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Phillip J. Smith and John T. Snow
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J. T. Snow, C. R. Church, and B. J. Barnhart

Abstract

The wall static pressure fields beneath tornado-like vortices have been investigated using a large vortex generator especially designed to model tornado cyclone airflow. Presented in nondimensional form, the data include both a series of radial profiles across the mean pressure field under a variety of flow conditions, and a detailed investigation of the magnitude of the central pressure as a function of swirl. The profiles clearly show the development of the intense vortical core from the no-swirl state, and the evolution of the core from a one-celled into a two-celled flow. For the experimental range examined, it is found that the greatest pressure deficits and largest pressure gradients (in the mean field) are associated with single-celled vortices. Strong evidence is found for the existence of a dynamically induced downdraft in the two-celled vortex.

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John A. Dutton, Leonard J. Pietrafesa, and John T. Snow
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R. L. Pauley, C. R. Church, and J. T. Snow

Abstract

Time-dependent features of the wall static pressure field beneath vortices modeled in a Ward-type vortex simulator have been investigated with emphasis on measurements of maximum surface pressure deficit. A pressure-measuring system was devised for this purpose which is capable of resolving important transient features of the surface pressure field in an essentially undistorted form, and measurement techniques were employed which reduced the influence of vortex wander. Measurements of maximum surface pressure deficits and their dependence on flow rate and geometry are presented, as well as a detailed study of the magnitudes of the maximum surface pressure deficits as a function of swirl ratio. Also presented are surface pressure distributions in individual subsidiary vortices in a multiple vortex flow.

The greatest deficit pressures are found to be associated with the penetration of the vortex breakdown to the surface. The magnitude of the surface pressure deficit is closely related to the. square of the mean vertical velocity of the upflow and also is dependent on swirl ratio. The pressure deficits in the subsidiary vortices presented are variable but range up to three times that found at the center of the “parent” vortex.

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C. R. Church, J. T. Snow, and E. M. Agee

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 Ward's 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 simulation and the similarity concept.

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E. M. Agee, J. T. Snow, and P. R. Clare

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. Tornadoes are observed to be left-turners or right-turners, depending on what portion along the cycloidal track touchdown and demise occur. A tornado family may he associated with a single vortex parent tornado cyclone which tends to produce longer tornado tracts that do not undergo appreciable turning. Tornado families, however, may be generated by multiple vortex tornado cyclone systems, and in this case tornado tracks follow a curtate cycloid, eventually turning into the cold air and dissipating. Another tornado can reappear, however, as the parent vortex caster returns cyclonically (along the cycloidal path) to more favorable environmental conditions for redevelopment. Finally, a summary is presented depicting the scales of possible multiple vortex systems associated with the tornado producing thunderstorm.

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R. A. Pielke, R. A. Stocker, A. J. Verdon, and J. T. Snow

Abstract

No abstract available.

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R. A. Stocker, R. A. Pielke, A. J. Verdon, and J. T. Snow

Abstract

On 12 May 1986, approximately 190 000 tagged helium balloons were released at 1330 EDT from locations scattered throughout the United States. An average of 4.5% of the balloon tags were recovered and sent to the American Geological Institute. A map of resultant balloon trajectories was then generated from the data. From an analysis of the surface and 500 mb maps on this data, synoptic conditions are defined and categorized. This along with the resultant motion map gives useful data for identifying the dispersion characteristics involved with a particular synoptic condition.

The ARL-ATAD model, a single-layer trajectory model, was run for the 12 May balloon launch for a PBL and 750-660 mb layer simulation from seven sites across the, United States. These were compared with the balloon resultant motion map. The PBL approach produced poor comparisons with two of the seven sites showing balloon and trajectory motions transporting plumes in opposite directions. The layer approach did much better with only the Los Angeles, California comparison showing disagreement. Yet oven with the layer approach, the ability of the ATAD model to predict plume transport is highly questionable due to the lateral dispersion evidenced by the balloon resultant motions.

This experiment is of great interest because of the uniqueness involved with the spatial extent and total number of released balloons.

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C. R. Church, J. T. Snow, G. L. Baker, and E. M. Agee

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 ratio. These data include measurements in both the quasi-irrotational outer flow and the more complex core region. Due to the similarity that exists between the model and tornadoes, these observations provide insight into the flow fields likely to be encountered in real events.

Particularly significant findings include the mapping of the transition points at which the flow converts from a single to a double helical vortex configuration, and from a double to a triple pattern, as a function of system parameters. Additionally, the velocity measurements show the development of a cylindrical shear zone at the outer edge of the core, which, through inertial instability, may lead to the multiple-vortex phenomenon.

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D. R. Smith, R. S. Weinbeck, I. W. Geer, J. T. Snow, K. M. Ginger, and J. M. Moran

Project ATMOSPHERE, the K–12 educational program of the American Meteorological Society, has been involved in teacher enhancement for four years. Summer workshops for teachers have been a primary component of the AMS K–12 educational initiatives since its inception. During the summer of 1994, Project ATMOSPHERE conducted four workshops: two of the workshops were for teachers in the Atmospheric Education Resource Agent program; another was for K–12 teachers, including one teacher each from Canada and Australia; and the fourth was for faculty members at community colleges or four-year undergraduate institutions. These workshops provide teachers at all levels with instruction on a variety of atmospheric topics, an introduction to the operational and research activities of the meteorological community, and exposure to atmospheric scientists and their facilities. Such workshops provide enriching experiences for educators who teach about weather and climate topics in their science classrooms.

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