Assessing impulses and decay of overshooting tops relative to supercell collapse using lightning and phased array radar data

The overshooting tops of seven tornadic supercell storms occurring in central Oklahoma were examined using data from the Oklahoma Lightning Mapping Array (LMA), the National Weather Radar Testbed Phase-Array Radar (PAR), and surrounding National Weather Service WSR88-D radars. The growth and decay of the overshooting tops in both the lightning and reflectivity data were noted relative to the times of tornadoes reported for each storm. The lightning signatures of the overshooting top were seen only in storms that reached at least 13 km MSL, typically seen only in the warm season when the tropopause tends to be higher. The rate of mapped lightning points in the overshooting top was much smaller than the rates typically seen for flashes lower in the storm and often consisted of isolated points separated by tens to hundreds of milliseconds. The high lightning associated with the overshooting top exhibited cycles of growth and decay similar to cycles seen in reflectivity, driven by the evolution of the storm's updraft. In most cases, the signature of an overshooting top appeared in radar data before it appeared in lightning data. The most concentrated areas of high lightning activity were immediately downstream of and within the overshooting top in areas having reflectivity of 20-40 dBZ. Of the 14 tornadoes that occurred during the storms we studied, 11 immediately followed or coincided with a decrease in the height or occurrences of lightning in the overshooting top, along with less noticeable decreases in the height of the radar echo top. Of the seven storms studied, six showed lightning impulses at the base of the overshooting top 3-25 min before the formation of the first tornado. The exception was a storm that produced only a weak tornado.