Doppler Spectra and Estimated Windspeed of a Violent Tornado

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  • 1 National Severe Storms Laboratory, NOAA, Norman, OK 73069
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Abstract

Presented in this Paper are Doppler spectra of a very large tornado that occurred on 22 May 1981 near Binger, Oklahoma. Tracking of the tornado was accomplished with the help of a novel “polar spectra display.” Bimodal tornado spectral signatures (TSS) were observed in about 40 scans. Direct measurements of maximum velocities from spectral skirts yielded a maximum tangential speed of 80 m s−1 (90 m s−1 relative to ground). A diameter of 1 km at 200 m above ground was deduced from a simplified model. Radial centrifuging of radar targets was estimated to be about 20 m s−1. With simple assumptions for radar target sizes and summation of forces, a beamwidth average convergence value of about 2.5 × 10−2 s−1 was calculated for the tornado boundary layer.

Tornado damage to trees and structures was subjectively rated on the Fujita damage scale. The windspeed range associated with the damage scale agreed well with the Doppler-estimated maximum windspeed when the tornado was large (1 km diameter). However, as the tornado diameter decreased, the Doppler-derived windspeed considerably underestimated that associated with the damage scale.

Abstract

Presented in this Paper are Doppler spectra of a very large tornado that occurred on 22 May 1981 near Binger, Oklahoma. Tracking of the tornado was accomplished with the help of a novel “polar spectra display.” Bimodal tornado spectral signatures (TSS) were observed in about 40 scans. Direct measurements of maximum velocities from spectral skirts yielded a maximum tangential speed of 80 m s−1 (90 m s−1 relative to ground). A diameter of 1 km at 200 m above ground was deduced from a simplified model. Radial centrifuging of radar targets was estimated to be about 20 m s−1. With simple assumptions for radar target sizes and summation of forces, a beamwidth average convergence value of about 2.5 × 10−2 s−1 was calculated for the tornado boundary layer.

Tornado damage to trees and structures was subjectively rated on the Fujita damage scale. The windspeed range associated with the damage scale agreed well with the Doppler-estimated maximum windspeed when the tornado was large (1 km diameter). However, as the tornado diameter decreased, the Doppler-derived windspeed considerably underestimated that associated with the damage scale.

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