A violent supercell tornado passed through the town of Spencer, SD on 30 May 1998 producing large gradients in damage severity, and the tornado was rated at F4 intensity by damage survey teams. A Doppler On Wheels (DOW) mobile radar vehicle followed this tornado, and observed the tornado at ranges between 1.7 km and 8.0 km during various stages of the tornado?s life. The DOW was deployed less than 4.0 km from the town of Spencer between 0134 UTC and 0145 UTC, and during this time period the tornado passed through Spencer and doppler velocity measurements approached 100 ms-1. Data gathered from the DOW during this time period contained high spatial resolution sample volumes of approximately 60x60x37m along with frequent volume updates every 45-50 s. Precise navigation of the radar data was obtained using on-board GPS data, visual observations, and gound-clutter reflectivity maps allowing the location of the DOW to be estimated to within 30m, and the orientation of the DOW to within about 0.1 degree.

The high resolution doppler velocity data gathered from low-level elevation scans, when sample volumes are between 20-40 m agl, are compared to extensive ground and arial damage surveys performed by the National Weather Service (NWS) and the National Institute of Standards and Technology (NIST). The single-doppler velocity volumes are interpolated to a cartesian grid, and the tornado velocity field is fitted with an axisymmetric vortex to provide a more complete two-dimensional velocity field which compensates for velocity components perpendicular to the radar beam as well as the translational motion of the tornado vortex. Both the original single-doppler velocity data and the interpolated velocity fields are compared with damage survey F-scale estimates at various locations in the town of Spencer. This comparison uses doppler velocity data as a proxy for F-scale wind strength, and several time-integrated F-scale fields are calculated for comparison against damage survey F-scale estimates. This comparison demonstrates some of the ambiguity inherent in both the F-scale rating system and doppler velocity measurements which are especially sensitive to large debris motion.