Mobile Doppler Radar Observations of a Tornado in a Supercell near Bassett, Nebraska, on 5 June 1999. Part II: Tornado-Vortex Structure

Howard B. Bluestein School of Meteorology, University of Oklahoma, Norman, Oklahoma

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Wen-Chau Lee Atmospheric Technology Division, National Center for Atmospheric Research,* Boulder, Colorado

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Michael Bell Atmospheric Technology Division, National Center for Atmospheric Research,* Boulder, Colorado

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Christopher C. Weiss School of Meteorology, University of Oklahoma, Norman, Oklahoma

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Andrew L. Pazmany Microwave Remote Sensing Laboratory, Department of Computer and Electrical Engineering, University of Massachusetts—Amherst, Amherst, Massachusetts

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Abstract

This is Part II of a paper detailing an analysis of high-resolution wind and reflectivity data collected by a mobile, W-band Doppler radar; the analysis depicts the near-surface life history of a tornado in a supercell in north-central Nebraska on 5 June 1999. The structure of the tornado vortex near the ground is described from a sequence of sector scans at 10–15-s intervals during much of the lifetime of the tornado. The formation of the tornado vortex near the ground is described in Part I.

The wind and reflectivity features in the tornado evolved on timescales of 10 s or less. A time history of the azimuthally averaged azimuthal and radial wind profiles and the asymmetric components of the azimuthal and radial wind fields in the tornado were estimated by applying the ground-based velocity track display (GBVTD) technique to the Doppler wind data. If the magnitude of the asymmetric part of the radial wind component were indeed much less than that of the azimuthal wind component (a necessary requirement for application of the GBVTD technique), then the azimuthal wind field was dominated by quasi-stationary wavenumber-2 disturbances for most of the lifetime of the tornado. The radius of maximum wind (RMW) contracted as the tornado intensified and increased as the tornado dissipated. Shorter-timescale oscillations in azimuthal wind speed and RMW were found that could be manifestations of inertial oscillations. Evidence was also found that the tornado vortex was two-celled when it was most intense. During the “shrinking stage,” the vortex remained relatively wide and intense, even though the condensation funnel had narrowed substantially.

Corresponding author address: Dr. Howard B. Bluestein, School of Meteorology, University of Oklahoma, 100 E. Boyd, Rm. 1310, Norman, OK 73019. Email: hblue@ou.edu

Abstract

This is Part II of a paper detailing an analysis of high-resolution wind and reflectivity data collected by a mobile, W-band Doppler radar; the analysis depicts the near-surface life history of a tornado in a supercell in north-central Nebraska on 5 June 1999. The structure of the tornado vortex near the ground is described from a sequence of sector scans at 10–15-s intervals during much of the lifetime of the tornado. The formation of the tornado vortex near the ground is described in Part I.

The wind and reflectivity features in the tornado evolved on timescales of 10 s or less. A time history of the azimuthally averaged azimuthal and radial wind profiles and the asymmetric components of the azimuthal and radial wind fields in the tornado were estimated by applying the ground-based velocity track display (GBVTD) technique to the Doppler wind data. If the magnitude of the asymmetric part of the radial wind component were indeed much less than that of the azimuthal wind component (a necessary requirement for application of the GBVTD technique), then the azimuthal wind field was dominated by quasi-stationary wavenumber-2 disturbances for most of the lifetime of the tornado. The radius of maximum wind (RMW) contracted as the tornado intensified and increased as the tornado dissipated. Shorter-timescale oscillations in azimuthal wind speed and RMW were found that could be manifestations of inertial oscillations. Evidence was also found that the tornado vortex was two-celled when it was most intense. During the “shrinking stage,” the vortex remained relatively wide and intense, even though the condensation funnel had narrowed substantially.

Corresponding author address: Dr. Howard B. Bluestein, School of Meteorology, University of Oklahoma, 100 E. Boyd, Rm. 1310, Norman, OK 73019. Email: hblue@ou.edu

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