Extended Abstract (104K)12.7
Three-dimensional stability analyses of tornado-like vortices with secondary circulations
David S. Nolan, University of Miami, Miami, FL; and M. T. Montgomery
Since the early work of Fujita, the multiple vortex phenomenon in tornadoes has been widely believed to be caused by dynamic instability of the azimuthal and vertical wind profiles as a function of radius from the vortex center. However, there is ample observational and numerical evidence that these profiles vary substantially with height, and that embedded within the rotational flow is a significant secondary circulation, with radial inflow at low levels and vertical flow up the vortex axis. This axial flow sometimes supports a "vortex breakdown," below which the flow has a laminar appearance, and above which there are large turbulent motions.
We further investigate these dynamics by extending vortex stability analysis in two ways: first, arbitrary variation with height is allowed for both the basic state and the associated eigenfunctions. Second, the secondary circulation is also included in the basic state. For simple tornado-like vortices generated from an axisymmetric model, unstable modes are shown to be localized just above the point of vortex expanion, as in a "drowned vortex jump." Instabilities in vortices with various profiles, and their dynamic mechanisms, will be presented.
Session 12, tornadogenesis
Thursday, 15 August 2002, 8:00 AM-10:00 AM
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