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Bogdan Antonescu, Jonathan G. Fairman Jr., and David M. Schultz

Abstract

On 24–25 June 1967 one of the most intense European tornado outbreaks produced extensive damage (approximately 960 houses damaged or destroyed) and resulted in 232 injuries and 15 fatalities in France, Belgium, and the Netherlands. The 24–25 June 1967 tornado outbreak shows that Europe is highly vulnerable to tornadoes. To better understand the impact of European tornadoes and how this impact changed over time, the question is raised, “What would happen if an outbreak similar to the 1967 one occurred 50 years later in 2017 over France, Belgium, and the Netherlands?” Transposing the seven tornado tracks from the June 1967 outbreak over the modern landscape would potentially result in 24 990 buildings being impacted, 255–2580 injuries, and 17–172 fatalities. To determine possible worst-case scenarios, the tornado tracks are moved in a systematic way around their observed positions and positioned over modern maps of buildings and population. The worst-case scenario estimates are 146 222 buildings impacted, 2550–25 440 injuries, and 170–1696 fatalities. These results indicate that the current disaster management policies and mitigation strategies for Europe need to include tornadoes, especially because exposure and tornado risk is anticipated to increase in the near future.

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Stephen F. Corfidi, Sarah J. Corfidi, and David M. Schultz

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The term elevated convection is used to describe convection where the constituent air parcels originate from a layer above the planetary boundary layer. Because elevated convection can produce severe hail, damaging surface wind, and excessive rainfall in places well removed from strong surface-based instability, situations with elevated storms can be challenging for forecasters. Furthermore, determining the source of air parcels in a given convective cloud using a proximity sounding to ascertain whether the cloud is elevated or surface based would appear to be trivial. In practice, however, this is often not the case. Compounding the challenges in understanding elevated convection is that some meteorologists refer to a cloud formation known as castellanus synonymously as a form of elevated convection. Two different definitions of castellanus exist in the literature—one is morphologically based (cloud formations that develop turreted or cumuliform shapes on their upper surfaces) and the other is physically based (inferring the turrets result from the release of conditional instability). The terms elevated convection and castellanus are not synonymous, because castellanus can arise from surface-based convection and elevated convection exists that does not feature castellanus cloud formations. Therefore, the purpose of this paper is to clarify the definitions of elevated convection and castellanus, fostering a better understanding of the relevant physical processes. Specifically, the present paper advocates the physically based definition of castellanus and recommends eliminating the synonymity between the terms castellanus and elevated convection.

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Sam Hardy, David M. Schultz, and Geraint Vaughan

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Major river flooding affected the United Kingdom in late September 2012 as a slow-moving extratropical cyclone brought over 150 mm of rain to parts of northern England and north Wales. The cyclone deepened over the United Kingdom on 24–26 September as a potential vorticity (PV) anomaly approached from the northwest, elongated into a PV streamer, and wrapped around the cyclone. The strength and position of the PV anomaly is modified in the initial conditions of Weather Research and Forecasting Model simulations, using PV surgery, to examine whether different upper-level forcing, or different phasing between the PV anomaly and cyclone, could have produced an even more extreme event. These simulations reveal that quasigeostrophic (QG) forcing for ascent ahead of the anomaly contributed to the persistence of the rainfall over the United Kingdom. Moreover, weakening the anomaly resulted in lower rainfall accumulations across the United Kingdom, suggesting that the impact of the event might be proportional to the strength of the upper-level QG forcing. However, when the anomaly was strengthened, it rotated cyclonically around a large-scale trough over Iceland rather than moving eastward as in the verifying analysis, with strongly reduced accumulated rainfall across the United Kingdom. A similar evolution developed when the anomaly was moved farther away from the cyclone. Conversely, moving the anomaly nearer to the cyclone produced a similar solution to the verifying analysis, with slightly increased rainfall totals. These counterintuitive results suggest that the verifying analysis represented almost the highest-impact scenario possible for this flooding event when accounting for sensitivity to the initial position and strength of the PV anomaly.

