Electrical discharges in the overshooting tops of thunderstorms

Previous studies have found that the vertical distribution of sources of very high frequency (VHF) signals from discharges mapped by Lightning Mapping Arrays typically have a secondary maximum in a storm's overshooting top. Low rates of these sources tend to occur continually throughout the lifetime of the overshooting top (OT), rather than sources occurring in the episodic distinct flashes observed at lower altitudes. This study examines the evolution of the VHF OT signature (VHF OT, defined here as a sustained period of >= 4 VHF sources per minute per 200 m layer above a storm's level of neutral buoyancy (LNB)) relative to the evolution of radar reflectivity and IR imagery of overshooting tops in three supercell and two multicell storms. The VHF OT began before OTs were detected in IR satellite imagery of the supercell storms. No OT was observed in IR imagery for either multicell storm, but this lack may have been due to the spatial and temporal resolution of the current IR imager. The VHF OT began near the time the 18 or 30 dBZ echo top rose above the LNB and ended near the time it fell below the LNB. There were too few radar volume scans of OTs in the multicell storms for a correlation analysis. In the supercell storms, however, the maximum altitude of VHF sources typically was less than or equal to the altitude of 18 dBZ echo tops and was correlated with these echo tops (linear correlation coefficient >= 0.86) during the period of the VHF OT.