21st Conf. on Severe Local Storms


Tornadogenesis and tornado-vortex structure in a supercell

Howard B. Bluestein, University of Oklahoma, Norman, OK; and C. C. Weiss, A. L. Pazmany, W. C. Lee, and M. Bell

On 5 June 1999 near Bassett, NE, a group from the Univ. of Oklahoma collected a dataset with the U. Mass. 3-mm wavelength mobile Doppler radar in which the entire life history of a tornado was documented. In a poster presentation at the last SLS conference, the qualitative aspects of this case were highlighted. In this presentation the more quantitative aspects are highlighted.

Circumstantial evidence is presented that 100-m scale cyclonic vortices that formed along the rear-flank downdraft gust front may have played a role in tornadogenesis: Tornadogenesis commenced when one such gust-front vortex reached the nose of a bulge in the gust front. It is speculated that such small-scale vortices may be the seeds for tornadoes within the larger-scale mesocyclone. There were a string of these vortices along the gust front, many of which were accompanied by tiny reflectivity eyes or swirls.

The Doppler velocity data were subjected to a ground-based velocity track display (GBVTD) analysis. The GBVTD technique, which has been applied extensively to hurricane-vortex radar data, allows one to extract information about the structure of vortices even though only Doppler velocities are known. It is shown how wavenumber-two disturbances in the tornado vortex predominated: The tornado vortex was not axisymmetric. Radial profiles of the azimuthally averaged tangential and radial wind and of vorticity are presented and discussed in light of vortex-sheet instability and barotropic instability theories. The temporal variation of core radius, maximum Doppler wind speed and shear, vorticity, and circulation are also presented and discussed.

Finally, it is shown how during the shrinking stage of a tornado the vortex remains intense and does not contract, even though the condensation funnel “ropes out.”

Session 12, tornadogenesis
Thursday, 15 August 2002, 8:00 AM-10:00 AM

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