Late-Spring Severe Blizzard Events over Eastern Romania: A Conceptual Model of Development

In this paper, the mechanism and model-representation of a late-spring severe blizzard event on eastern Romania are studied. The mechanism relies on the coupled contribution of the tropospheric ageostrophic circulations associated to jet streaks. These circulations: (1) interact under local and regional forcing (sea surface temperature, topography and latent heat) and (2) feedback on enhancing an upper-level jet’s secondary streak, leading to a persistent, severe event. The enhanced secondary jet streak appears only for developing systems that lead to extreme impact, as shown by 40 years of knowledge of late-spring severe blizzards over the area. It is shown that actual regional high-resolution models are able to represent the occurrence and the mechanism of late-spring severe blizzard events, thus increasing the confidence on their ability to represent current and future climate extreme variability. Understanding the preconditioning of dynamic and thermodynamic processes indicated by this analysis could be useful in supporting the operational forecast analysis.

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

Monthly Weather Review ◽

10.1175/mwr-d-21-0014.1 ◽

2021 ◽

Author(s):

Manda B. Chasteen ◽

Steven E. Koch

Keyword(s):

Large Scale ◽

Vertical Wind Shear ◽

Convective Systems ◽

Upper Level ◽

Large Scale Flow ◽

Jet Streaks ◽

Upper Level Jet ◽

Jet Structure ◽

Tornado Outbreaks ◽

Jet Streak

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.

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Cyclogenesis Downstream of Extratropical Transition Analyzed by Q-Vector Partitioning Based on Flow Geometry

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-14-0023.1 ◽

2014 ◽

Vol 71 (11) ◽

pp. 4204-4220 ◽

Cited By ~ 14

Author(s):

Michael Riemer ◽

Marlene Baumgart ◽

Sven Eiermann

Keyword(s):

Vertical Motion ◽

Geostrophic Wind ◽

Extratropical Transition ◽

Secondary Importance ◽

Upper Level ◽

Jet Streaks ◽

Balanced Dynamics ◽

Jet Streak ◽

Q Vector

Abstract During extratropical transition (ET), tropical cyclones exert a significant impact on the midlatitude circulation. Archetypical features of this impact are jet streak formation, amplification of the downstream trough, and modification of the associated downstream cyclogenesis. This study investigates the relative importance of the jet streak and the upper-level trough for cyclone development by quantifying the respective contributions to midtropospheric vertical motion using the Q-vector partitioning by J. C. Jusem and R. Atlas. Their framework is here extended from quasigeostrophic theory to alternative balance. The Q vector under alternative balance involves the nondivergent wind, instead of the geostrophic wind, and therefore represents more accurately the balanced dynamics associated with vertical motion, in particular downstream of ET where the flow often exhibits significant curvature associated with the amplified trough. An idealized ET scenario and three real cases, the cyclones downstream of Hanna (2008), Choi-wan (2008), and Jangmi (2009), are analyzed. In all cases, the trough plays a prominent role in cyclone development. The jet streak plays a prominent, favorable role in the idealized ET scenario and downstream of Hanna. In contrast, the role of the jet streak downstream of Choi-wan is clearly of secondary importance. Interestingly, downstream of Jangmi the jet streak has a prominent but detrimental impact. It is concluded that amplified jet streaks associated with ET have the potential to be of significant importance for downstream cyclone development. The few cases considered in this study, however, point to a large case-to-case variability of the role of the jet streak.

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Climatology of Storm Reports Relative to Upper-Level Jet Streaks

Weather and Forecasting ◽

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 ◽

Jet Streak

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.

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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 ◽

Jet Streak

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.

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Relationship between sea surface temperature anomalies over tropical Atlantic in late spring and East Asian summer monsoon

10.5194/egusphere-egu2020-6293 ◽

2020 ◽

Author(s):

Joong-Bae Ahn ◽

Yeon-Woo Choi

Keyword(s):

