Nonsupercell Tornadoes Research Articles

Analysis of a multiple non-supercell tornado event in Mexico

Multiple tornado events frequently occur in different parts of the world. A region that seems favorable for recurring multiple tornado events due to its geographical location and topography is the eastern sector of the Trans-Mexican Volcanic Belt. A notable multiple tornado event that occurred in this region is investigated. It consisted of five simultaneous tornadoes during the convergence of three air masses: a cold air mass displaced from the north with moderately high relative humidity and specific humidity, a comparatively warm air mass with moderate relative humidity from the southeast, and a hotter air mass with low relative humidity from the southwest. This situation generated a line of intense horizontal wind shear through which the five tornadoes occurred. Numerical simulations revealed three cyclonic vortices and other minor disturbances along the shear line. Vertical cross-sections show rising air in these disturbances suggesting a cyclonic character. The results indicate that the relatively warm mass from the southeast moves over the one associated with a cold front coming from the north, generating multiple cyclonic rotation zones. Experiments with Lagrangian particles revealed vertical movements in the cyclonic rotation zones implying that the maximum heights of the tornadoes varied between 1000 and 1500 m. Moisture flux convergence calculations display structures resembling Kelvin-Helmholtz instabilities. This research proposes a conceptual model where the convergence of masses induced by topography can lead to multiple tornado generation.

Quantifying the Separation of Enhanced ZDR and KDP Regions in Nonsupercell Tornadic Storms

AbstractTornadoes associated with nonsupercell storms present unique challenges for forecasters. These tornadic storms, although often not as violent or deadly as supercells, occur disproportionately during the overnight hours and the cool season—times when the public is more vulnerable. Additionally, there is significantly lower warning skill for these nonsupercell tornadoes compared to supercell tornadoes. This study utilizes dual-polarization Weather Surveillance Radar-1988 Doppler (WSR-88D) data to analyze nonsupercell tornadic storms over a three-and-a-half-year period focused on the mid-Atlantic and southeastern United States. A signature found in a large number of cases is the separation of low-level specific differential phase KDP and differential reflectivity ZDR enhancement regions, thought to arise owing to size sorting. This study employs a new method to define the “separation vector,” which comprises the distance separating the enhancement regions and the direction from the KDP enhancement region to the ZDR enhancement region, measured relative to storm motion. While there is some variation between cases, preliminary results show that the distribution of separation distance between the enhancement regions is centered around 3–4 km and tends to maximize around the time of tornadogenesis. A preferred quadrant for separation direction is found between parallel and 90° to the right of storm motion and is most orthogonal near the time of tornadogenesis. Further, it is shown that, for a given separation distance, separation direction increasing from 0° toward 90° is associated with increased storm-relative helicity.

Analysis of a non-supercell tornado in Sombor on July 10, 2014

Around 14:00 UTC, on July 10, 2014 a non-supercell tornado-a landspout, formed and occurred within a convective cloudiness in the S-SW upper air flow and moved over Sombor (45°46' N, 19°06' E, h=89 m). Synoptic conditions, type of synoptic situation which caused the landspout event, CAPE, LI indices and adiabatic stability of the atmosphere have been analysed in this paper. An analysis of sounding measurements and satellite images has also been done. Based on field survey analysis and using the Fujita scale method (F-scale), damage estimates have been shown in order to determine the tornado strength. The three main objectives of this research are: 1) analysis of atmospheric synoptic conditions that caused the landspout development in northern Backa using the data obtained from upper air and surface maps; 2) investigation into the satellite images data and radiosounding measurements; 3) to determine the evolution of the parent cloud from which the landspout developed using these methods. This article therefore represents a contribution to understanding of atmospheric conditions which favour a suitable synoptic and thermodynamic environment for a landspout genesis and development.

Kinematic Observations of Misocyclones along Boundaries during IHOP

Abstract During the International H2O Project, mobile radars collected high-resolution data of several 0.5–2-km-wide vertically oriented vortices (or misocyclones) along at least five mesoscale airmass boundaries. This study analyzes the properties of the misocyclones in three of these datasets—3, 10, and 19 June 2002—to verify findings from finescale numerical models and other past observations of misocyclones and to further the understanding of the role that they play in the initiation of deep moist convection and nonsupercell tornadoes. Misocyclones inflect or disjoint the swath of low-level convergence along each boundary to varying degrees depending on the size of their circulations. When several relatively large misocyclones are next to each other, the shape of low-level convergence along each boundary is arranged into a staircase pattern. Mergers of misocyclones are an important process in the evolution of the vorticity field, as a population of small vortices consolidates into a smaller number of larger ones. Additionally, merging misocyclones may affect the mixing of thermodynamic fields in their vicinity when the merger axis is perpendicular to the boundary. Misocyclones interact with linear and cellular structures in the planetary boundary layers (PBLs) of the air masses adjacent to each boundary. Cyclonic low-level vertical vorticity generated by both types of structures makes contact with each boundary and sometimes is incorporated into preexisting misocyclones. Intersections of either type of PBL structure with the boundary result in strengthened pockets of low-level convergence and, typically, strengthened misocyclones.

