Multiscale aspects of the 26–27 April 2011 tornado outbreak. Part II: Environmental modifications and upscale feedbacks arising from latent processes

AbstractOne of the most prolific tornado outbreaks ever documented occurred on 26–27 April 2011 and comprised three successive episodes of tornadic convection that culminated with the development of numerous long-track, violent tornadoes over the southeastern U.S. during the afternoon of 27 April. This notorious afternoon supercell outbreak was preceded by two quasi-linear convective systems (hereafter QLCS1 and QLCS2), the first of which was an anomalously severe nocturnal system that rapidly grew upscale during the previous evening. In this Part II, we use a series of RUC 1-h forecasts and output from convection-permitting WRF-ARW simulations configured both with and without latent heat release to investigate how environmental modifications and upscale feedbacks produced by the two QLCSs contributed to the evolution and exceptional severity of this multi-episode outbreak.QLCS1 was primarily responsible for amplifying the large-scale flow pattern, inducing two upper-level jet streaks, and promoting secondary surface cyclogenesis downstream from the primary baroclinic system. Upper-level divergence markedly increased after QLCS1 developed, which yielded strong isallobaric forcing that rapidly strengthened the low-level jet (LLJ) and vertical wind shear over the warm sector and contributed to the system’s upscale growth and notable severity. Moreover, QLCS2 modified the mesoscale environment prior to the supercell outbreak by promoting the downstream formation of a pronounced upper-level jet streak, altering the midlevel jet structure, and furthering the development of a highly ageostrophic LLJ over the Southeast. Collectively, the flow modifications produced by both QLCSs contributed to the notably favorable shear profiles present during the afternoon supercell outbreak.

Multiscale aspects of the 26–27 April 2011 tornado outbreak. Part I: Outbreak chronology and environmental evolution

Monthly Weather Review ◽

10.1175/mwr-d-21-0013.1 ◽

2021 ◽

Author(s):

Manda B. Chasteen ◽

Steven E. Koch

Keyword(s):

Mississippi Valley ◽

Environmental Evolution ◽

Rossby Wave Breaking ◽

Convective Systems ◽

South Central ◽

Rapid Flow ◽

Low Level Jet ◽

Upper Level ◽

Tornado Outbreaks ◽

AbstractOne of the most prolific tornado outbreaks ever documented occurred on 26–27 April 2011 and comprised three successive episodes of tornadic convection that primarily impacted the southeastern U.S., including two quasi-linear convective systems (hereafter QLCS1 and QLCS2) that preceded the notorious outbreak of long-track, violent tornadoes spawned by numerous supercells on the afternoon of 27 April. The ~36-h period encompassing these three episodes was part of a longer multiday outbreak that occurred ahead of a slowly moving upper-level trough over the Rocky Mountains. In this Part I, we detail how the environment evolved to support this extended outbreak, with particular attention given to the three successive systems that each exhibited a different morphology and severity.The amplifying upper-level trough and attendant jet streak resulted from a Rossby wave breaking event that yielded a complex tropopause structure and supported three prominent shortwave troughs that sequentially moved into the south-central U.S. QLCS1 formed ahead of the second shortwave and was accompanied by rapid flow modifications, including considerable low-level jet (LLJ) intensification. The third shortwave moved into the lee of the Rockies early on 27 April to yield destabilization behind QLCS1 and support the formation of QLCS2, which was followed by further LLJ intensification and helped to establish favorable deep-layer shear profiles over the warm sector. The afternoon supercell outbreak commenced following the movement of this shortwave into the Mississippi Valley, which was attended by a deep tropopause fold, cold front aloft, and dryline that promoted two prominent bands of tornadic supercells over the Southeast.

