscillations initiated by a change of the surface thermal forcing during the evening transition. External parameters are the free-atmospheric geostrophic wind, ambient atmospheric stratification, surface buoyancy forcing, and slope angle. The governing momentum and buoyancy balance equations are written in slope-following coordinates, and solved numerically in the Boussinesq approximation. The surface forcing is prescribed in a form of surface buoyancy or buoyancy flux, both of which are slope-uniform but change in time. LLJs over slopes are contrasted with LLJs over flat terrain.

Slope-induced effects essentially modify the entire structure of nocturnal LLJs. The shape of the LLJ wind profile over a slope is characterized by a sharper and larger-magnitude maximum. The presence of the slope causes the along-slope advection of environmental potential temperature during the night. This advection can reignite static instability in the LLJ flow developing after the evening transition. The resulting turbulence leads to a complete or partial remix of the boundary layer flow and drastically changes the appearance of the LLJ in terms of its shape and vertical position. A pronounced nighttime jet can also develop from the daytime convective boundary layer in the absence of any free-atmospheric geostrophic forcing. The daytime flow preconditioning, an important precursor of the nocturnal LLJ development, plays an especially important role in LLJs over a slope.

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

Abstract

Radiosonde measurements from the Plains Elevated Convection At Night (PECAN) 2015 field campaign are used to diagnose mesoscale vertical motions near nocturnal convection initiation (CI). These CI events occur in distinctly different environments including ones with 1) strong forcing for ascent associated with a synoptic cold front and midtropospheric short wave, 2) nocturnal low-level jets interacting with weaker quasi-stationary fronts, or 3) the absence of a surface front or boundary altogether. Radiosonde-derived vertical motion profiles in each of these CI environments are characterized by low- to midtropospheric ascent. The representativeness of these vertical motion profiles is supported by distributions of corresponding mesoscale averages from model-produced 0–6-h ensemble forecasts. Thermodynamic data from radiosondes are then analyzed along with selected model ensemble members to elucidate the role of the vertical motions on subsequent CI. In a case with strong forcing for mesoscale ascent, vertical motions facilitated CI by reducing convection inhibition (CIN). However, in the majority of cases, weaker but persistent vertical motions contributed to the development of elevated, approximately saturated layers with lapse rates greater than moist adiabatic. Such layers have negligible CIN and, thereby, the capacity to support CI even without strong finescale triggering mechanisms in the environment. This aspect may distinguish much central U.S. nocturnal CI from typical daytime CI. The elevated unstable layers occur in disparate large-scale environments, but a common aspect of their development is mesoscale ascent in the presence of warm advection, which results in upward transports of moisture (contributing to local increases of moist static energy) with adiabatic cooling above.

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Sean Stelten and William A. Gallus Jr.

Abstract

The prediction of convective initiation remains a challenge to forecasters in the Great Plains, especially for elevated events at night. This study examines a subset of 287 likely elevated nocturnal convective initiation events that occurred with little or no direct influence from surface boundaries or preexisting convection over a 4-month period of May–August during the summer of 2015. Events were first classified into one of four types based on apparent formation mechanisms and location relative to any low-level jet. A climatology of each of the four types was performed focusing on general spatial tendencies over a large Great Plains domain and initiation timing trends. Simulations from five convection-allowing models available during the Plains Elevated Convection At Night (PECAN) field campaign, along with four versions of a 4-km Weather Research and Forecasting (WRF) Model, were used to examine the predictability of these types of convective initiation. A dual-peak pattern for initiation timing was revealed, with one peak near 0400 UTC and another around 0700 UTC. The times and prominence of each peak shifted depending on the region analyzed. Positive thermal advection by the geostrophic wind was present in the majority of events for three types but not for the type occurring without a low-level jet. Models were more deficient with location than timing for the five PECAN models, with the four 4-km WRF Models showing similar location errors and problems with initiating convection at a lower altitude than observed.

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Thomas R. Parish and Richard D. Clark

Abstract

Extensive measurements were made of the summertime Great Plains low-level jet (LLJ) in central Kansas during June and July 2015 as a component of the Plains Elevated Convection at Night (PECAN) field study. Here, the authors describe the early phase of the LLJ development on 20 June 2015. Half-hourly soundings were launched to monitor the progress of the jet. An airborne mission was also conducted using the University of Wyoming King Air research aircraft. Vertical sawtooth patterns were flown along a fixed track at 38.7°N between longitudes 98.9° and 100.3°W to document changes in the potential temperature and wind profiles. Ageostrophic winds during the LLJ formation were also assessed. In addition, a high-resolution numerical simulation of the 20 June 2015 LLJ case was conducted using the Weather Research and Forecasting Model. Observations and model results show that the early stage of development consisted of a rapid increase in wind speed in the hours just after sunset with less pronounced directional change. The LLJ evolution is similar to that expected from an inertial oscillation of the ageostrophic wind following the stabilization of the near-surface layer.