Observational Analysis of the 27 May 1997 Central Texas Tornadic Event. Part II: Tornadoes

AbstractThe 27 May 1997 central Texas tornadic event has been investigated in a two-part observational study. As demonstrated in Part I, the 1D environment associated with this event was unfavorable for significant (≥F2) tornadoes. Yet, the storm complex produced at least six significant tornadoes, including one rated F5 (the Jarrell, Texas, tornado). The purpose of this article is to examine the spatiotemporal interrelationships between tornadoes, preexisting boundaries, antecedent low-level mesocyclones, convective cells, and midlevel mesocyclones. It is shown that each of the six observed tornadoes that produced greater than F0 damage formed along the storm-generated gust front, not along preexisting boundaries. Half of these tornadoes formed on the distorted gust front, the portion of the storm-generated gust front whose orientation was deformed largely by the horizontal shear across the cold front. The remaining three tornadoes developed at the gust front cusp (the persistent gust front inflection located at the northeast end of the gust front distortion). Unlike the tornadoes south of the gust front cusp, these tornadoes are found to be associated with antecedent mesocyclones located in the low levels above the boundary layer. Furthermore, these mesocyclonic tornadoes are found to be larger and more destructive than the three nonmesocyclonic tornadoes. The formation of the Jarrell tornado is found to occur as a nearly stationary convective cell became collocated with a south-southwestward-moving low-level mesocyclone near the gust front cusp—a behavior that resembles the formation of nonsupercell tornadoes. It is argued that the back-building propagation/maintenance of the storm complex enabled this juxtaposition of convective cells with vorticity along the distorted gust front and may have therefore enabled tornado formation. Each of the convective cells without midlevel mesocyclones was found to remain farther from the boundaries than the mesocyclonic cells. Since the cells nearest to the boundaries were longer lived than the remaining cells, it is argued that cells near the boundaries were mesocyclonic because the boundaries yielded cells that were more likely to support temporally coherent midlevel rotation.

Central Illinois Cold Air Funnel Outbreak

Abstract Numerous cold air funnels developed in close proximity over central Illinois on 23 May 1988 as the core of an upper-level cutoff low pressure center passed over the region. Five separate funnels were observed by one of the authors from a single location over a period of 33 min. Images of these funnels captured from video are presented, one showing two funnels extending toward one another from the same cloud and two others illustrating stages in the life cycle of one of the funnels. Surface and upper air analyses, radar data, and two soundings taken at the time of the outbreak are presented to illustrate the environment in which these funnels formed. The family of cold air funnels occurred along a weak stationary front. The fact that so many funnels occurred along the same line is suggestive of a vortex sheet breaking down due to the development of horizontal shearing instability. If this is the case, then the cold air funnels observed on 23 May are similar dynamically to nonsupercell tornado fami...

Nonhydrostatic Barotropic Instability: Applicability to Nonsupercell Tornadogenesis

Abstract This paper reports a barotropic instability analysis of nonhydrostatic columnar disturbances in a layer of homogeneous fluid over a flat bottom surface. The extended form of Hamilton's principle of least action is used to obtain the canonical form of Green–Naghdi equations for this model. The basic state has a monotonic shear flow and a variable depth. For values of the parameters supposedly relevant to an environment of nonsupercell tornadoes (NST), the nonhydrostatic effect is found to have significant impacts. It is capable of sufficiently slowing down some short gravity waves so that they resonantly interact with vorticity waves of the same wavelengths in the shear zone to form strongly unstable gravity–vorticity hybrid modes. Their instability properties are by and large compatible with the observed counterparts of NST, such as growth rate, phase speed, length scale, aspect ratio, structure, and energetics. Those results are robust for a reasonable range of each key parameter. Apart from a n...

The Numerical Simulation of Nonsupercell Tornadogenesis. Part III: Parameter Tests Investigating the Role of CAPE, Vortex Sheet Strength, and Boundary Layer Vertical Shear

Abstract Nonsupercell tornadogenesis has been investigated in a three-part numerical study. Building on the results of Parts I and II, Part III addresses the sensitivity of nonsupercell tornadogenesis to variations in convective available potential energy (CAPE), outflow boundary vortex sheet strength, and boundary layer vertical shear. A three-dimensional, nonhydrostatic, quasi-compressible convective cloud model has been employed to examine nonsupercell tornado (NST) development in an environment typical of the Colorado high plains. A strong relationship was shown to exist between the magnitude of the environmental CAPE and the structure and intensity of the misocyclones and nonsupercell tornadoes that developed. As CAPE was increased from 0 to 1700 J kg−1, the simulated vortices markedly contracted and intensified. Multiple CAPE thresholds were identified that yielded markedly different vortex intensity. The highest CAPE runs produced NST families with peak ground-relative surface winds of ∼47 m s−1. V...

