Robert J. Trapp

Changes in severe thunderstorm environment frequency during the 21st century caused by anthropogenically enhanced global radiative forcing.

Severe thunderstorms comprise an extreme class of deep convective clouds and produce high-impact weather such as destructive surface winds, hail, and tornadoes. This study addresses the question of how severe thunderstorm frequency in the United States might change because of enhanced global radiative forcing associated with elevated greenhouse gas concentrations. We use global climate models and a high-resolution regional climate model to examine the larger-scale (or “environmental”) meteorological conditions that foster severe thunderstorm formation. Across this model suite, we find a net increase during the late 21st century in the number of days in which these severe thunderstorm environmental conditions (NDSEV) occur. Attributed primarily to increases in atmospheric water vapor within the planetary boundary layer, the largest increases in NDSEV are shown during the summer season, in proximity to the Gulf of Mexico and Atlantic coastal regions. For example, this analysis suggests a future increase in NDSEV of 100% or more in locations such as Atlanta, GA, and New York, NY. Any direct application of these results to the frequency of actual storms also must consider the storm initiation.

The Bow Echo and MCV Experiment: Observations and Opportunities

The Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) is a research investigation using highly mobile platforms to examine the life cycles of mesoscale convective systems. It represents a combination of two related investigations to study (a) bow echoes, principally those that produce damaging surface winds and last at least 4 h, and (b) larger convective systems that produce long-lived mesoscale convective vortices (MCVs). The field phase of BAMEX utilized three instrumented research aircraft and an array of mobile ground-based instruments. Two long-range turboprop aircraft were equipped with pseudo-dual-Doppler radar capability, the third aircraft was a jet equipped with dropsondes. The aircraft documented the environmental structure of mesoscale convective systems (MCSs), observed the kinematic and thermodynamic structure of the convective line and stratiform regions (where rear-inflow jets and MCVs reside), and captured the structure of mature MCVs. The ground-based instruments augmented sou...

Robust increases in severe thunderstorm environments in response to greenhouse forcing

Significance Severe thunderstorms are one of the primary causes of catastrophic loss in the United States. However, the response of such storms to elevated greenhouse forcing has remained highly uncertain. We use an ensemble of global climate model experiments to probe the severe thunderstorm response. We find that this ensemble exhibits robust increases in the occurrence of severe thunderstorm environments over the eastern United States. In addition, the simulated changes in the atmospheric environment indicate an increase in the number of days supportive of the spectrum of convective hazards, with the suggestion of a possible increase in the number of days supportive of tornadic storms. Given current vulnerabilities, such increases imply increasing risk of thunderstorm-related damage if global warming continues. Although severe thunderstorms are one of the primary causes of catastrophic loss in the United States, their response to elevated greenhouse forcing has remained a prominent source of uncertainty for climate change impacts assessment. We find that the Coupled Model Intercomparison Project, Phase 5, global climate model ensemble indicates robust increases in the occurrence of severe thunderstorm environments over the eastern United States in response to further global warming. For spring and autumn, these robust increases emerge before mean global warming of 2 °C above the preindustrial baseline. We also find that days with high convective available potential energy (CAPE) and strong low-level wind shear increase in occurrence, suggesting an increasing likelihood of atmospheric conditions that contribute to the most severe events, including tornadoes. In contrast, whereas expected decreases in mean wind shear have been used to argue for a negative influence of global warming on severe thunderstorms, we find that decreases in shear are in fact concentrated in days with low CAPE and therefore do not decrease the total occurrence of severe environments. Further, we find that the shift toward high CAPE is most concentrated in days with low convective inhibition, increasing the occurrence of high-CAPE/low-convective inhibition days. The fact that the projected increases in severe environments are robust across a suite of climate models, emerge in response to relatively moderate global warming, and result from robust physical changes suggests that continued increases in greenhouse forcing are likely to increase severe thunderstorm occurrence, thereby increasing the risk of thunderstorm-related damage.

Low-level mesovortices within squall lines and bow echoes. Part II: Their genesis and implications

This two-part study proposes a fundamental explanation of the genesis, structure, and implications of lowlevel, meso-g-scale vortices within quasi-linear convective systems (QLCSs) such as squall lines and bow echoes. Such ‘‘mesovortices’’ are observed frequently, at times in association with tornadoes. Idealized experiments with a numerical cloud model show that significant low-level mesovortices develop in simulated QLCSs, especially when the environmental vertical wind shear is above a minimum threshold and when the Coriolis forcing is nonzero. As illustrated by a QLCS simulated in an environment of moderate vertical wind shear, mesovortexgenesis is initiated at low levels by the tilting, in downdrafts, of initially crosswise horizontal baroclinic vorticity. Over a 30-min period, the resultant vortex couplet gives way to a dominant cyclonic vortex as the relative and, more notably, planetary vorticity is stretched vertically; hence, the Coriolis force plays a direct role in the low-level mesovortexgenesis. A downward-directed vertical pressure-gradient force is subsequently induced within the mesovortices, effectively segmenting the previously (nearly) continuous convective line. In moderate-to-strong environmental shear, the simulated QLCSs evolve into bow echoes with ‘‘straight line’’ surface winds found at the bow-echo apex and additionally in association with, and in fact induced by, the lowlevel mesovortices. Indeed, the mesovortex winds tend to be stronger, more damaging, and expand in area with time owing to a mesovortex amalgamation or ‘‘upscale’’ vortex growth. In weaker environmental shear—in which significant low-level mesovortices tend not to form—damaging surface winds are driven by a rear-inflow jet that descends and spreads laterally at the ground, well behind the gust front.