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Russ S. Schumacher, David M. Schultz, and John A. Knox

Abstract

Convective snowbands moved slowly over Wyoming and northern Colorado on 16–17 February 2007 and produced up to 71 mm (2.8 in.) of snow that was unpredicted by operational numerical weather prediction models and human forecasters. The northwest–southeast-oriented bands lasted for over 6 h, comprising both a single major band (more than 30 km wide) and multiple minor bands (about 10 km wide). The convective bands initiated within the ascending branch of a secondary circulation associated with both near-surface and elevated frontogenesis, but the bands remained nearly stationary while the near-surface frontogenesis moved quickly equatorward. The bands occurred downstream of complex terrain on the anticyclonic-shear side of a midlevel jet streak, where conditional, dry symmetric (negative potential vorticity), and inertial (negative absolute vorticity) instabilities were present.

To determine the mechanisms responsible for the development and organization of these bands, simulations using a convection-permitting numerical model are conducted. In contrast to the operational models, these simulations are able to produce convective bands in the same area and at about the same time as that observed. The simulated bands occurred in an environment with a nearly well-mixed, baroclinic boundary layer, positive convective available potential energy, and widespread negative potential vorticity. Individual bands initiated on the low-momentum side of vorticity banners downstream of mountains, and in association with frontogenetical ascent along two baroclinic zones. In addition, ascent caused by both frontogenesis and banded moist convection produced additional narrow regions of negative vorticity by transporting low-momentum air upward and creating strong horizontal gradients in wind speed. This event is similar to other observed instances of banded convection in the western United States on the anticyclonic-shear side of strong mid- and upper-tropospheric jets in environments lacking large-scale saturation. In contrast, these events differ from previously published banded precipitation events in the comma head of extratropical cyclones and downstream of mountains where large-scale saturation is present.

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David M. Schultz, Daniel Keyser, and Lance F. Bosart

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Observational and modeling studies documented in the literature indicate that the large-scale flow has an important effect on the structure and evolution of low-level fronts in midlatitude cyclones. The purpose of this paper is to address the role of the large-scale flow on low-level cyclone/frontal structure and evolution through a combined observational and idealized modeling approach.

Analyses of two observed cyclone cases embedded in large-scale diffluence and confluence, respectively, are presented to illustrate two possible cyclone/frontal structures and evolutions. Specifically, the cyclone moving into a diffluent, high-amplitude ridge becomes meridionally elongated and possesses a strong meridionally oriented cold front and a weak warm front. The cold front rotates into the warm front, forming an occluded front in the manner of the Norwegian cyclone model, as indicated by the narrowing of the thermal ridge connecting the warm sector to the cyclone center. In contrast, the cyclone moving into confluent, low-amplitude zonal flow becomes zonally elongated and possesses strong zonally oriented warm and bent-back fronts and a weak cold front. The frontal structure in this case is reminiscent of the Shapiro–Keyser cyclone model, exhibiting a fracture between perpendicularly oriented cold and warm fronts (i.e., the so-called frontal T-bone structure).

The idealized simulations employ a nondivergent barotropic model in which potential temperature is treated as a passive tracer. When a circular vortex acts on an initially zonally oriented baroclinic zone, cold and warm fronts, a frontal fracture, a bent-back front, and eventually a Norwegian-like occlusion develop. When a circular vortex is placed in a diffluent background flow, the vortex and frontal zones become meridionally elongated, and the evolution resembles the Norwegian occlusion with a narrowing thermal ridge. When a circular vortex is placed in a confluent background flow, the vortex and frontal zones become zonally elongated, and the evolution resembles the Shapiro–Keyser model with a frontal fracture, frontal T-bone, and bent-back front. Although the idealized model qualitatively reproduces many of the frontal features found in the observed cyclones analyzed in the present study, one significant difference is that the maximum potential temperature gradient and frontogenesis along the cold and warm fronts may differ by a factor of 2 or more in the observed cases, but remain equal along the cold and warm fronts throughout the idealized model simulations. Possible reasons for this asymmetry in the strength of the observed cold and warm fronts are discussed.