The Numerical Simulation of Nonsupercell Tornadogenesis. Part II: Evolution of a Family of Tornadoes along a Weak Outflow Boundary

Nonsupercell tornadogenesis along a weak outflow boundary has been simulated using a three-dimensional moist convective cloud model. Thermodynamic conditions similar to those observed for nonsupercell tornado (NST) events of the High Plains were utilized in the model initialization. As the ensemble system of storm, outflow boundary, and leading edge vortices evolve, six distinct life cycle stages for the development and decay of NSTs are documented that span a period of about 35 min. Consistent with the results of Part I of this numerical study, vortex sheet dynamics exert considerable control over the outflow leading edge. The progression of pretornadic life cycle stages serves to concentrate vertical vorticity effectively along the outflow boundary in discrete misocyclone circulations aligned in a 3-km wavelength pattern. The organization of larger-scale misocyclones and ultimate intensification to initial tornadic intensity occurs coincident with the rapid development of deep convection overhead. The strongest members of a family of NSTs that develop in the model maintain ground-relative surface wind speeds of greater than 30 m s−1 for approximately 11 min within which wind speeds meet F1 severity criteria for 6 min. The mature vortices reach the midlevels of the moist convection and display deep, rotationally induced axial downdrafts. The rapid transition to a predominant downdraft character for the storm complex and to an outflow dominated subcloud air mass heralds the onset of NST dissipation. The NST evolution simulated here compares very favorably to observational NST studies. The misocyclones are shown to provide an asymmetric pattern of convective forcing along the outflow boundary, which supports the formation of deep moist convection directly over them. Vertical vorticity in the boundary layer misocyclones is redistributed upward into the midlevels of the moist convection (∼6 km) by developing deep updrafts. The mature vortices are maintained by vertical vorticity transported from a vortex sheet located along the outflow boundary and by vertical vorticity produced from the tilting of horizontal vorticity in the inflow region southeast of the NSTs. Low-level vortex stretching is the dominant vorticity tendency term as the vortices intensify to and maintain tornadic strength. No significant vertical tilting of baroclinically generated horizontal vorticity was indicated even after convective downdraft-associated new outflow pools formed in the environment surrounding the misocyclones. These new outflow pools play a major role in NST intensification by increasing convergence and resultant vortex stretching along the periphery of the tornadic circulations. A comparative simulation with rain production turned off, which precluded the possibility of new outflow development, revealed that NST intensity was roughly 25% greater in the baseline simulation. In an additional comparative simulation where no moist convection was allowed to develop, the resultant misocyclones markedly lacked the coherent organization and intensity of the misocyclones and NSTs of the baseline simulation and no NST-strength vortices developed. A six-stage “refined” model of NST development and decay is presented.

The Genesis of Three Nonsupercell Tornadoes Observed with Dual-Doppler Radar

Dual-Doppler radar analyses of three tornadoes associated with a multicellular line of storms are presented. The F2–F3 intensity tornadoes occurred on 15 June 1988 near Denver, Colorado, during the Terminal Doppler Weather Radar (TDWR) Project. These tornadoes developed from misocyclones of no larger than 2 km in diameter that formed along the collision of two surface outflows. The misocyclones were observed to build in height and intensify with time, coincident with rapid storm growth overhead. All three misocyclones were clearly associated with the maximum storm updrafts. Downdrafts and associated outflows did not play a role in the formation of one of the tornadoes, but may have contributed to the genesis of the other two tornadoes. It is clear that a downdraft is not a necessary condition for the formation of a nonsupercell tornado, but when present, likely plays a role in determining the timing and intensity of the tornado. This is achieved by the downdraft and outflow causing an increase in the magnitude of the low-level convergence and updraft. Vertical vorticity production terms were examined for each tornado. Given the close proximity in time and space of the tornadoes, there was surprising variability in the magnitudes and locations of the stretching, tilting, and advection terms for each tornado. In general, however, the predominant contribution to positive vertical vorticity and tornadogenesis was from vorticity stretching in the 0.2–2.0-km layer resulting from intensification of low-level convergence and storm updrafts. Above 2.0 km, increased vertical vorticity resulted from a redistribution of low-level vorticity vertically. Small areas of positive vorticity tilting were found within the regions of large streamwise vorticity just prior to tornadogenesis but not during the formative stages of the mesocyclones, amplifying the already strong contributions to tornadogenesis from vertical stretching of the vortices. The spatial resolution of the data presented here is as high as any documented in tornado literature. However, limitations in what features are actually resolvable became strikingly apparent and are discussed in the paper.

Non-supercell Tornadoes

Abstract Analyses of tornadoes that are not associated with supercells are presented. The database for this study was collected during CINDE (Convention INitiation and Downburst Experiment), a field project operated during the summer of 1987 in Colorado. A total of 27 visual vortices were studied. They appeared to form as shear instabilities along radar detected convergence lines. The circulations initiated at low levels generally in the absence of precipitation echo. Subsequently as these vortices propagated along the convergence line they appeared to strengthen to tornadic intensity when they became colocated with the updraft of a rapidly developing storm. It is hypothesized that vortex stretching is responsible for intensifying the initial rotation. Although these tornadoes were weaker than those accompanied by strong midlevel mesocyclones, estimates of their strength suggest damage capability as high as F2. The implications for operational radars to detect these types of phenomena were identified. The...