Understanding WSR-88D signatures for the 3 May 1999 Oklahoma City tornado

The 3 May 1999 Oklahoma City storm was unique from a radar perspective, having a long-track violent tornado pass within close range of several Doppler radars, having a detailed damage survey conducted immediately after the event, and having high-quality visual observations. The tornado passed within 10 nmi of three Doppler radars: the National Weather Service WSR-88D in Twin Lakes (KTLX), the OSF WSR-88D Test Radar (KCRI), and the Terminal Doppler Weather Radar at Will Rodgers World Airport. Base data from all three radars are used in this study. For a portion of the tornado lifetime, very-high resolution Doppler on Wheels data exist and are compared to the WSR-88D and TDWR analyses. Having multiple radars at close range to a large tornado affords an opportunity to investigate: 1) how the radar signatures relate to tornado damage and 2) radar sampling issues associated with tornadoes and the complex flow within which they are embedded.

The radars displayed strong, well-defined signatures along the track of the violent May 3rd Oklahoma City tornado. The traditional tornado proxy of strong and localized velocity difference (Tornadic Vortex Signature; TVS) compared favorably with damage locations and documented changes in tornado intensity (F-Scale). The diameter of the TVSs seen by the radars are wider than the tornado width even when the TVS is spread across several azimuths. Several hypotheses are being investigated to explain the spatial characteristics of the TVS. A unique maximum in reflectivity values also occurred as the tornado moved through Bridge Creek and the Oklahoma City metropolitan area. The localized reflectivity signature is believed to emanate from large amounts of debris being lofted to high heights by the tornado.

A better understanding of the tornado’s velocity and reflectivity signatures will lead to better radar operational applications in tornado warning decision making.