The numerical simulation of non-supercell tornadogenesis
Lee, Bruce Donald
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https://hdl.handle.net/2142/21887
Description
Title
The numerical simulation of non-supercell tornadogenesis
Author(s)
Lee, Bruce Donald
Issue Date
1995
Doctoral Committee Chair(s)
Wilhelmson, Robert B.
Department of Study
Atmospheric Sciences
Discipline
Atmospheric Sciences
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Atmospheric Science
Language
eng
Abstract
Non-supercell tornado is a name given to a class of tornadoes which occur with deep moist convection not displaying the strong mid-level rotation found in supercell thunderstorms. In recent years the severe storms community has acknowledged that a significant portion of the small tornadoes occurring across North America every year are associated with storms not displaying strong mid-level rotation before tornado occurrence. This research represents the first high resolution three-dimensional modeling effort to successfully simulate the genesis processes of non-supercell tornadoes occurring along thunderstorm outflow boundaries.
Two numerical models were employed in this investigation. A model without moist microphysics was used to examine the underlying processes that create pre-tornadic misocyclone circulations along thunderstorm outflow boundaries. The second model, which includes moist microphysics, was used to examine the full moist non-supercell tornadogenesis problem. Composite environmental conditions from a number of northeast Colorado non-supercell tornado cases were used to initialize the moist simulations. Model results indicate that vortex sheet dynamics play an important role in concentrating vertical vorticity and eddy kinetic energy along the outflow boundary leading edge. Simulations demonstrate that the misocyclones control the updraft distribution along the outflow boundary which has important implications for nowcasting storm initiation and possible tornado locations. A tornadic circulation evolves in the moist model which has a life of approximately 7 minutes. Peak ground relative winds exceed F1 severity levels for a 4.5 minute span. Strong vertical vorticity at low-levels precedes significant rotation aloft which is consistent with the observational model for non-supercell tornadogenesis. The observational model's vortex stretching hypothesis for tornado intensification has been confirmed. Model results indicate that new precipitation induced cold pools intensify the tornadic circulation. A refined model for non-supercell tornadogenesis has been created.
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