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Aaron Johnson, Xuguang Wang, and Samuel Degelia

Abstract

Multiscale ensemble-based data assimilation and forecasts were performed in real time during the Plains Elevated Convection At Night (PECAN) field experiment. A 20-member ensemble of forecasts at 4-km grid spacing was initialized daily at both 1300 and 1900 UTC, together with a deterministic forecast at 1-km grid spacing initialized at 1300 UTC. The configuration of the GSI-based data assimilation and forecast system was guided by results presented in Part I of this two-part study. The present paper describes the implementation of the real-time system and the extensive forecast products that were generated to support the unique interests of PECAN researchers. Subjective and objective verification of the real-time forecasts from 1 June through 15 July 2015 is conducted, with an emphasis on nocturnal mesoscale convective systems (MCSs), nocturnal convective initiation (CI), nocturnal low-level jets (LLJs), and bores on the nocturnal stable layer. Verification of nocturnal precipitation during overnight hours, a proxy for MCSs, shows both greater skill and spread for the 1300 UTC forecasts than the 1900 UTC forecasts. Verification against observed soundings reveals that the forecast LLJs systematically peak, veer, and dissipate several hours before the observations. Comparisons with bores that passed over an Atmospheric Emitted Radiance Interferometer reveal an ability to predict borelike features that is greatly improved at 1-km, compared with 4-km, grid spacing. Objective verification of forecast CI timing reveals strong sensitivity to the PBL scheme but an overall unbiased ensemble.

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Dylan W. Reif and Howard B. Bluestein

Abstract

A nocturnal maximum in rainfall and thunderstorm activity over the central Great Plains has been widely documented, but the mechanisms for the development of thunderstorms over that region at night are still not well understood. Elevated convection above a surface frontal boundary is one explanation, but this study shows that many thunderstorms form at night without the presence of an elevated frontal inversion or nearby surface boundary.

This study documents convection initiation (CI) events at night over the central Great Plains from 1996 to 2015 during the months of April–July. Storm characteristics such as storm type, linear system orientation, initiation time and location, and others were documented. Once all of the cases were documented, surface data were examined to locate any nearby surface boundaries. The event’s initiation location relative to these boundaries (if a boundary existed) was documented. Two main initiation locations relative to a surface boundary were identified: on a surface boundary and on the cold side of a surface boundary; CI events also occur without any nearby surface boundary. There are many differences among the different nocturnal CI modes. For example, there appear to be two main peaks of initiation time at night: one early at night and one later at night. The later peak is likely due to the events that form without a nearby surface boundary. Finally, a case study of three nocturnal CI events that occurred during the Plains Elevated Convection At Night (PECAN) field project when there was no nearby surface boundary is discussed.

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Bart Geerts, David Parsons, Conrad L. Ziegler, Tammy M. Weckwerth, Michael I. Biggerstaff, Richard D. Clark, Michael C. Coniglio, Belay B. Demoz, Richard A. Ferrare, William A. Gallus Jr., Kevin Haghi, John M. Hanesiak, Petra M. Klein, Kevin R. Knupp, Karen Kosiba, Greg M. McFarquhar, James A. Moore, Amin R. Nehrir, Matthew D. Parker, James O. Pinto, Robert M. Rauber, Russ S. Schumacher, David D. Turner, Qing Wang, Xuguang Wang, Zhien Wang, and Joshua Wurman

Abstract

The central Great Plains region in North America has a nocturnal maximum in warm-season precipitation. Much of this precipitation comes from organized mesoscale convective systems (MCSs). This nocturnal maximum is counterintuitive in the sense that convective activity over the Great Plains is out of phase with the local generation of CAPE by solar heating of the surface. The lower troposphere in this nocturnal environment is typically characterized by a low-level jet (LLJ) just above a stable boundary layer (SBL), and convective available potential energy (CAPE) values that peak above the SBL, resulting in convection that may be elevated, with source air decoupled from the surface. Nocturnal MCS-induced cold pools often trigger undular bores and solitary waves within the SBL. A full understanding of the nocturnal precipitation maximum remains elusive, although it appears that bore-induced lifting and the LLJ may be instrumental to convection initiation and the maintenance of MCSs at night.

To gain insight into nocturnal MCSs, their essential ingredients, and paths toward improving the relatively poor predictive skill of nocturnal convection in weather and climate models, a large, multiagency field campaign called Plains Elevated Convection At Night (PECAN) was conducted in 2015. PECAN employed three research aircraft, an unprecedented coordinated array of nine mobile scanning radars, a fixed S-band radar, a unique mesoscale network of lower-tropospheric profiling systems called the PECAN Integrated Sounding Array (PISA), and numerous mobile-mesonet surface weather stations. The rich PECAN dataset is expected to improve our understanding and prediction of continental nocturnal warm-season precipitation. This article provides a summary of the PECAN field experiment and preliminary findings.

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