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oai:www.ideals.illinois.edu:2142/23060 2023-07-10 col_2142_5131 col_2142_14839 com_2142_5130 com_2142_14838 com_2142_13459 com_2142_8903

eng Copyright 1990 Wicker, Louis John Geophysics Physics, Atmospheric Science Computer Science A numerical simulation of a tornado-scale vortex in a three-dimensional cloud model text Wilhelmson, Robert B. Wicker, Louis John 2011-05-07T14:00:42Z 2011-05-07T14:00:42Z 10000-01-01 1990 "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. Made available in DSpace on 2011-05-07T14:00:42Z (GMT). No. of bitstreams: 2 license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5) 9114463.pdf: 9240524 bytes, checksum: 7e509b143dde6981dc3ebc90df383eb9 (MD5) Previous issue date: 1990 Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding (hding2@illinois.edu) on 2011-05-07T15:01:54Z Item is restricted indefinitely. Restriction data tranferred 2014-07-01T11:29:24-05:00 Original Data Group with Access UIUC Users [automated] Release Date: none Reason: ETDs are only available to UIUC Users without author permission ETDs are only available to UIUC Users without author permission U of I Only AAI9114463 (UMI)AAI9114463 http://hdl.handle.net/2142/23060 Atmospheric Sciences Atmospheric Sciences University of Illinois at Urbana-Champaign Dissertation Ph.D.