Tornadoes from Squall Lines and Bow Echoes. Part I: Climatological Distribution

The primary objective of this study was to estimate the percentage of U.S. tornadoes that are spawned annually by squall lines and bow echoes, or quasi-linear convective systems (QLCSs). This was achieved by examining radar reflectivity images for every tornado event recorded during 1998–2000 in the contiguous United States. Based on these images, the type of storm associated with each tornado was classified as cell, QLCS, or other. Of the 3828 tornadoes in the database, 79% were produced by cells, 18% were produced by QLCSs, and the remaining 3% were produced by other storm types, primarily rainbands of landfallen tropical cyclones. Geographically, these percentages as well as those based on tornado days exhibited wide variations. For example, 50% of the tornado days in Indiana were associated with QLCSs. In an examination of other tornado attributes, statistically more weak (F1) and fewer strong (F2–F3) tornadoes were associated with QLCSs than with cells. QLCS tornadoes were more probable during the winter months than were cells. And finally, QLCS tornadoes displayed a comparatively higher and statistically significant tendency to occur during the late night/early morning hours. Further analysis revealed a disproportional decrease in F0–F1 events during this time of day, which led the authors to propose that many (perhaps as many as 12% of the total) weak QLCSs tornadoes were not reported.

Low-Level Mesovortices within Squall Lines and Bow Echoes. Part I: Overview and Dependence on Environmental Shear

This two-part study proposes fundamental explanations of the genesis, structure, and implications of lowlevel meso-g-scale vortices within quasi-linear convective systems (QLCSs) such as squall lines and bow echoes. Such ‘‘mesovortices’’ are observed frequently, at times in association with tornadoes. Idealized simulations are used herein to study the structure and evolution of meso- g-scale surface vortices within QLCSs and their dependence on the environmental vertical wind shear. Within such simulations, significant cyclonic surface vortices are readily produced when the unidirectional shear magnitude is 20 m s 21 or greater over a 0‐2.5- or 0‐5-km-AGL layer. As similarly found in observations of QLCSs, these surface vortices form primarily north of the apex of the individual embedded bowing segments as well as north of the apex of the larger-scale bow-shaped system. They generally develop first near the surface but can build upward to 6‐8 km AGL. Vortex longevity can be several hours, far longer than individual convective cells within the QLCS; during this time, vortex merger and upscale growth is common. It is also noted that such mesoscale vortices may be responsible for the production of extensive areas of extreme ‘‘straight line’’ wind damage, as has also been observed with some QLCSs. Surface vortices are also produced for weaker shears but remain shallow, weak, and short-lived. Although similar in size and strength to mesocyclones associated with supercell storms, and also sometimes producing similar hooklike structures in the rain field, it is also shown that the present vortices are quite distinct, structurally and dynamically. Most critically, such vortices are not associated with long-lived, rotating updrafts at midlevels and the associated strong, dynamically forced vertical accelerations, as occur within supercell mesocyclones.

Buyer Beware: Some Words of Caution on the Use of Severe Wind Reports in Postevent Assessment and Research

Abstract Postevent damage surveys conducted during the Bow Echo and Mesoscale Convective Vortex Experiment demonstrate that the severe thunderstorm wind reports in Storm Data served as a poor characterization of the actual scope and magnitude of the surveyed damage. Contrasting examples are presented in which a few reports grossly underrepresented a significant event (in terms of property damage and actual areal coverage of damage), while a large number of reports overrepresented a relatively less significant event. Explanations and further discussion of this problem are provided, as are some of the implications, which may include a skewed understanding of how and when systems of thunderstorms cause damage. A number of recommendations pertaining to severe wind reporting are offered.

Observations of Nontornadic Low-Level Mesocyclones and Attendant Tornadogenesis Failure during VORTEX*

Abstract Three storms intercepted during the Verification of the Origins of Rotation in Tornadoes Experiment generated a moderate-to-strong mesocyclone within the lowest several hundred meters above the ground and qualitatively appeared capable of tornadogenesis, yet did not produce a tornado. Such novel observations of what is considered“tornadogenesis failure” are documented and used to show the insufficiency of a low-level mesocyclone for tornadogenesis. Possible modes of failure are discussed.

A Reassessment of the Percentage of Tornadic Mesocyclones

Abstract A large set of data collected by numerous Weather Surveillance Radar-1988 Doppler (WSR-88D) units around the United States was analyzed to reassess the percentage of tornadic mesocyclones. Out of the 5322 individual mesocyclone detections that satisfied the relatively stringent WSR-88D Mesocyclone Detection Algorithm objective criteria, only 26% were associated with tornadoes. In terms of height or altitude of mesocyclone base, 15% of midaltitude mesocyclone detections were tornadic, and more than 40% of low-altitude mesocyclone detections (e.g., those with bases ≤ 1000 m above radar level) were tornadic. These results confirm that a low-altitude mesocyclone is much more likely to be associated with a tornado than is a midaltitude mesocyclone, and more generally, that the percentage of tornadic mesocyclones is indeed lower than previously thought.

Transient response of severe thunderstorm forcing to elevated greenhouse gas concentrations

] We investigate the transient response of severe-thunderstorm forcing to the time-varying greenhouse gasconcentrations associated with the A1B emissions scenario.Using a five-member ensemble of global climate modelexperiments, we find a positive trend in such forcing withinthe United States, over the period 1950–2099. The rate ofincrease varies by geographic region, depending on (i) low-level water vapor availability and transport, and (ii) thefrequencyofsynoptic-scalecyclonesduringthewarmseason.Our results indicate that deceleration of the greenhouse gasemissions trajectory would likely result in slower increasesin severe thunderstorm forcing.