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Anthony W. Lyza
and
Matthew D. Flournoy

Abstract

Numerous questions remain regarding the influence of environmental inhomogeneities on supercell evolution. Motivated by this topic, this study associates cell-merger occurrence with supercell evolution and tornado production during the prolific 27–28 April 2011 outbreak in the U.S. Southeast. This event included 29 discrete supercells that produced 102 tornadoes and featured 300 cell mergers. Cell-merger frequency increased for supercells that initiated farthest east, possibly owing to changes in overall convective coverage over time. There is some signal for stronger mesocyclones to be associated with more mergers in the primary supercell’s forward flank. There is also a slight tendency for supercells that encounter more cell mergers to produce tornadoes more quickly, especially for those that formed away from a significant zonal boundary. However, there is a slight tendency for supercells spawning the longest-lived tornadoes (especially those with durations over 60 min) to be associated with fewer cell mergers during the 15-min window preceding tornadogenesis. Of particular importance, a significant inverse relationship exists between premerger mesocyclone strength and the subsequent change in mesocyclone strength during the merger (i.e., weaker mesocyclones tended to strengthen as a result of the merger, and vice versa). These findings highlight the influence that cell mergers can have on supercell evolution and tornado production—even within an incredibly volatile environment—and motivate future work exploring the physical processes involved and ways to translate these findings into experimental techniques or guidance for operational forecasters.

Significance Statement

The prolific 27–28 April 2011 supercell tornado outbreak in the U.S. Southeast featured 29 supercells that produced 102 tornadoes. This study analyzes mergers between these tornadic supercells and 300 weaker cells to determine if the mergers corresponded with important supercell characteristics. This appeared to be the case during this event; cell-merger events tended to be associated with tornadic periods of the supercells’ life cycles and influenced low- and midlevel mesocyclone strength, supercell evolutionary time scales, and subsequent tornado duration. These results are important for (i) better understanding of different supercell evolutionary paths in similar background environments and (ii) motivating future work in investigating experimental products related to these findings.

Free access
Anthony W. Lyza
and
Kevin R. Knupp
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Anthony W. Lyza
and
Kevin R. Knupp

Abstract

The effects of terrain on tornadoes are poorly understood. Efforts to understand terrain effects on tornadoes have been limited in scope, typically examining a small number of cases with limited observations or idealized numerical simulations. This study evaluates an apparent tornado activity maximum across the Sand Mountain and Lookout Mountain plateaus of northeastern Alabama. These plateaus, separated by the narrow Wills Valley, span ~5000 km2 and were impacted by 79 tornadoes from 1992 to 2016. This area represents a relative regional statistical maximum in tornadogenesis, with a particular tendency for tornadogenesis on the northwestern side of Sand Mountain. This exploratory paper investigates storm behavior and possible physical explanations for this density of tornadogenesis events and tornadoes. Long-term surface observation datasets indicate that surface winds tend to be stronger and more backed atop Sand Mountain than over the adjacent Tennessee Valley, potentially indicative of changes in the low-level wind profile supportive to storm rotation. The surface data additionally indicate potentially lower lifting condensation levels over the plateaus versus the adjacent valleys, an attribute previously shown to be favorable for tornadogenesis. Rapid Update Cycle and Rapid Refresh model output indicate that Froude numbers for the plateaus in tornadic environments are likely supportive of enhanced low-level flow over the plateaus, which further indicates the potential for favorable wind profile changes for tornado production. Examples of tornadic storms rapidly acquiring increased low-level rotation while reaching the plateaus of northeast Alabama are presented. The use of this background to inform the VORTEX-SE 2017 field campaign is discussed.

Full access
Anthony W. Lyza
,
Barrett T. Goudeau
, and
Kevin R. Knupp

Abstract

A tornado outbreak occurred across the Southeast United States on 13–14 April 2019, during the Verification of the Origins of Rotation in Tornadoes Experiment–Southeast (VORTEX-SE) Meso18-19 experiment. Among the most noteworthy events was a pair of large tornadoes in Monroe County, Mississippi, near the Columbus Air Force Base (GWX) Weather Surveillance Radar–1988 Doppler (WSR-88D). The second tornado, near the Greenwood Springs community, formed within the “no data” region near the radar and passed about 900 m to its east, rapidly strengthening into an intense tornado. This tornado produced forest devastation and electrical infrastructure damage up to at least EF4 intensity. The maximum radial velocity from GWX was 81.5 m s−1 (182 mph) in a resolution volume centered at 56 m (183 ft) above radar level. This paper presents a damage survey of the Greenwood Springs tornado and compares this assessment to the GWX data. A displacement of the maximum forest damage from the maximum radial velocity, despite the radar beam sampling <100 m ARL, is documented, as well as other likely effects of debris loading by the tornado on the observed radar signatures. The radar observations are placed into context with past mobile radar studies to illustrate the unique nature of this dataset. The relationship between radar data and damage observations, the implications for tornado structure in rough terrain and land cover, and the use of forest damage and radar data in tornado intensity estimation are discussed.