A Preliminary Study of Severe Wind-Producing MCSs in Environments of Limited Moisture

10.1175/waf947.1 ◽

2006 ◽

Vol 21 (5) ◽

pp. 715-734 ◽

Cited By ~ 9

Author(s):

Stephen F. Corfidi ◽

Sarah J. Corfidi ◽

David A. Imy ◽

Allen L. Logan

Keyword(s):

Convective Initiation ◽

Composite Surface ◽

Convective Systems ◽

Mesoscale Convective ◽

Upper Level ◽

Jet Streaks ◽

Cell Propagation ◽

Wind Profiles ◽

Upper Level Jet ◽

Limited Moisture

Abstract An examination of severe wind-producing mesoscale convective systems that occur in environments of very limited moisture is presented. Such systems, herein referred to as low-dewpoint derechos (LDDs), are difficult to forecast as they form in regions where the level of convective instability is well below that normally associated with severe convective weather. Using a dataset consisting of 12 LDDs that affected various parts of the continental United States, composite surface and upper-level analyses are constructed. These are used to identify factors that appear to be associated with LDD initiation and sustenance. It is shown that LDDs occur in mean kinematic and thermodynamic patterns notably different from those associated with most derechos. LDDs typically form along or just ahead of cold fronts, in the exit region of strong, upper-level jet streaks. Based on the juxtaposition of features in the composite analysis, it appears that linear forcing for ascent provided by the front, and/or ageostrophic circulations associated with the jet streak, induce the initial convective development where the lower levels are relatively dry, but lapse rates are steep. This convection subsequently grows upscale as storm downdrafts merge. The data further suggest that downstream cell propagation follows in the form of sequential, downwind-directed microbursts. Largely unidirectional wind profiles promote additional downwind-directed storm development and system sustenance until the LDD ultimately moves beyond the region supportive of forced convective initiation.

Environments of Northeast U.S. Severe Thunderstorm Events from 1999 to 2009

10.1175/waf-d-12-00042.1 ◽

2014 ◽

Vol 29 (1) ◽

pp. 3-22 ◽

Cited By ~ 10

Author(s):

Melissa M. Hurlbut ◽

Ariel E. Cohen

Keyword(s):

Large Scale ◽

Weather Forecasting ◽

Convective Available Potential Energy ◽

Severe Weather ◽

Severe Thunderstorms ◽

Weather Events ◽

Upper Level ◽

Upper Level Jet ◽

Lapse Rates ◽

Jet Structure

Abstract An investigation of the environments and climatology of severe thunderstorms from 1999 through 2009 across the northeastern United States is presented. A total of 742 severe weather events producing over 12 000 reports were examined. Given the challenges that severe weather forecasting can present in the Northeast, this study is an effort to distinguish between the more prolific severe-weather-producing events and those that produce only isolated severe weather. The meteorological summer months (June–August) are found to coincide with the peak severe season. During this time, 850–500- and 700–500-hPa lapse rates, mixed layer convective inhibition (MLCIN), and downdraft convective available potential energy (DCAPE) are found to be statistically significant in discriminating events with a large number of reports from those producing fewer reports, based on observed soundings. Composite synoptic pattern analyses are also presented to spatially characterize the distribution of key meteorological variables associated with severe weather events of differing magnitudes. The presence of a midlevel trough and particular characteristics of its tilt, along with an accompanying zone of enhanced flow, are found in association with the higher-report severe weather events, along with cooler midlevel temperatures overlaying warmer low-level temperatures (i.e., contributing to the steeper lapse rates). During the meteorological fall and winter months (September–February), large-scale ascent is often bolstered by the presence of a coupled upper-level jet structure.

Examining the Relationship between Tropopause Polar Vortices and Tornado Outbreaks

10.1175/waf-d-21-0058.1 ◽

2021 ◽

Author(s):

Matthew T. Bray ◽

Steven M. Cavallo ◽

Howard B. Bluestein

Keyword(s):

Wind Shear ◽

Large Scale ◽

Gulf Of Alaska ◽

Static Stability ◽

The Arctic ◽

The North ◽

Jet Streaks ◽

Significant Patterns ◽

Tornado Outbreaks ◽

AbstractMid-latitude jet streaks are known to produce conditions broadly supportive of tornado outbreaks, including forcing for large-scale ascent, increased wind shear, and decreased static stability. Although many processes may initiate a jet streak, we focus here on the development of jet maxima by interactions between the polar jet and tropopause polar vortices (TPVs). Originating from the Arctic, TPVs are long-lived circulations on the tropopause, which can be advected into the mid-latitudes. We hypothesize that when these vortices interact with the jet, they may contribute supplemental forcing for ascent and shear to tornado outbreaks, assuming other environmental conditions supportive of tornado development exist. Using a case set of significant tornado outbreak days from three states—Oklahoma, Illinois, and Alabama—we show that a vortex-jet streak structure is present (within 1250 km) in around two-thirds of tornado outbreaks. These vortices are commonly Arctic in origin (i.e., are TPVs) and are advected through a consistent path of entry into the mid-latitudes in the week before the outbreak, moving across the Northern Pacific and into the Gulf of Alaska before turning equatorward along the North American coast. These vortices are shown to be more intense and longer-lived than average. We further demonstrate that statistically significant patterns of wind shear, quasi-geostrophic forcing for ascent, and low static stability are present over the outbreak regions on the synoptic scale. In addition, we find that TPVs associated with tornadic events occur most often in the spring and are associated with greater low-level moisture when compared to non-tornadic TPV cases.

