Initiation Mechanisms of Nocturnal Convection without Nearby Surface Boundaries over the Central and Southern Great Plains during the Warm Season

2018 ◽  
Vol 146 (9) ◽  
pp. 3053-3078 ◽  
Author(s):  
Dylan W. Reif ◽  
Howard B. Bluestein
Keyword(s):  
Case Studies ◽  
Great Plains ◽  
Field Experiments ◽  
Warm Season ◽  
Surface Boundary ◽  
Spatiotemporal Resolution ◽  
Southern Great Plains ◽  
Convective Systems ◽  
Low Level Jet ◽  
Convection Initiation

Abstract The number of case studies in the literature of nocturnal convection has increased during the past decade, especially those that utilize high-spatiotemporal-resolution datasets from field experiments such as the International H2O Project (IHOP_2002) and Plains Elevated Convection at Night (PECAN). However, there are few case studies of events for convection initiation without a nearby surface boundary. These events account for approximately 25% of all nocturnal convection initiation (CI) events. Unique characteristics of these events include a peak initiation time later at night, a preferred initiation location in northern Kansas and southern Nebraska, and a preferred north–south orientation to linear convective systems. In this study, four case studies of convection that is initiated without a nearby surface boundary are detailed to reveal a number of possible initiation mechanisms, including quasigeostrophic-aided ascent, elevated ascent associated with convergent layers (of unknown causes), the low-level jet, and gravity waves. The case studies chosen illustrate the wide variety of synoptic-scale conditions under which these events can occur.

scholarly journals A 20-Year Climatology of Nocturnal Convection Initiation over the Central and Southern Great Plains during the Warm Season

2017 ◽  
Vol 145 (5) ◽  
pp. 1615-1639 ◽  
Author(s):  
Dylan W. Reif ◽  
Howard B. Bluestein
Keyword(s):  
Great Plains ◽  
Warm Season ◽  
Thunderstorm Activity ◽  
Surface Boundary ◽  
Initiation Time ◽  
Southern Great Plains ◽  
Surface Data ◽  
Convection Initiation ◽  
Storm Characteristics

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.

scholarly journals The 2015 Plains Elevated Convection at Night Field Project

2017 ◽  
Vol 98 (4) ◽  
pp. 767-786 ◽  
Author(s):  
Bart Geerts ◽  
David Parsons ◽  
Conrad L. Ziegler ◽  
Tammy M. Weckwerth ◽  
Michael I. Biggerstaff ◽  
...  
Keyword(s):  
Great Plains ◽  
Climate Models ◽  
Convective Available Potential Energy ◽  
Warm Season ◽  
Lower Troposphere ◽  
Convective Systems ◽  
Predictive Skill ◽  
Mesoscale Convective ◽  
The Rich ◽  
Convection Initiation

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.

The Role of Upper-Level Coupling on Great Plains Low-Level Jet Structure and Variability

2020 ◽  
Vol 77 (12) ◽  
pp. 4317-4335
Author(s):  
D. Alex Burrows ◽  
Craig R. Ferguson ◽  
Lance F. Bosart
Keyword(s):  
Great Plains ◽  
Atmospheric Stability ◽  
Warm Season ◽  
Inertial Oscillation ◽  
Low Level ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Southern Central ◽  
Low Level Jet ◽  
Upper Level

AbstractThe Great Plains (GP) southerly nocturnal low-level jet (GPLLJ) is a dominant contributor to the region’s warm-season (May–September) mean and extreme precipitation, wind energy generation, and severe weather outbreaks—including mesoscale convective systems. The spatiotemporal structure, variability, and impact of individual GPLLJ events are closely related to their degree of upper-level synoptic coupling, which varies from strong coupling in synoptic trough–ridge environments to weak coupling in quiescent, synoptic ridge environments. Here, we apply an objective dynamic classification of GPLLJ upper-level coupling and fully characterize strongly coupled (C) and relatively uncoupled (UC) GPLLJs from the perspective of the ground-based observer. Through composite analyses of C and UC GPLLJ event samples taken from the European Centre for Medium-Range Weather Forecasts’ Coupled Earth Reanalysis of the twentieth century (CERA-20C), we address how the frequency of these jet types, as well as their inherent weather- and climate-relevant characteristics—including wind speed, direction, and shear; atmospheric stability; and precipitation—vary on diurnal and monthly time scales across the southern, central, and northern subregions of the GP. It is shown that C and UC GPLLJ events have similar diurnal phasing, but the diurnal amplitude is much greater for UC GPLLJs. C GPLLJs tend to have a faster and more elevated jet nose, less low-level wind shear, and enhanced CAPE and precipitation. UC GPLLJs undergo a larger inertial oscillation (Blackadar mechanism) for all subregions, and C GPLLJs have greater geostrophic forcing (Holton mechanism) in the southern and northern GP. The results underscore the need to differentiate between C and UC GPLLJs in future seasonal forecast and climate prediction activities.