Surface Temperature ◽

Sea Surface Temperature ◽

Summer Monsoon ◽

East Asian Summer Monsoon ◽

Asian Summer Monsoon ◽

East Asian ◽

Late Spring ◽

Sea Surface ◽

Tropical Atlantic ◽

Asian Summer

This study investigates the relationship between the preceding late spring Sea Surface Temperature (SST) over the tropical Atlantic and the East Asian Summer Monsoon (EASM) based on the observational data and Coupled Model Intercomparison Project Phase 5 (CMIP5) historical simulations. The results show that warm (cold) tropical Atlantic SST (TASST) during May tends to be followed by a strong (weak) EASM with positive (negative) precipitation anomalies over the subtropical frontal area. Evidence is also provided that the atmospheric teleconnections propagating in both east and west directions are the key mechanisms linking the EASM with the preceding May TASST. That is, the warm TASST anomaly during late spring can persist through the subsequent summer, which, in turn, induces the Gill-type Rossby wave response in the eastern Pacific, exciting the westward relay of the Atlantic signal, as well as the eastward propagation of the Rossby wave along the jet stream. Furthermore, the westward (eastward) propagating teleconnection signal may induce the anomalous anticyclone in the lower troposphere over the Philippine Sea (anomalous tropospheric anticyclone with barotropic structure over the Okhotsk Sea). The anomalous anticyclonic circulation over the Philippine Sea (Okhotsk Sea) brings warm and humid (cold) air to higher latitudes (lower latitudes). These two different types of air mass merge over the Baiu-Meiyu&#8211;Changma region, causing the enhanced subtropical frontal rainfall. To support the observational findings, CMIP5 historical simulations are also utilized. Most state-of-the-art CMIP5 models can simulate this relationship between May TASST and the EASM.Reference: Choi, Y., Ahn, J. Possible mechanisms for the coupling between late spring sea surface temperature anomalies over tropical Atlantic and East Asian summer monsoon. Clim Dyn 53,&#160;6995&#8211;7009 (2019) doi:10.1007/s00382-019-04970-3Acknowledgment: This work was funded by the Korea Meteorological Administration Research and Development Program under Grant KMI2018-01213.&#160;

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

Weather and Forecasting ◽

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.

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Influence of Sea Surface Temperature on a Mesoscale Convective System producing Extreme Rainfall over the Yellow Sea

Monthly Weather Review ◽

10.1175/mwr-d-20-0335.1 ◽

2021 ◽

Author(s):

Yunhee Kang ◽

Jong-Hoon Jeong ◽

Dong-In Lee

Keyword(s):

Surface Temperature ◽

Sea Surface Temperature ◽

Yellow Sea ◽

Extreme Rainfall ◽

Sea Surface ◽

The Yellow Sea ◽

Convective System ◽

Mesoscale Convective ◽

Upper Level ◽

Convective Cells

AbstractAn extreme rainfall-producing linear mesoscale convective systems (MCSs) occurred over the Yellow Sea, Korea, on 13 August 2012, producing 430 mm of rainfall in less than 12 h, causing devastating flash floods and landslides. To understand the causative processes underlying this event, we examined the linear MCSs formation and development mechanisms using observations and cloud-resolving models. The organized linear MCSs produced extreme rainfall at Gunsan in a favorable large-scale environment. The synoptic environment showed the stationary surface front elongating from China to Korea; a southwesterly low-level jet transported the warm, moist air from low latitudes towards the front. In the upper-level synoptic environment, the trough and entrance regions of the upper-level jet were north of the heavy rainfall, promoting the development of convection. The extreme rainfall over the Gunsan area resulted from the back-building mode of the MCSs, in which new convective cells continued to pass over the same area while the entire convective system was nearly stationary. The sea surface temperature (SST) during the extreme rainfall events was abnormally 1°C higher than the 30-year climatological mean, and a local warm pool (>28.5°C) existed where the convective cells were continuously initiated. Cloud-resolving models simulated the low-level convergence, and the latent heat flux was large in the local warm SST field. These induced MCSs formation and development, contributing to a significant rainfall increase over the Yellow Sea. The terrain’s influence on the large rainfall amount in the area was also noted.

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A cut-off low in southern South America: dynamic and thermodynamic processes

Revista Brasileira de Meteorologia ◽

10.1590/s0102-77862011000400001 ◽

2011 ◽

Vol 26 (4) ◽

pp. 503-514 ◽

Cited By ~ 7

Author(s):

Alejandro Anibal Godoy ◽

Norma Edit Possia ◽

Claudia Marcela Campetella ◽

Yanina García Skabar

Keyword(s):

Lower Troposphere ◽

Pressure System ◽

Horizontal Advection ◽

The Andes ◽

The Pacific ◽

Divergence Effect ◽

The Pacific Ocean ◽

Cutoff Low ◽

Upper Level ◽

Thermodynamic Processes

The dynamic and thermodynamic processes involved in the life cycle of a cutoff low occurred in March 2007 are studied. These processes are analyzed using the vorticity and thermodynamic equations and a set of analyses generated with the BRAMS model. The main processes that explain the segregation of the subtropical part of the trough are the horizontal advection of cyclonic vorticity at high levels and warm horizontal advection at middle levels, both over the Pacific Ocean extending south to the Patagonia region, building the ridge located upstream of the trough. Increased intensity of the upper level low pressure system is mainly explained by intensification of the ridges down and upstream. The divergence effect is opposed to the horizontal advection of vorticity which explains the stagnation of the cut-off low windward of the Andes. The decay stage is dominated by warm vertical advection. Assuming the conservation of potential vorticity the analysis of parcel trajectories , allowed detecting the entrance of stratospheric air to middle levels of the lower troposphere around the cut-off low.

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