Role of the Low-Frequency Deformation Field on the Explosive Growth of Extratropical Cyclones at the Jet Exit. Part II: Baroclinic Critical Region

Journal of the Atmospheric Sciences ◽

10.1175/jas3729.1 ◽

2006 ◽

Vol 63 (8) ◽

pp. 1982-1995 ◽

Cited By ~ 26

Author(s):

G. Rivière ◽

A. Joly

Keyword(s):

Critical Region ◽

High Frequency ◽

Large Scale ◽

Intermediate Phase ◽

Low Frequency ◽

Atmospheric Flow ◽

Upper Level ◽

Large Scale Flow ◽

Upper Level Jet

Abstract Midlatitude cyclones tend to develop strongly in specific locations relative to the large-scale flow, such as jet-exit zones. Here, the approach developed in Part I that highlights the role of large-scale deformation in constraining the location of such events is continued. The atmospheric flow is decomposed into a high- and low-frequency part separating large and synoptic scales. A new low-frequency diagnostic has been introduced, called effective deformation Δm. It is defined as σ2m − ζ2m, where σm is the low-frequency deformation magnitude and ζm is the low-frequency vorticity. While Part I focused on large-scale conditions inducing an intermediate phase of barotropic growth, the present paper concentrates on other configurations that rather prevent this phase from happening. This large-scale circulation is characterized by the presence of a strong zonal upper-level jet and a lower-level jet that are meridionally quite far from each other over the Atlantic but close to one another in the eastern Atlantic region. As high-frequency disturbances are trapped by the effective deformation of the low-frequency jets, the increasing closeness of the two jets associated with that of the two effective deformation fields computed in the lower and upper levels defines a region called the baroclinic critical region where upper high-frequency disturbances and surface cyclones may strongly interact baroclinically. The increased baroclinic energy collection resulting from this constrained configuration change is outlined. An analysis of the explosive growth of the Christmas wind storms of 1999 and of mid-December 2004 provides different realizations of this configuration and associated mechanism.

Climatology of Storm Reports Relative to Upper-Level Jet Streaks

10.1175/2009waf2222216.1 ◽

2009 ◽

Vol 24 (4) ◽

pp. 1032-1051 ◽

Cited By ~ 23

Author(s):

Adam J. Clark ◽

Christopher J. Schaffer ◽

William A. Gallus ◽

Kaj Johnson-O’Mara

Keyword(s):

Vertical Velocity ◽

Conceptual Models ◽

Temporal Trends ◽

Vertical Motion ◽

Upper Level ◽

Jet Streaks ◽

Upper Level Jet ◽

Four Quadrant ◽

The Right ◽

Abstract Using quasigeostrophic arguments and numerical simulations, past works have developed conceptual models of vertical circulations induced by linear and curved jet streaks. Because jet-induced vertical motion could influence the development of severe weather, these conceptual models, especially the “four quadrant” model for linear jet streaks, are often applied by operational forecasters. The present study examines the climatology of tornado, hail, and severe wind reports relative to upper-level jet streaks, along with temporal trends in storm report frequencies and changes in report distributions for different jet streak directions. In addition, composite fields (e.g., divergence, vertical velocity) are analyzed for jet streak regions to examine whether the fields correspond to what is expected from conceptual models of curved or linear jet streaks, and whether the fields help explain the storm report distributions. During the period analyzed, 84% of storm reports were associated with upper-level jet streaks, with June–August having the lowest percentages. In March and April the left-exit quadrant had the most storm reports, while after April the right-entrance quadrant was associated with the most reports. Composites revealed that tornado and hail reports are concentrated in the jet-exit region along the major jet axis and in the right-entrance quadrant. Wind reports have similar maxima, but the right-entrance quadrant maximum is more pronounced. Upper-level composite divergence fields generally correspond to what would be expected from the four-quadrant model, but differences in the magnitudes of the vertical velocity between the quadrants and locations of divergent–convergent centers may have resulted from jet curvature. The maxima in the storm report distributions are not well collocated with the maxima in the upper-level divergence fields, but are much better collocated with low-level convergence maxima that exist in both exit regions and extend into the right-entrance region. Composites of divergence–convergence with linear, cyclonic, and anticyclonic jet streaks also generally matched conceptual models for curved jet streaks, and it was found that wind reports have a notable maximum in the right-entrance quadrant of both anticyclonic and linear jet streaks. Finally, it was found that the upper-level divergence and vertical velocity in all jet-quadrants have a tendency to decrease as jet streak directions shift from SSW to NNW.

