Dissipation Characteristics of Tornadic Vortex Signatures Associated with Long-Duration Tornadoes

Radar data from a large sample of 2012–16 supercell cases are investigated to identify common tornadic vortex signature (TVS) behaviors prior to tornado dissipation. Based on the results of past case studies, four characteristics of TVSs associated with tornado dissipation are identified: decrease in TVS intensity, change in the storm-relative motion of the TVS, increase in TVS inclination, and increase in TVS horizontal displacement from the main storm updraft. Since all four behaviors rely on changes to TVS characteristics in time, only cases in which the reported tornado was at least 20 min in duration and the final 20 min of its life cycle occurred within 60 km of a WSR-88D system are examined. These two conditions ensure that the aforementioned quantities can be determined within 500 m of the surface at several time periods despite the relatively coarse spatial and temporal resolution of WSR-88D systems. It is found that prior to dissipation, TVSs became increasingly less intense, tended to move rearward in a storm-relative sense (usually westward), and became increasingly more separated from the approximate location of the main storm updraft. There was no clear signal in the relationship between tornado tilt as measured in inclination angle and TVS dissipation. The sensitivity of the results to the methods used and prospects for future nowcasting of tornado life cycles also are discussed.