John T. Snow

Characteristics of Tornado-Like Vortices as a Function of Swirl Ratio: A Laboratory Investigation

Abstract The investigation of tornado vortex dynamics by means of a laboratory simulation is described. Based on observations from nature and an examination of the Navier-Stokes equations, a laboratory simulator of the Ward type has been constructed. This simulator generates various vortex configurations as a function of swirl ratio, radial Reynolds number and aspect ratio. Configurations which are described are 1) a single laminar vortex; 2) a single vortex with breakdown bubble separating the upper turbulent region from the lower laminar region; 3) a fully developed turbulent core, where the breakdown bubble penetrates to the bottom of the experimental chamber; 4) vortex transition to two intertwined helical vortices; and 5) examples of higher order multiple-vortex configurations that form in the core region. Hot-film anemometry measurements of the magnitude of the velocity vector and inflow (swirl) angle have been obtained in a sequence of flows characterized by progressively increasing values of swirl...

Tornado Vortex Simulation at Purdue University

Abstract A 4 m wide and 7 m tall tornado vortex generator (including exhaust fan and duct work) has been constructed at Purdue University that operates on a principle similar to that of the earlier machine modeled by Ward (1972). Characteristics of the Purdue simulator are described, as well as the corresponding modifications and improvements that have been made to Wards machine. Selected photographs of vortex configurations obtained in the simulator demonstrate the ability of the machine to achieve vortex breakdown and multiple vortex configuration. A radial-axial profile of velocity magnitudes (using hot-film anemometry) has been obtained for the state of vortex breakdown characterized by two interlocking helical spiral vortices. This preliminary result shows the potential that the experimental system offers for obtaining quantitative information about the flow field of selected vortex configurations. Multiple vortex phenomena in the thunderstorm-tornado system are examined in light of the laboratory s...

Intense Atmospheric Vortices Associated with a 1000 MW Fire

Abstract Observations of vortices of various types produced in a large thermal plume are described. The apparatus used to generate the plume is the Meteotron, an array of 105 fuel oil burners with a total heat output of approximately 1000 MW. Three types of vortices have been observed: 1) large counter-rotating rolls in the downstream plume, 2) intense small-scale vortices resembling very strong dust devils seen at the surface on the downwind side of the plume, and 3) very large columnar vortices produced when the lower portion of the plume goes into rotation as a whole. Three mechanisms leading to the concentration of vorticity necessary to produce these vortex types are discussed. These include tilting and stretching of horizontal vorticity present in the environmental wind field, generation of vorticity within the plume by the action of buoyancy and drag forces, and convergence of preexisting background vorticity from the environment. It is concluded, based on these observations and physical considerat...

Tornadoes in France

Abstract In the period 1680–1988, 107 significant tornadoes in the Fujita scale categories F2–F5 have occurred in France. These include 49 such events in the historical period 1680–1959, and 58 events in the modem period 1960–1988. Estimates of the temporal and spatial climatological distributions of significant tornadoes in France have been developed that suggest June and August are the months with the greatest number of such tornadoes; 1600–1700 UTC is the interval in which occurrence is most likely, with a secondary maximum in likelihood between 1800 and 1900 UTC; the northwestern quarter of the country is the region where a significant tornado is most likely to occur. A second, much smaller area with several observations is evident in the far south-center portion of France, near the Mediterranean coast; two significant tornadoes can be expected in France each year; the mean area stricken by such a tornado is about 4 km2; France has a mean risk probability of a significant tornado occurring at a point ...

Mitigation against extreme windstorms

This paper treats various manifestations of atmospheric wind in relation to the effects these have on human safety and comfort. We concentrate on more recent advances in our knowledge of the geophysical structure and behavior of extreme windstorms. Particular attention is given to severe thunderstorms and their attendant mesoscale offspring, tornadoes and downbursts, as well as the extratropical and tropical cyclones. It appears that the highest credible wind speed estimates in any of these windstorms so far are for tornadoes, about 135 m s−1; these have been derived from photogrammetric analyses of eyewitness photography and remote sensing from in situ and portable Doppler radars. On the other hand, it is found from extensive aerial and ground poststorm damage surveys that the vast majority of all tornadoes have peak wind speeds no higher than those measured by sparse surface networks and reconnaissance aircraft in mature hurricanes. New in situ and remote sensing (Doppler radars) weather observing networks currently being deployed across the United States and other countries, combined with greater public awareness, may significantly alter our current assessment of the climatology of extreme windstorms in these areas. Advances in the field of wind engineering are shown to afford cost-effective techniques for mitigating against extreme windstorms, including tornadoes. However, the rather unique sociopolitical framework of building codes and practices in the United States presently hinders effective technology transfer and mitigation practice. Important implications of these findings accrue to forecasts and warnings of forest fires and airborne dispersal (loss of containment) of toxic materials, including nuclear processing by-products.

