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John T. Snow

Abstract

Geophysical columnar vortices such as tornadoes, waterspouts and dust devils are frequently observed to have one or more cylindrical sheaths of dust concentric with the axis of symmetry. The mechanisms by which such sheaths form have previously been investigated by assuming a balance between inward drag force (due to inward radial motion of the fluid) and outward centrifugal form (due to rotation of the particles around the vortex). However, the strong radial inflow required to establish this balance is confined to the surface inflow layer. In the upper two thirds of the vortex core, where the sheaths are most frequently observed, the radial component of fluid motion is very weak and may be outward. In this study, an alternative approach is presented wherein the drag forces arising from radial motion of the fluid are assumed negligible. The particles are thus continuously centrifuged out of the core. It is shown for four representative profiles of the tangential velocity component of the fluid that a particle sheath will form. The time required for its formation, the location of the sheath, and its evolution in time are in agreement with the available field evidence. Also, a two-celled vortex flow field is shown to produce a two-sheath structure. However, the inner sheath is a transient feature, so it is argued that the observed patterns of multiple concentric sheaths are probably due to the combined effects of the lifting of puffs of particles aloft by the vertical motion field while at the same time the particles are centrifuged out of the core.

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John T. Snow

Abstract

The occurrence of subsidiary vortices (multiple vortices) in intense tornadoes is hypothesized as being due to the development of an inertial instability in a two-celled vortex. The instability is taken to develop within a cylindrical shear layer surrounding a central stagnant subcore. The possible validity of this hypothesis is examined by means of a simple linear stability analysis. The special case of the stability of a rotating flow within strongly sheared azimuthal velocity to non-axisymmetric (cylinder symmetric) disturbances is solved in detail. It is found that the results of this highly simplified vortex model show good qualitative agreement with actual observations. The observed sequence of destabilization of progressively higher wavenumber modes, the shifting of the most unstable mode to large wavenumbers, and the hysteresis effects found in laboratory simulations can all be argued from the model. The results are shown to reduce to the corresponding classical findings of Rayleigh in the appropriate co-limits of small curvature and/or large wavenumber. Streamline patterns for two critical neutral modes are presented and discussed.

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John T. Snow
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John T. Snow
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Michael A. Magsig
and
John T. Snow

Abstract

On 7 May 1995, during VORTEX 95, the Tornado Debris Project at the University of Oklahoma collected debris transported up to 190 km (120 mi) in association with an isolated tornadic thunderstorm which moved over north Texas and southern/central Oklahoma. Analysis of WSR-88D radar reflectivity data along with eyewitness accounts suggest that plumes observable in the radar reflectivity field contained debris that were transported long distances by this storm. The three main clusters of debris can be explained by at least three different modes of deposition: rear-flank deposition, left-flank deposition, and forward-flank deposition. Trajectory estimates outside the storm based on terminal fall speeds of the collected debris suggest much of the debris did not travel as far as it would if it had left the storm at upper levels in a dry state. Direct and indirect precipitation effects are diagnosed as potential factors limiting the distance transported.

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John T. Snow
and
Donald E. Lund

Abstract

No abstract available.

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Christopher R. Church
and
John T. Snow

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.

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David R. Smith
and
John T. Snow
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Phillip J. Smith
and
John T. Snow
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John T. Snow
and
Randal L. Pauley

Abstract

No abstract available.

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