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 form Witchita Falls, Texas. To better represent the influence of surface friction on the vortex flow, a simple surface layer parameterization 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 over a several minute period. The inclusion of the surface layer parameterization increases the low-level velocity convergence. Surface vertical vorticity is greater than 0.43s-1 for thirty seconds and greater than 0.3 s-1 for several minutes. During tornadogenesis, pressures at the surface fall 3-4mb in thirty seconds and a pressure gradient develops of over 7mb from the outer edge of the tornado to the center. A vortex tube extends from the surface to over 2.5km 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. Analysis 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.