scholarly journals Coupling between Large-Scale Atmospheric Processes and Mesoscale Land–Atmosphere Interactions in the U.S. Southern Great Plains during Summer. Part II: Mean Impacts of the Mesoscale

2004 ◽  
Vol 5 (6) ◽  
pp. 1247-1258 ◽  
Author(s):  
Christopher P. Weaver
Keyword(s):  
Case Studies ◽  
Time Scales ◽  
Great Plains ◽  
Large Scale ◽  
Spatial Scales ◽  
Surface Heterogeneity ◽  
Atmospheric Modeling ◽  
Mesoscale Circulations ◽  
Southern Great Plains ◽  
Atmospheric Processes

Abstract This is Part II of a two-part study of mesoscale land–atmosphere interactions in the summertime U.S. Southern Great Plains. Part I focused on case studies drawn from monthlong (July 1995–97), high-resolution Regional Atmospheric Modeling System (RAMS) simulations carried out to investigate these interactions. These case studies were chosen to highlight key features of the lower-tropospheric mesoscale circulations that frequently arise in this region and season due to mesoscale heterogeneity in the surface fluxes. In this paper, Part II, the RAMS-simulated mesoscale dynamical processes described in the Part I case studies are examined from a domain-averaged perspective to assess their importance in the overall regional hydrometeorology. The spatial statistics of key simulated mesoscale variables—for example, vertical velocity and the vertical flux of water vapor—are quantified here. Composite averages of the mesoscale and large-scale-mean variables over different meteorological or dynamical regimes are also calculated. The main finding is that, during dry periods, or similarly, during periods characterized by large-scale-mean subsidence, the characteristic signature of surface-heterogeneity-forced mesoscale circulations, including enhanced vertical motion variability and enhanced mesoscale fluxes in the lowest few kilometers of the atmosphere, consistently emerges. Furthermore, the impact of these mesoscale circulations is nonnegligible compared to the large-scale dynamics at domain-averaged (200 km × 200 km) spatial scales and weekly to monthly time scales. These findings support the hypothesis that the land– atmosphere interactions associated with mesoscale surface heterogeneity can provide pathways whereby diurnal, mesoscale atmospheric processes can scale up to have more general impacts at larger spatial scales and over longer time scales.

scholarly journals A Mechanistically Credible, Poleward Shift in Warm-Season Precipitation Projected for the U.S. Southern Great Plains?

2017 ◽  
Vol 30 (20) ◽  
pp. 8275-8298 ◽  
Author(s):  
Melissa S. Bukovsky ◽  
Rachel R. McCrary ◽  
Anji Seth ◽  
Linda O. Mearns
Keyword(s):  
Great Plains ◽  
Climate Models ◽  
Climate Model ◽  
Global Climate Model ◽  
Regional Climate ◽  
Warm Season ◽  
Dry Season ◽  
Wet Season ◽  
Southern Great Plains ◽  
Poleward Shift

Abstract Global and regional climate model ensembles project that the annual cycle of rainfall over the southern Great Plains (SGP) will amplify by midcentury. Models indicate that warm-season precipitation will increase during the early spring wet season but shift north earlier in the season, intensifying late summer drying. Regional climate models (RCMs) project larger precipitation changes than their global climate model (GCM) counterparts. This is particularly true during the dry season. The credibility of the RCM projections is established by exploring the larger-scale dynamical and local land–atmosphere feedback processes that drive future changes in the simulations, that is, the responsible mechanisms or processes. In this case, it is found that out of 12 RCM simulations produced for the North American Regional Climate Change Assessment Program (NARCCAP), the majority are mechanistically credible and consistent in the mean changes they are producing in the SGP. Both larger-scale dynamical processes and local land–atmosphere feedbacks drive an earlier end to the spring wet period and deepening of the summer dry season in the SGP. The midlatitude upper-level jet shifts northward, the monsoon anticyclone expands, and the Great Plains low-level jet increases in strength, all supporting a poleward shift in precipitation in the future. This dynamically forced shift causes land–atmosphere coupling to strengthen earlier in the summer, which in turn leads to earlier evaporation of soil moisture in the summer, resulting in extreme drying later in the summer.