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Geraint Vaughan, Bogdan Antonescu, David M. Schultz, and Christopher Dearden

Abstract

Deep convection frequently occurs on the eastern side of upper-level troughs, or potential vorticity (PV) anomalies. This is consistent with uplift ahead of a cyclonic PV anomaly, and consequent reduction in static stability and increase of convective available potential energy (CAPE). Nevertheless, the causal link between upper-level PV and deep convection has not been proven, and given that lift, moisture, and instability must all be present for deep convection to occur it is not clear that upper-level forcing is sufficient. In this paper a convective rainband that intensified ahead of a cyclonic PV anomaly in an environment with little CAPE (~10 J kg−1) is examined to determine the factors responsible for its intensification. The key feature was a low-level convergence line, arising from the remnants of an occluded front embedded in the low-level cyclonic flow. The rainband’s intensity and morphology was influenced by the remnants of a tropopause fold that capped convection at midlevels in the southern part of the band, and by a reduction in upper-level static stability in the northern part of the band that allowed the convection to reach the tropopause. Ascent ahead of the trough appears to have played only a minor role in conditioning the atmosphere to convection: in most cases the ascending airstream had previously descended in the flow west of the trough axis. Thus, simple “PV thinking” is not capable of describing the development of the rainband, and it is concluded that preexisting low-level wind and humidity features played the dominant role.

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W. James Steenburgh, David M. Schultz, and Brian A. Colle

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Mesoscale-model simulations are used to examine the structure and dynamics of a gap-outflow event over the Gulf of Tehuantepec, Mexico, that was associated with a surge of cold air along the eastern slopes of the Sierra Madre. The simulated gap-outflow winds emerged from Chivela Pass, reached a maximum speed of 25 m s−1, and turned anticyclonically as they fanned out over the gulf. Northerly winds were also able to ascend the mountains east, and to a lesser extent west, of Chivela Pass, indicating that the movement of cold air across the Sierra Madre was not confined to the pass. A mesoscale pressure ridge was aligned along the axis of the gap-outflow jet, which was flanked to the west by an anticyclonic eddy, and to the east by a weaker cyclonic eddy.

A model-derived trajectory along the axis of the outflow jet traced an inertial path, with anticyclonic curvature produced primarily by the Coriolis acceleration. The cross-flow pressure-gradient acceleration along this trajectory was negligible because it followed the axis of the mesoscale pressure ridge. Trajectories west (east) of the jet axis experienced stronger (weaker) anticyclonic curvature than expected from inertial balance because the cross-flow pressure-gradient acceleration produced by the mesoscale pressure ridge reinforced (opposed) the anticyclonic deflection by the Coriolis acceleration. As a result of these directional variations in the cross-flow pressure-gradient acceleration, a fanlike wind pattern was observed rather than a narrow jet.

Because of the large changes in SST and surface roughness that are observed during these gap-outflow events, better representation of these effects might improve future mesoscale-model simulations. Such improvements could be accomplished through coupled atmosphere–ocean mesoscale modeling, which could also be used to advance understanding of the oceanography of the gulf.

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Russ S. Schumacher, David M. Schultz, and John A. Knox

