The 8 June 1995 McLean, Texas, Storm. Part II: Cyclic Tornado Formation, Maintenance, and Dissipation

David C. Dowell NOAA/National Severe Storms Laboratory and Cooperative Institute for Mesoscale Meteorological Studies, Norman, Oklahoma

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Howard B. Bluestein School of Meteorology, University of Oklahoma, Norman, Oklahoma

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Abstract

On 8 June 1995, scientists participating in the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) collected a unique dataset with the Electra Doppler Radar (ELDORA). The ELDORA observations document the sequential life cycles of storm-scale circulations associated with three large tornadoes in a supercell thunderstorm near McLean, Texas. A qualitative description of the evolution of the storm was provided in Part I of this paper.

During the first stage of development of each storm-scale circulation, interaction of the updraft with the environmental low-level horizontal vorticity produced a vorticity column that increased in intensity with height. As the vortex matured, vorticity increased greatly at low levels (i.e., below 2 km AGL) and exceeded that aloft. Each tornadic vortex was located near the rear side of the updraft, where the surrounding low-level horizontal vorticity was modified locally, most likely by weak baroclinity within the storm. Tilting of low-level horizontal vorticity into the vertical, followed by stretching of the vertical vorticity, occurred in the air parcels that entered the rear portion of the main storm updraft from its left (as viewed in the direction of storm motion). Although the region of tilting was near the interface of the main updraft and that portion of the downdraft to the left of the updraft, there is no direct evidence in the observations (above 500 m AGL) of generation of cyclonic vertical vorticity by tilting in the downdraft itself.

For this storm, the cyclic tornadogenesis process was associated with a mismatch between the horizontal motion of successive tornadoes and the horizontal velocity of the main storm-scale updraft and downdraft. Low-level updraft-relative flow seemed to be the most important factor in determining tornado motion.

Current affiliation: National Center for Atmospheric Research, Boulder, Colorado

Corresponding author address: David C. Dowell, National Center for Atmospheric Research, P. O. Box 3000, Boulder, CO 80307-3000. Email: ddowell@ucar.edu

Abstract

On 8 June 1995, scientists participating in the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) collected a unique dataset with the Electra Doppler Radar (ELDORA). The ELDORA observations document the sequential life cycles of storm-scale circulations associated with three large tornadoes in a supercell thunderstorm near McLean, Texas. A qualitative description of the evolution of the storm was provided in Part I of this paper.

During the first stage of development of each storm-scale circulation, interaction of the updraft with the environmental low-level horizontal vorticity produced a vorticity column that increased in intensity with height. As the vortex matured, vorticity increased greatly at low levels (i.e., below 2 km AGL) and exceeded that aloft. Each tornadic vortex was located near the rear side of the updraft, where the surrounding low-level horizontal vorticity was modified locally, most likely by weak baroclinity within the storm. Tilting of low-level horizontal vorticity into the vertical, followed by stretching of the vertical vorticity, occurred in the air parcels that entered the rear portion of the main storm updraft from its left (as viewed in the direction of storm motion). Although the region of tilting was near the interface of the main updraft and that portion of the downdraft to the left of the updraft, there is no direct evidence in the observations (above 500 m AGL) of generation of cyclonic vertical vorticity by tilting in the downdraft itself.

For this storm, the cyclic tornadogenesis process was associated with a mismatch between the horizontal motion of successive tornadoes and the horizontal velocity of the main storm-scale updraft and downdraft. Low-level updraft-relative flow seemed to be the most important factor in determining tornado motion.

Current affiliation: National Center for Atmospheric Research, Boulder, Colorado

Corresponding author address: David C. Dowell, National Center for Atmospheric Research, P. O. Box 3000, Boulder, CO 80307-3000. Email: ddowell@ucar.edu

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