scholarly journals Characteristics of Deep Convective Systems and Initiation during Warm Seasons over China and Its Vicinity

2021 ◽  
Vol 13 (21) ◽  
pp. 4289
Author(s):  
Yang Li ◽  
Yubao Liu ◽  
Yun Chen ◽  
Baojun Chen ◽  
Xin Zhang ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
South China ◽  
Warm Season ◽  
Statistical Characteristics ◽  
The Tibetan Plateau ◽  
Spatiotemporal Resolution ◽  
Convective Systems ◽  
Seasonal And Diurnal Variations ◽  
Deep Convective Systems ◽  
Guizhou Plateau

The spatiotemporal statistical characteristics of warm-season deep convective systems, particularly deep convective systems initiation (DCSI), over China and its vicinity are investigated using Himawari-8 geostationary satellite measurements collected during April-September from 2016 to 2020. Based on a satellite brightness temperature multiple-threshold convection identification and tracking method, a total of 47593 deep convective systems with lifetimes of at least 3 h were identified in the region. There are three outstanding local maxima in the region, located in the southwestern, central and eastern Tibetan Plateau and Yunnan-Guizhou Plateau, followed by a region of high convective activities in South China. Most convective systems are developed over the Tibetan Plateau, predominantly eastward-moving, while those developed in Yunnan-Guizhou Plateau and South China mostly move westward and southwestward. The DSCI occurrences become extremely active after the onset of the summer monsoon and tend to reach a maximum in July and August, with a diurnal peak at 11–13 LST in response to the enhanced solar heating and monsoon flows. Several DCSI hotspots are identified in the regions of inland mountains, tropical islands and coastal mountains during daytime, but in basins, plains and coastal areas during nighttime. DCSI over land and oceans exhibits significantly different sub-seasonal and diurnal variations. Oceanic DCSI has an ambiguous diurnal variation, although its sub-seasonal variation is similar to that over land. It is demonstrated that the high spatiotemporal resolution satellite dataset provides rich information for understanding the convective systems over China and vicinity, particularly the complex terrain and oceans where radar observations are sparse or none, which will help to improve the convective systems and initiation nowcasting.

scholarly journals Convection Initiation Caused by Heterogeneous Low-Level Jets over the Great Plains

2018 ◽  
Vol 146 (8) ◽  
pp. 2615-2637 ◽  
Author(s):  
Joshua G. Gebauer ◽  
Alan Shapiro ◽  
Evgeni Fedorovich ◽  
Petra Klein
Keyword(s):  
Great Plains ◽  
Three Dimensional ◽  
Warm Season ◽  
The United States ◽  
Dimensional Structure ◽  
Nonuniform Heating ◽  
Low Level ◽  
Low Level Jets ◽  
Convection Initiation ◽  
Capping Inversion

AbstractObservations 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.

Bores Observed during IHOP_2002: The Relationship of Bores to the Nocturnal Environment

2017 ◽  
Vol 145 (10) ◽  
pp. 3929-3946 ◽  
Author(s):  
Kevin R. Haghi ◽  
David B. Parsons ◽  
Alan Shapiro
Keyword(s):  
Great Plains ◽  
Deep Convection ◽  
Horizontal Wind ◽  
Wave Trapping ◽  
Southern Great Plains ◽  
Direction Of Movement ◽  
Low Level Jet ◽  
Relationship Of ◽  
The Relationship ◽  
Scorer Parameter

This study documents atmospheric bores and other convergent boundaries in the southern Great Plains’ nocturnal environment during the IHOP_2002 summer campaign. Observational evidence demonstrates that convective outflows routinely generate bores. Statistically resampled flow regimes, derived from an adaptation of hydraulic theory, agree well with observations. Specifically, convective outflows within the observed environments are likely to produce a partially blocked flow regime, which is a favorable condition for generating a bore. Once a bore develops, the direction of movement generally follows the orientation of the bulk shear vector between the nose of the nocturnal low-level jet and a height of 1.5 or 2.5 km AGL. This relationship is believed to be a consequence of wave trapping through the curvature of the horizontal wind with respect to height. This conclusion comes after analyzing the profile of the Scorer parameter. Overall, these findings provide an impetus for future investigations aimed at understanding and predicting nocturnal deep convection over this region.

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