Abstract

On 16–17 February 2007, snowbands formed in the lee of the Rocky Mountains in Wyoming, Colorado, and Nebraska on the anticyclonic-shear side of a midlevel jet streak. Two types of bands were prevalent: a longer, wider band associated with frontogenesis along an equatorward-moving cold front (major band) and multiple shorter, narrower bands farther poleward (minor bands). To understand how the upstream terrain affected the occurrence and intensity of the bands, multiple mesoscale model simulations were performed in which the terrain was incrementally smoothed. The evolutions of the synoptic patterns were similar in all simulations that included topography, but the synoptic pattern differed and no bands developed in a simulation with a flat land surface. These results allowed a focus on the changes to the banded precipitation due to the terrain resolution. Remarkably, although the exact location of the bands differed from run to run, the bands in all simulations with topography were in roughly the same region where they occurred on 16–17 February 2007. The major band was associated with frontogenesis along an equatorward-moving cold front that became stalled against the terrain. The minor bands formed from the release of conditional, symmetric, and inertial instabilities by ascent up the large-scale topography, rather than by ascent up specific small-scale topographic features. Because the bands were not tied to specific terrain features, these results suggest that the precise location of the minor bands is unpredictable.

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David M. Schultz, Hans Volkert, Bogdan Antonescu, and Huw C. Davies

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Tor Bergeron was a key member of the Bergen School of Meteorology that developed some of the most influential contributions to synoptic analysis in the twentieth century: airmass analysis, polar-front theory, and the Norwegian cyclone model. However, the eventual success of these so-called Bergen methods of synoptic analysis was not guaranteed. Concerns and criticisms of the methods—in part from the lack of referencing to prior studies, overly simplified conceptual models, and lack of real data in papers by J. Bjerknes and Solberg—were inhibiting worldwide adoption. Bergeron’s research output in the 1920s was aimed at addressing these concerns. His doctoral thesis, written in German, was published as a journal article in Geofysiske Publikasjoner in 1928. Here, an accessible and annotated English translation is provided along with a succinct overview of this seminal study. Major interlaced themes of Bergeron’s study were the first comprehensive description of the Bergen methods: a vigorous defense of cyclogenesis as primarily a lower-tropospheric process as opposed to an upper-tropospheric–lower-stratospheric one; a nuanced explanation of the assertion that meteorology constituted a distinct and special scientific discipline; and, very understandably, a thorough account of Bergeron’s own contributions to the Bergen School. His contributions included identifying how deformation results in frontogenesis and frontolysis, classifying the influence of aerosols on visibility, and explaining the role of the ambient conditions in the onset of drizzle as opposed to rain showers—a distinction that led the formulation of the Wegener–Bergeron–Findeisen process.

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Jari-Petteri Tuovinen, Jenni Rauhala, and David M. Schultz

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The environmental characteristics and convective mode of significant hailstorms (those storms producing reported hail 5 cm or larger in diameter) in Finland during 1972–2011 were analyzed. Altogether, 23 significant-hail-day environments were analyzed by modifying radiosonde data from proximity soundings in the observed data archives of the Finnish Meteorological Institute. Convective parameters derived from the environmental soundings were compared between a set of significant-hail soundings and a null set of nonsevere-thunderstorm soundings. A subset of 13 significant-hail days was examined using data from a network of Doppler radars during 1999–2011. Convective-storm mode and storm characteristics (e.g., hook echo, bounded weak-echo region) were determined for the 18 significant-hail-producing storms during these days. Most (78%) of these storms producing significant hail in Finland occurred with supercells. Of the significant-hail days, 39% (9 out of 23) did not have the minimum of 15 m s−1 of deep-layer (0–6 km) shear commonly expected for supercells. Convective parameters of significant-hail and thunderstorm-day environments were substantially different from each other. Specifically, significant-hail environments had a mean most-unstable convective available potential energy (MUCAPE) of 1464 J kg−1 and deep-layer shear of 17.5 m s−1, whereas thunderstorm days had a MUCAPE of 593 J kg−1 and deep-layer shear of 10.2 m s−1. Larger hail was associated with higher values of MUCAPE. The lifetimes and track lengths of significant-hail-producing storms were related to the convective mode and storm environment. Specifically, larger deep-layer shear seemed to support longer lifetimes and track lengths. Nonsupercells had shorter lifetimes, shorter stormtrack lengths, and lower speeds than supercells. The value of deep-layer shear was smaller for nonsupercells than for supercells. Discrete supercells had higher speeds, longer lifetimes, and longer track lengths than cluster supercells.

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