Rapid-Scan, Polarimetric Radar Observations of the Dissipation of a Violent Tornado on 9 May 2016 Near Sulphur, Oklahoma

Mobile polarimetric X-band Doppler radar data obtained by the University of Oklahoma’s Rapid X-band Polarimetric radar (RaXPol) system are used to analyze a long-lived tornado during its dissipation phase; the tornado occurred on 9 May 2016 near Sulphur, OK. The focus of this study is on changes in TVS and storm-scale polarimetric features as the tornado weakened rapidly leading up to its eventually dissipation. Past work has identified a number of TVS characteristics, including intensity, tilt, and storm-relative motion that change, often dramatically, prior to tornado dissipation. In addition, there is some evidence that tornadoes sometimes undergo a prolonged dissipation process in time and height, occurring first in the 1-2 km layer. However, in most studies these dissipation behaviors have not been observed with rapid-scan mobile radar systems, preventing a detailed look at how they evolve over very short time scales. In contrast, few studies have examined at all whether changes in storm-scale polarimetric characteristics may be used to portend tornado dissipation, though there is some evidence that hook echo median drop sizes may increase as tornadoes dissipate.

In this study, TVS and polarimetric features observed by RaXPol (with ~25 s volumetric updates) are analyzed during the final ~20 min in the life cycle of the Sulphur, OK tornado. In addition, data from the KTLX WSR-88D are used to estimate bulk hook echo drop size distributions (DSDs) leading up to tornado dissipation. Among the features that will be discussed include: a rapid time-height dissipation that lacks obvious directionality, large changes in 0-1 km TVS tilt, a long-lived low-reflectivity ribbon that occasionally feeds directly into a large tornadic debris signature, several low-correlation coefficient (CC) bands attendant to the TDS, and regular, simultaneous oscillations in storm-relative motion and assumed hook echo median drop diameter. Possible connections are made among the oscillations, rapid TVS intensity decline, low-CC bands, and internal momentum surges. The potential impact of oscillatory behaviors on prospects for future nowcasting of tornado dissipation also will be discussed.