Significance Statement

This study showcases radar and damage observations of an intense tornado in a forested region of Mississippi. The formation of the tornado within 1 km of a WSR-88D allowed for near-surface radar observations to be collected as significant tree destruction was occurring. Doppler velocities below 60 m above radar level (ARL), near tree canopy top, exceeded 80 m s−1. Tree damage patterns were complicated while the tornado was near maximum intensity. The most severe tree damage was notably displaced from the highest radar-observed velocities, despite the radar sampling as low as 45 m ARL. These findings highlight challenges in utilizing radar data to estimate tornado intensity and structure, particularly in a region of relatively high surface and terrain roughness.

Full access
Anthony W. Lyza
,
Matthew D. Flournoy
, and
Erik N. Rasmussen

Abstract

An historic outbreak of tornadoes impacted a large swath of the eastern United States on 26–28 April 2011. The most severe series of tornadoes was associated with numerous classic supercell thunderstorms that developed across the Southeast during the afternoon and evening of 27 April and continued into the predawn of 28 April. This study documents characteristics of these storms with respect to tornado production and mesocyclone strength during different periods of each storm’s life cycle. The supercells initiated in four quasi-distinct spatiotemporal regions, with each cluster exhibiting slightly different evolutionary traits and tornado production. These included differences in the mean times between convection initiation and the time of first tornadogenesis for each supercell, as well as variations in overall and significant tornado production. This suggests that mesoscale environmental differences, such as proximity to a mesoscale boundary, and/or storm-scale events strongly influenced the variety of supercell evolutionary paths that were observed during this event, even in the presence of a synoptic-scale background environment extremely favorable for supercell and tornado production. The azimuthal shear products from the Multi-Year Reanalysis of Remotely Sensed Storms database perform well in discriminating between mesocyclones associated with ongoing weak, strong, and violent tornadoes during the event. Furthermore, mean azimuthal shear values during pre-tornadic (e.g., within 30 min of tornadogenesis) and tornadic phases are significantly larger than those during nontornadic phases. This warrants further study of azimuthal shear characteristics in different environments and its potential usefulness in aiding real-time forecasting efforts.

Significance Statement

This study documents the prolific supercell tornado outbreak that occurred in the southeastern United States on 27–28 April 2011. We associate tornado families with their parent supercells and use a radar-derived database to quantify changes in mesocyclone strength. We show that a variety of supercell evolutionary paths occurred during the event that were somewhat distinct based on where and when each supercell initiated. We also find significant differences between supercell intensity, characterized using azimuthal shear as a measure of mesocyclone strength, during nontornadic periods as opposed to the 30-min window prior to tornadogenesis. These findings are relevant for both researchers and operational forecasters and motivate future work to better understand relationships and processes influencing supercells and their background environments.

Free access
Matthew D. Flournoy
,
Anthony W. Lyza
,
Martin A. Satrio
,
Madeline R. Diedrichsen
,
Michael C. Coniglio
, and
Sean Waugh

Abstract

In this study, we present a climatology of observed cell mergers along the paths of 342 discrete, right-moving supercells and their association with temporal changes in low-level mesocyclone strength (measured using azimuthal shear). Nearly one-half of the examined supercells experience at least one cell merger. The frequency of cell merger occurrence varies somewhat by geographical region and the time of day. No general relationship exists between cell merger occurrence and temporal changes in low-level azimuthal shear; this corroborates prior studies in showing that the outcome of a merger is probably sensitive to storm-scale and environmental details not captured in this study. Interestingly, we find a significant inverse relationship between premerger azimuthal shear and the subsequent temporal evolution of azimuthal shear. In other words, stronger low-level mesocyclones are more likely to weaken after cell mergers and weaker low-level mesocyclones are more likely to strengthen. We also show that shorter-duration cell merger “events” (comprising multiple individual mergers) are more likely to be associated with a steady or weakening low-level mesocyclone whereas longer-duration cell merger events (3–4 individual mergers) are more likely to be associated with a strengthening low-level mesocyclone. These findings suggest what physical processes may influence the outcome of a merger in different scenarios and that the impact of these processes on low-level mesocyclone strength may change depending on storm maturity. We establish a baseline understanding of the supercell–cell merger climatology and highlight areas for future research in how to better anticipate the outcomes of cell mergers.

