Multiple Tornadoes Research Articles

High-Resolution Dual-Doppler Analyses of the 29 May 2001 Kress, Texas, Cyclic Supercell

Abstract On 29 May 2001, Doppler on Wheels radars collected data on a supercell near Kress, Texas. The supercellular storm, cyclic in nature, produced multiple mesocyclones throughout its lifetime. Dual-Doppler syntheses were conducted using a grid spacing of 100 m, resulting in the highest-resolution observational analysis of a cyclic supercell to date. In addition, collection of data from ground-based radar allowed for the analysis of near-ground features irresolvable with airborne radar, providing another advantage over previous studies. The syntheses revealed a number of evolving low-level mesocyclones over the observation period of 900 s. While nontornadic during the synthesis period, the supercell exhibited evidence of strong (vertical vorticity greater than 10−2 s−1) low-level circulation with classic cyclic structure and multiple tornadoes beginning 3600 s later. A comparison between the current results, conceptual models, and previous lower-resolution analyses is presented. A striking similarity exists between the cyclic evolution of the Kress storm during the synthesis time period and other previous cyclic conceptual models. However, differences did exist between the Kress storm and previously studied tornadic storms. Analysis showed that the rear-flank downdraft provided the only surface boundary associated with low-level mesocyclogenesis. Other characteristics, including forward-flank gust front structure and the orientation of low-level horizontal vorticity, also differed. In addition, there was a general lack of surface convergence associated with the forward-flank reflectivity gradient, yet convergence associated with the forward-flank gust front increased with height. Finally, a large component of crosswise horizontal vorticity was found to exist throughout the supercell environment, within both the inflow and outflow. Incorporating these differences, an attempt was made to identify possible mechanisms responsible for the lack of tornadogenesis during the synthesis time period.

Doppler Radar and Lightning Network Observations of a Severe Outbreak of Tropical Cyclone Tornadoes

Data from a single WSR-88D Doppler radar and the National Lightning Detection Network are used to examine in detail the characteristics of the convective storms that produced a severe tornado outbreak within Tropical Storm Beryl's remnants on 16 August 1994. Comparison of the radar data with reports of tornadoes suggests that only 13 cells produced the 29 tornadoes that were documented in Georgia and the Carolinas on that date. Six of these cells spawned multiple tornadoes, and the radar data confirm the presence of miniature supercells. One of the cells was identifiable on radar for 11 hours, spawning tornadoes over a time period spanning approximately 6.5 hours. Several other tornadic cells also exhibited great longevity, with cell lifetimes greater than ever previously documented in a landfalling tropical cyclone tornado event, and comparable to those found in major midlatitude tornadic supercell outbreaks. Time-height analyses of the three strongest tornadic supercells are presented in order to document storm kinematic structure and to show how these storms appear at different ranges from a WSR-88D radar. In addition, cloud-to-ground (CG) lightning data are examined for the outbreak, the most intense tropical cyclone tornado event studied thus far. Although the tornadic cells were responsible for most of Beryl's CG lightning, flash rates were only weak to moderate, even in the most intense supercells, and in all the tornadic storms the lightning flashes were almost entirely negative in polarity. A few of the single-tornado storms produced no detectable CG lightning at all. In the stronger cells, there is some evidence that CG lightning rates decreased during tornadogenesis, as has been documented before in some midlatitude tornadic storms. A number of the storms spawned tornadoes just after producing their final CG lightning flashes. Surprisingly, both peak currents and positive flash percentages were larger in Beryl s nontornadic storms than in the tornadic ones. Despite some intriguing patterns, the CG lightning behavior in this outbreak remains mostly inconsistent and ambiguous, and offers only secondary value for warning guidance. The present findings argue in favor of the implementation of observing systems capable of continuous monitoring of total lightning activity in storms.

Barron County, Wisconsin, Multiple Tornadoes and Hailstorms of 11 September 1990

Abstract Four tornadoes, of F1–F2 intensity, occurred over Barron County, Wisconsin, on the evening of 11 September 1990. The tornadoes were associated with a slow-moving thunderstorm cluster that developed along a warm front, and all occurred within 17 km of Rice Lake over a 2-hour and 15-minute period. National Weather Service radar data indicate that the tornadoes probably were associated with mesocyclones. Hail up to 7 cm in diameter and damaging winds also were reported over Barron County and three adjoining counties. Forecasting of slow-moving thunderstorms within an environment capable of producing mesocyclones and tornadoes remains an important forecast problem. Analysis of features that produced quasi-stationary thunderstorms and a vertical wind profile sufficient to generate mesocyclones hopefully will improve recognition and forecasting of similar events in the future.