Barotropic Regeneration of Upper-Level Synoptic Disturbances in Different Configurations of the Zonal Weather Regime

Journal of the Atmospheric Sciences ◽

10.1175/2008jas2603.1 ◽

2008 ◽

Vol 65 (10) ◽

pp. 3159-3178 ◽

Cited By ~ 10

Author(s):

Gwendal Rivière

Keyword(s):

Large Scale ◽

Deformation Field ◽

Regeneration Process ◽

Barotropic Model ◽

Weather Regime ◽

The North ◽

The Difference ◽

Upper Level ◽

Composite Maps ◽

Large Scale Flow

Barotropic dynamics of upper-tropospheric midlatitude disturbances evolving in different configurations of the zonal weather regime (i.e., in different zonal-like large-scale flows) were studied using observational analyses and barotropic model experiments. The contraction stage of upper-level disturbances that follows their elongation stage leads to an increase of eddy kinetic energy that is called the barotropic regeneration process in this text. This barotropic mechanism is studied through notions of barotropic critical regions (BtCRs) and effective deformation that have been introduced in a previous paper. The effective deformation field is equal to the difference between the square of the large-scale deformation magnitude and the square of the large-scale vorticity. Regions where the effective deformation is positive correspond to regions where the large-scale flow tends to strongly stretch synoptic disturbances. A BtCR is an area separating two large-scale regions of positive effective deformation, one located upstream and on the south side of the jet and the other downstream and on the north side. Such a region presents a discontinuity in the orientation of the dilatation axes and is a potential area where the barotropic regeneration process may occur. Winter days presenting a zonal weather regime in the 40-yr ECMWF Re-Analysis dataset are decomposed, via a partitioning algorithm, into different configurations of the effective deformation field at 300 hPa. A six-cluster partition is obtained. Composite maps of the barotropic generation rate for each cluster exhibit a succession of negative and positive values on both sides of the BtCRs. It confirms statistically that the barotropic regeneration mechanism occurs preferentially about BtCRs. Numerical experiments using a forced barotropic model on the sphere are performed. Each experiment consists of adding a synoptic-scale perturbation to one of the zonal-like jet configurations found in observations, which is kept fixed with time. The combined effects of the effective deformation and nonlinearities are shown to be crucial to reproduce the barotropic regeneration process about BtCRs.

Cause Analysis on Eastward Movement of Southwest China Vortex and Its Induced Heavy Rainfall in South China

Advances in Meteorology ◽

10.1155/2015/481735 ◽

2015 ◽

Vol 2015 ◽

pp. 1-22 ◽

Cited By ~ 9

Author(s):

Yongren Chen ◽

Yueqing Li ◽

Tianliang Zhao

Keyword(s):

South China ◽

Heavy Rainfall ◽

Southwest China ◽

Dominant Role ◽

Vertical Wind Shear ◽

Heavy Rain ◽

Convective Systems ◽

Rainfall Occurrence ◽

Positive Vorticity ◽

Upper Level Jet

The movement of southwest China vortex (SWV) and its heavy rainfall process in South China had been investigated during June 11–14, 2008. The results show that under the steering of upper-level jet (ULJ) and mid-level westerly trough, SWV moved eastward from southern Sichuan Plateau, across eastern Yunnan-Guizhou Plateau to South China, forming an obvious heavy rain belt. SWV developed in the large storm-relative helicity (SRH) environment, as environmental wind field continuously transferred positive vorticity to it to support its development. The thermodynamic structures of distinctive warm (cold) advections in front (rear) of the SWV movement are also important factors for the SWV evolutions with a southwest low-level jet (LLJ) and vertical wind shear. SWV development was associated with the distributions of negative MPV1 (the barotropic item of moist potential vorticity) and positive MPV2 (the baroclinic item of it). The MPV1 and MPV2 played the dominant role in the formation and the evolution of SWV, respectively. The mesoscale convective systems (MCSs) frequently occurred and persisted in water vapor convergence areas causing the severe heavy rainfall. The areas of high moist helicity divergence and heavy rainfall are consistent, and the moist helicity divergence could be a good indicator for heavy rainfall occurrence.