Significance Statement

A common assumption in idealized supercell simulations is that the background environment is homogeneous. Cells merging into a primary supercell represent one of many ways in which the environment might be significantly inhomogeneous. This study analyzes the paths of 342 supercells with a particular focus on how cell merger occurrence influences the strength of the low-level mesocyclone. Almost one-half of all supercells experience at least one cell merger. Supercells are more likely to weaken after a cell merger event if the premerger mesocyclone was strong or if the merger event is relatively short, and vice versa for the likelihood for a supercell to strengthen. These findings are important for those interested in short-term predictions of supercell evolution in response to cell mergers and suggest what dynamic processes may play a role in governing these relationships.

Full access
Anthony W. Lyza
,
Todd A. Murphy
,
Barrett T. Goudeau
,
Preston T. Pangle
,
Kevin R. Knupp
, and
Ryan A. Wade

Abstract

The Sand Mountain and Lookout Mountain Plateaus in northeastern Alabama have been established as a regional relative maximum in tornadogenesis reports within the southeastern United States. Investigation of long-term surface datasets has revealed (i) stronger and more backed winds atop Sand Mountain than over the Tennessee Valley, and (ii) measured cloud-base heights are lower to the surface atop Sand Mountain than over the Tennessee Valley. These observations suggest that low-level wind shear and lifting condensation level (LCL) height changes may lead to conditions more favorable for tornadogenesis atop the plateaus than over the Tennessee Valley. However, prior to fall 2016, no intensive observations had been made to further investigate low-level flow or thermodynamic changes in the topography of northeastern Alabama. This paper provides detailed analysis of observations gathered during VORTEX-SE field campaign cases from fall 2016 through spring 2019. These observations indicate that downslope winds form along the northwest edge of Sand Mountain in at least some severe storm environments in northeastern Alabama. Wind profiles gathered across northeastern Alabama indicate that low-level helicity changes can be substantial over small distances across different areas of the topographic system. LCL height changes often scale to changes in land elevation, which can be on the order of 200–300 m across northeastern Alabama.

Free access
Craig A. Clark
,
Travis J. Elless
,
Anthony W. Lyza
,
Bharath Ganesh-Babu
,
Dana M. Koning
,
Alexander R. Carne
,
Holly A. Boney
,
Amanda M. Sink
,
Sarah K. Mustered
, and
Justin M. Barrick

Abstract

This study has investigated the spatiotemporal structure and changes in Lake Michigan snowfall for the period 1950–2013. With data quality caveats acknowledged, a larger envelope of stations was included than in previous studies to explore the data using time series analysis, principal component analysis, and geographic information systems. Results indicate warming in recent decades, a near-dearth of serial correlation, midwinter dependence on teleconnection patterns, strong sensitivity of snowfall to temperature, peak snowfall variability and dependence on temperature within the lake-effect belt, an increasing fraction of seasonal snowfall occurring from December to February, and temporal behavior consistent with the previously reported trend reversal in snowfall.

Full access
Karen A. Kosiba
,
Anthony W. Lyza
,
Robert J. Trapp
,
Erik N. Rasmussen
,
Matthew Parker
,
Michael I. Biggerstaff
,
Stephen W. Nesbitt
,
Christopher C. Weiss
,
Joshua Wurman
,
Kevin R. Knupp
,
Brice Coffer
,
Vanna C. Chmielewski
,
Daniel T. Dawson
,
Eric Bruning
,
Tyler M. Bell
,
Michael C. Coniglio
,
Todd A. Murphy
,
Michael French
,
Leanne Blind-Doskocil
,
Anthony E. Reinhart
,
Edward Wolff
,
Morgan E. Schneider
,
Miranda Silcott
,
Elizabeth Smith
,
Joshua Aikins
,
Melissa Wagner
,
Paul Robinson
,
James M. Wilczak
,
Trevor White
,
David Bodine
,
Matthew R. Kumjian
,
Sean M. Waugh
,
A. Addison Alford
,
Kim Elmore
,
Pavlos Kollias
, and
David D. Turner

Abstract

Quasi-linear convective systems (QLCSs) are responsible for approximately a quarter of all tornado events in the U.S., but no field campaigns have focused specifically on collecting data to understand QLCS tornadogenesis. The Propagation, Evolution, and Rotation in Linear System (PERiLS) project was the first observational study of tornadoes associated with QLCSs ever undertaken. Participants were drawn from more than 10 universities, laboratories, and institutes, with over 100 students participating in field activities. The PERiLS field phases spanned two years, late winters and early springs of 2022 and 2023, to increase the probability of intercepting significant tornadic QLCS events in a range of large-scale and local environments. The field phases of PERiLS collected data in nine tornadic and nontornadic QLCSs with unprecedented detail and diversity of measurements. The design and execution of the PERiLS field phase and preliminary data and ongoing analyses are shown.

Open access