A numerical simulation of a tornado-scale vortex in a three-dimensional cloud model
Wicker, Louis John
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https://hdl.handle.net/2142/23060
Description
Title
A numerical simulation of a tornado-scale vortex in a three-dimensional cloud model
Author(s)
Wicker, Louis John
Issue Date
1990
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)
Geophysics
Physics, Atmospheric Science
Computer Science
Language
eng
Abstract
"One of the more spectacular and elusive events of nature is the tornado. Usually spawned by a highly organized, lasting, and rotating thunderstorm called a ""supercell"", tornadoes are one of the most destructive atmospheric phenomena. Tornadoes almost always have length and time scales smaller than the measurable scales within the observing network of surface stations, conventional radar, Doppler radar and satellites. Therefore direct observations of tornadoes and their parent features are rarely obtained. Consequently, understanding of these phenomena will generally have to come from theoretical work, laboratory experiments, and numerical simulations."
In this thesis we seek to understand the process of tornadogenesis within the context of a fully three-dimensional cloud model. Very high horizontal and vertical resolution is used to capture a developing tornado-scale vortex during the simulation of a strongly rotating supercell storm simulated within the 3 April 1964 environment from Witchita Fall, Texas. To better represent the influence of surface friction on the vortex flow, a simple surface layer parametrization of the vertical fluxes of horizontal momentum is added to the model.
Results from the simulation show that a tornado-scale vortex forms along the western edge of the mesocyclone, intensifies and rotates cyclonically around the center of the mesocyclone over a several minute period. The inclusion of the surface layer parameterization increases the low-level velocity convergence. Surface vertical vorticity is greater than 0.43 s$\sp{-1}$ for thirty seconds and greater than 0.3 s$\sp{-1}$ for several minutes. During tornadogenesis, pressures at the surface fall 3-4 mb in thirty seconds and a pressure gradient develops of over 7 mb from the outer edge of the tornado to the center. A vortex tube extends from the surface to over 2.5 km aloft and tilts to the northwest. Analyses show that tornadogenesis occurs when the vertical velocity gradients along the western side of the mesocyclone increase and that the principle mechanism for intensifying the vertical vorticity is convergence. Analyses also show that the development of the occlusion updraft along the western edge of the mesocyclone is related to advection of warm air southwestward over the gust front and the lowering of pressure aloft within the mesocyclone core.
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