On 3 May 1999, a supercell thunderstorm, with a pronounced hook echo and an intense cyclonic circulation, was identified by the Norman, OK WSR-88D radar. This thunderstorm, observed by the DOW from 0310-0328 UTC, produced an exceptionally large and powerful tornado with ~800 m in peak wind radius and peak wind speed exceeding 100 m/s. Multiple vortices were identified embedded in the parent tornado circulation. Scanning was conducted through ~85° azimuthal sectors at 12 stepped elevation angles from 0-17°. The volume scan time is ~1 minute; therefore, a total of 16 volumes of single Doppler radar data is collected. The core region of the tornado moved to a range of ~3.5-4.5 km from the DOW, resulting in a radar beam width of 65 m, and 25-37.5 m gate spacing. Staggered pulse repetition frequency resulted in a Nyquist velocity of ±128 m/s.

The tornado structure is deduced using the velocity track display (VTD) technique that has been applied successfully in many tropical cyclones. The axisymmetric and asymmetric structure of the Mulhall tornado can be deduced and show coherent structures over time. The swirl ratio can also be estimated from the axisymmetric structures and the values are consistent with those in the lab. simulations of tornadoes with multiple vortices. This is the first time that the 3-D structure of a tornado is revealed by single Doppler radar observations.