Understanding Intermodel Variability in Future Projections of a Sahelian Storm Proxy and Southern Saharan Warming

Journal of Climate ◽

10.1175/jcli-d-20-0382.1 ◽

2021 ◽

Vol 34 (2) ◽

pp. 509-525

Author(s):

David P. Rowell ◽

Rory G. J. Fitzpatrick ◽

Lawrence S. Jackson ◽

Grace Redmond

Keyword(s):

Large Scale ◽

Climate Models ◽

Global Climate ◽

Vertical Wind Shear ◽

Global Climate Models ◽

Tropospheric Temperature ◽

Storm Activity ◽

Convective Systems ◽

The North ◽

Projected Changes

AbstractProjected changes in the intensity of severe rain events over the North African Sahel—falling from large mesoscale convective systems—cannot be directly assessed from global climate models due to their inadequate resolution and parameterization of convection. Instead, the large-scale atmospheric drivers of these storms must be analyzed. Here we study changes in meridional lower-tropospheric temperature gradient across the Sahel (ΔTGrad), which affect storm development via zonal vertical wind shear and Saharan air layer characteristics. Projected changes in ΔTGrad vary substantially among models, adversely affecting planning decisions that need to be resilient to adverse risks, such as increased flooding. This study seeks to understand the causes of these projection uncertainties and finds three key drivers. The first is intermodel variability in remote warming, which has strongest impact on the eastern Sahel, decaying toward the west. Second, and most important, a warming–advection–circulation feedback in a narrow band along the southern Sahara varies in strength between models. Third, variations in southern Saharan evaporative anomalies weakly affect ΔTGrad, although for an outlier model these are sufficiently substantive to reduce warming here to below that of the global mean. Together these uncertain mechanisms lead to uncertain southern Saharan/northern Sahelian warming, causing the bulk of large intermodel variations in ΔTGrad. In the southern Sahel, a local negative feedback limits the contribution to uncertainties in ΔTGrad. This new knowledge of ΔTGrad projection uncertainties provides understanding that can be used, in combination with further research, to constrain projections of severe Sahelian storm activity.

The Dynamic and Thermodynamic Structures Associated with a Series of Heavy Precipitation Events over China during January 2008

10.1175/2010waf2222335.1 ◽

2010 ◽

Vol 25 (4) ◽

pp. 1124-1141 ◽

Cited By ~ 16

Author(s):

Xiaohui Shi ◽

Xiangde Xu ◽

Chungu Lu

Keyword(s):

Large Scale ◽

East Coast ◽

Heavy Precipitation ◽

Chinese History ◽

Physical Processes ◽

Ice Storms ◽

Upper Level ◽

Upper Level Jet ◽

The Western Pacific ◽

Precipitation Events

Abstract In the winter of 2008, China experienced once-in-50-yr (or once in 100 yr for some regions) snow and ice storms. These storms brought huge socio economical impacts upon the Chinese people and government. Although the storms had been predicted, their severity and persistence were largely underestimated. In this study, these cases were revisited and comprehensive analyses of the storms’ dynamic and thermodynamic structures were conducted. These snowstorms were also compared with U.S. east coast snowstorms. The results from this study will provide insights on how to improve forecasts for these kinds of snowstorms. The analyses demonstrated that the storms exhibited classic patterns of large-scale circulation common to these types of snowstorms. However, several physical processes were found to be unique and thought to have played crucial roles in intensifying and prolonging China’s great snowstorms of 2008. These include a subtropical high over the western Pacific, an upper-level jet stream, and temperature and moisture inversions. The combined effects of these dynamic and thermodynamic structures are responsible for the development of the storms into one of the most disastrous events in Chinese history.