Analysis of a Nontornadic Storm during VORTEX 95

Roger M. Wakimoto Department of Atmospheric Sciences, University of California, Los Angeles, Los Angeles, California

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Huaqing Cai Department of Atmospheric Sciences, University of California, Los Angeles, Los Angeles, California

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Abstract

Analysis of a supercell storm that did not produce a tornado near Hays, Kansas, is presented. A well-defined midlevel mesocyclone was apparent throughout most of the storm’s life cycle. Numerous shallow circulations were observed along the rear-flank gust front during the data collection period. Six of these circulations strengthened into intense low-level mesocyclones. Each of these mesocyclones failed to produce a tornado. The strongest low-level mesocyclone, referred to as vortex #4, underwent a life cycle that was consistent with other tornadic mesocyclones documented in the literature. These results illustrate that the presence of a long-lived mesocyclone at low levels is not sufficient for tornadogenesis to occur.

The kinematic structure of the low-level mesocyclone that did not produce a tornado is compared with a tornadic mesocyclone from another storm in order to understand the characteristic differences between these circulations. The results lead to the conclusion that the presence of a low-level mesocyclone, occlusion downdraft, and updraft/downdraft structure that spirals cyclonically around the circulation are not sufficient conditions for tornadogenesis. Retrieved perturbation pressure and buoyancy fields are used to examine the forcing mechanism of the occlusion downdraft. A downward-directed pressure gradient appears to be the primary forcing mechanism of this downdraft. Perturbation temperature retrievals suggest that the occlusion downdraft is accompanied by a warm core.

Corresponding author address: Dr. Roger Wakimoto, UCLA, 405 Hilgard Ave., Los Angeles, CA 90095-1565.

Email: roger@atmos.ucla.edu

Abstract

Analysis of a supercell storm that did not produce a tornado near Hays, Kansas, is presented. A well-defined midlevel mesocyclone was apparent throughout most of the storm’s life cycle. Numerous shallow circulations were observed along the rear-flank gust front during the data collection period. Six of these circulations strengthened into intense low-level mesocyclones. Each of these mesocyclones failed to produce a tornado. The strongest low-level mesocyclone, referred to as vortex #4, underwent a life cycle that was consistent with other tornadic mesocyclones documented in the literature. These results illustrate that the presence of a long-lived mesocyclone at low levels is not sufficient for tornadogenesis to occur.

The kinematic structure of the low-level mesocyclone that did not produce a tornado is compared with a tornadic mesocyclone from another storm in order to understand the characteristic differences between these circulations. The results lead to the conclusion that the presence of a low-level mesocyclone, occlusion downdraft, and updraft/downdraft structure that spirals cyclonically around the circulation are not sufficient conditions for tornadogenesis. Retrieved perturbation pressure and buoyancy fields are used to examine the forcing mechanism of the occlusion downdraft. A downward-directed pressure gradient appears to be the primary forcing mechanism of this downdraft. Perturbation temperature retrievals suggest that the occlusion downdraft is accompanied by a warm core.

Corresponding author address: Dr. Roger Wakimoto, UCLA, 405 Hilgard Ave., Los Angeles, CA 90095-1565.

Email: roger@atmos.ucla.edu

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