Association between NSSL Mesocyclone Detection Algorithm-Detected Vortices and Tornadoes

Abstract Nearly 100 000 vortex detections produced by the Mesocyclone Detection Algorithm (MDA) are analyzed to gain insight into the effectiveness of the detection algorithm in identifying various types of tornado-producing events. Radar and algorithm limitations prevent raw vortex detections from being very useful without further discrimination. Filtering techniques are developed to remove spurious vortex detections and discriminate between vortices that are and are not related to mesocyclones. To investigate whether various vortex detections (and their attributes) are associated with severe weather phenomena, they are compared with available tornado reports to determine if detections with certain types of attributes can be associated with tornadic events. Tornado reports are used since the ground truth tornado set is more reliable than other databases of severe weather phenomena. Basic skill scores and more advanced principal component methods are used to quantify the correlation between vortex detecti...

Fallout of Debris from Tornadic Thunderstorms: A Historical Perspective and Two Examples from VORTEX

Abstract Preliminary results of an investigation of debris lofted by tornadoes, its long-distance transport by thunderstorms, and its subsequent fallout are reported. The authors begin with a review of historical accounts, including the unique study of the 1984 Barneveld, Wisconsin, tornado by Anderson, which shows that long-distance transport and fallout of debris have occurred and that distances involved have been significant. The authors then report first-hand evidence of these phenomena in two events with F2 tornadoes: the Tuskahoma, Oklahorna, tornado of 25 April 1994 and the Gainesville, Texas, tornadoes of 26 April 1994. In both cases, traceable material in the form of canceled checks, bills-of-sale, invoices, and legal documents were reported to the authors, who were able to locate the source locations for several of these items. The authors close with some conjectures on the implications of these first findings.

Measurements of Axial Pressures in Tornado-like Vortices

Abstract The results of a series of measurements of centerline pressure deficit in tornado-like vortices are described. These measurements were undertaken for the purpose of determining 1) how the magnitude of the central pressure deficit in a columnar vortex varies with height, and 2) what functional relationships exist between them deficits and the dynamic and geometric parameters characterizing the flow. The results graphically show the complicated variation of central pressure deficit with height in both laminar and turbulent vortices In low-swirl vortices, the largest deficits are found aloft, not at surface. Further, the low-swirl vortices have generally greater central pressure deficits than moderate-swirl events. The greatest deficits are tied to the approach of the vortex breakdown to the lower surface. The data also indicate a cubic dependence of the central pressure deficit on applied circulation.

Considerations in Exploring Laboratory Tornadolike Vortices with a Laser Doppler Velocimeter

Abstract A laser Doppler velocimeter was used to collect datasets consisting of means and turbulence intensities of radial and tangential velocity components for two vortex morphologies within a Ward-type tornado vortex chamber. This paper focuses on problems encountered in the application of the laboratory technique and the applied solutions. Among these are the nature and origin of particles for backscattering laser light, establishing and maintaining boundary conditions, lack of coincidence between the mean position of the vortex axis and that of the vortex chamber, and the nonstationary nature of the vortex. The impact of the axial offset and wander of the vortex on velocity measurements is examined using realistic velocity profiles based on laboratory data and parameters governing vortex wander based on video imagery. It is found that the tangential speed maximum was underestimated by at least 15% due to an inherent averaging process and that the core radius was overestimated by a factor of nearly 50%.

The Dynamic Response of a Wind Measuring System

Abstract The response of a simple wind measuring system to a step increase in wind speed is examined. The commercially available system consists of a cup anemometer and related signal conditioning electronics. It is shown that for small increases in wind speed, the response of the system is governed by the anemometer, while for large increases, the response is determined by the signal conditioning. A second-order model is developed to simulate this behavior and a critical magnitude for the step increase in speed is proposed to characterize the system response. It is argued that the observed behavior may be a significant constraint on the application of the system to certain types of wind measurements.