Two Examples of Operational Tornado Warnings Using Doppler Radar Data

Abstract The National Weather Service Forecast Office in Denver, Colorado; has access to a variety of new observational datasets via an advanced, interactive workstation. This paper describes the impact of one of these datasets—Doppler radar—on the real-time operational capabilities in the forecast office. In particular, the forecasts during two tornadic events are emphasized. One case involved a supercell thunderstorm with a well-defined mesocyclone. The other case was one of multiple tornadoes which were not associated with a supercell. The forecasters were able to achieve significant lead times on their tornado warnings by applying appropriate conceptual models of tornadogenesis to the Doppler radar capabilities which were new to the operational environment.

Oklahoma Thunderstorms on 29–30 April 1970. Part II: Radar-Observed Merger of Twin Hook Echoes

National Severe Storms Laboratory radar film reveals twin hook echoes on the southeast flank of a tornadic thunderstorm (Storm G) that struck Oklahoma City on 30 April 1970. Only one mesocyclone is apparent in surface wind analyses (that associated with G's supercell updraft), but both tornado cyclones produced tornadoes. As the two hook echoes merged into one larger hook, damage diminished. Divergence (−2.6 × 10−3 s−1) shows an overall decrease in updraft strength during the merger. Maximum vorticity (1.3 × 10−3 s−1) overtook the developing hook echoes and increased slightly in response to an accelerating downdraft impulse from the storm's southwest flank. The multiple-tornado cyclones that developed along the accelerating gust front are believed due to vortex sheet rollup induced partly by the horizontal momentum of the downdraft air and partly by an unusual distribution of pressure-gradient forces behind the gust front. Merger of the two tornado cyclones is believed related to updraft propagation along convergent axes of maximum pressure gradients. One tornado cyclone contained multiple tornadoes orbiting about its center. Analyzed maximum vorticity/convergence ratio, updraft radius estimated from radar signatures and inflow layer depth are used to approximate the ratio of updraft perimeter tangential velocity to mean updraft speed (swirl ratio) which in turn is used to estimate radius of turbulent vortex core (about which multiple vortices orbit). The relationship used to estimate core radii from swirl ratio is that published by Davies-Jones who used Ward's experimental results. For the observed tornado cyclone, independent radius estimates range from 0.68 to 0.84 km, and bracket the 0.7 km radius deduced from damage path characteristics. It is concluded that divergence and vorticity maxima analyzed from wind observations 11 km apart reflect the kinematic properties of the inflowing air that determine tornado character. Evolutions of convergence and vorticity centers for this and three other tornado cyclones suggest that a tornado cyclone's ability to produce vortices in the surface (friction) layer depends on ambient vorticity exceeding a threshold value 10−3 to 10−2 s−1 on a scale from 25 to 2.5 km.

Oklahoma Thunderstorms on 29–30 April 1970. Part III: Tornado Characteristics Inferred from Damage Tracks

Newspaper accounts and damage tracks surveyed by National Severe Storms Laboratory staff reveal characteristics of three tornadoes that struck the Oklahoma City area shortly after midnight on 30 April 1970. Few details emerge from surveying the first tornado (Storm F) except that its path crossed mostly open country and passed several kilometers northwest of Oklahoma City. Twin tornado cyclones (Mustang and Camelot), observed an hour later in Storm G, apparently produced multiple tornadoes that moved rapidly (80 m s−1) around the southern half of larger cyclonic circulations (hook echoes). Sporadic damage mostly at roof level suggests that the tornadoes often did not extend to the ground. Typically, debris was strewn in straight paths at an angle to the path of the tornado cyclone, suggesting tornado spin velocity less than speed of forward movement. Geometric relationship between a portion of one damage path and the path and forward speed of the tornado cyclone are the basis for estimating maximum winds at the ground to have been less than 160 m s−1. The nature of the structural damage suggests maximum winds were about 105 m s−1. It is concluded that the $6.3 million damage done by these storms was caused by relatively weak tornadoes (probably less than 25 m s−1 spin velocity) embedded within rapidly rotating tornado cyclones.

Multiple Vortex Features in the Tornado Cyclone and the Occurrence of Tornado Families

Abstract Analyses are presented of the tornado tracks for four tornado families affecting Indiana and neighboring states on 3 April 1974. The study by Agee et al.(1975)on the occurrence of multiple suction vortices in the tornado has been used by further extending Ward's (1972) multiple vortex phenomenon to the scale of the tornado cyclone. This has allowed an interpretation of multiple tornado events by means of which consecutive tornado damage paths may be frequently recognized as segments of cycloidal tracks for multiple vortices occurring within the larger tornado cyclone. A tornado cyclone, most likely within the right rear quadrant of the severe thunderstorm, may contain two, three, or more smaller scale vortices (mini tornado cylones) that revolve cyclonically about the center of the. parent tornado cyclone as the entire system translates along with the thunderstorm. It is shown that these centers of action have cycloidal tracks that can be matched with the damage paths of tornado families. Tornado...