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John T. Snow
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John T. Snow
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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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Jean Dessens and John T. Snow

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

  1. June and August are the months with the greatest number of such tornadoes;
  2. 1600–1700 UTC is the interval in which occurrence is most likely, with a secondary maximum in likelihood between 1800 and 1900 UTC;
  3. 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;
  4. two significant tornadoes can be expected in France each year;
  5. the mean area stricken by such a tornado is about 4 km2;
  6. France has a mean risk probability of a significant tornado occurring at a point of about 1.5 × 10−5 per year, a value some 15 times lower than the Great Plains of the United States.

During the cold season November–March, tornadoes are most frequently observed in northwest France. During the warm season April–October, they axe most frequently observed in the interior of the country.

An examination of the meteorological situations associated with 21 cases suggests that there is a distinctive synoptic pattern for each season. Analyses of tornado-producing situations show that French tornadoes usually occur where an unstable surface layer of modified air that originated in the Mediterranean is overlaid at midlevels by maritime air originating over the Atlantic. The instability in the surface layer develops while the air is in southern France. Such situations are characterized by high degrees of conditional instability between surface and midlevels in the troposphere as reflected in a steep lapse of wet-bulb potential temperature. However, case studies of individual events suggest that for a tornado to occur, the instability of the surface layer must be further enhanced by local warming and moistening The appearance of a secondary low-pressure center on or near a cold front advancing from the west is an additional favorable condition for the onset of severe weather.

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Randal L. Ipauley and John T. Snow

Abstract

No abstract available.

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David R. Smith and John T. Snow
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John T. Snow and Donald E. Lund

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No abstract available.

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John T. Snow and Randal L. Pauley

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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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