Tuesday, 11 February 2003: 8:45 AM
Radar measurements of extreme winds in tornadoes and comparisons with observed damage
Joshua Wurman, University of Oklahoma, Norman, OK; and D. Burgess, D. C. Dowell, Y. Richardson, M. A. Magsig, and C. R. Alexander
Peak windspeeds of 125-144 m/s, based on DOW Doppler measurements, have
been calculated for the 3 May 1999 Oklahoma City tornado, representing
the highest windspeeds ever measured (with caveats concerning height and
duration discussed below). Windspeeds of 125 m/s have been calculated
for the Mulhall tornado during the same outbreak and 116 m/s for the
Spencer, South Dakota tornado of 30 May 1998. Damage surveys in the
wakes of these tornadoes have revealed F5, F4, and F4 damage
respectively. While the DOWs have been recording extremely high
windspeeds, the wind engineering community has been simultaneously
reaching the conclusion that windspeeds of ~120 m/s are not necessary to
produce the worst, F-5, damage that has been surveyed. However,
comparison between DOW, and other radar, measurements with wind
estimates derived from damage estimates, or even not-yet-existent
surface in situ observations, is problematic. DOW radar measurements
occur nearly instantaneously, representing integration periods of
0.01-0.02 s. Crude space-time conversions across the radar resolution
volumes of the highest windspeed measurements result in time-scales of
0.2 s. 1/4 mile winds, corresponding to several second duration wind
gusts are associated with F-scale estimates. Furthermore, DOW tornado
measurements rarely are collected below 20 m agl. Standard surface
measurements occur at 10 m agl, and Fujita-scale wind estimates apply at
the height of damaged objects, usually about 3-5 m agl. Further
complicating the issue is the fact that radars measure the motion of
debris, which may be significantly slower than that of the surrounding
air. However, it is likely that debris impacts are one major cause of
damage. Finally, spectral width measurements of 15-20 m/s suggest that a
significant fraction of the debris may be moving at speeds up to 30 m/s
above the mean, perhaps over 150 m/s in the strongest measured tornadoes.
Careful interpretation of DOW windspeed measurements suggests that the
radar observations and engineering estimates are not inconsistent.
In the Spencer, South Dakota tornado of 30 May 1998, DOW Doppler
measurements at intervals of approximately 25-30 m, and a tornado
windspeed model are compared to a house-by-house F-scale mapping.
Observed damage is compared to reconstructed time histories of windspeed
occurring at various points. Locations impacted by the core flow region
experience two separate, but short duration, periods of extreme winds
while locations just to the south of this region experience these winds
for a longer time. Winds, and damage, are significantly lower on the
north side of the track. Comparisons of observed winds and damage in the
3 May 1999 tornado may be presented if available.
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