| Intracloud (IC) lightning has been characterized using electric field waveforms and the temporal evolution of the flash, which are used to define an active stage. Of- ten, IC lightning will be characterized based on pulses in the electric field waveform due to the radiation portion of the electric field. These pulse characterizations have focused on pulse amplitude and frequency during the active stage in IC lightning, occurring early in the lightning flash. Further, little or no optical emission is associated, in previous studies, with the early portion of IC lightning; however, the early stage is typically identified as the most active portion of the IC waveform. With the objective of expanding how the active stage is characterized using optical emission and a spatio-temporal analysis of electric field waveforms with a focus on radiation field pulses, this study addresses IC lightning leader processes in a comprehensive fusion of electric field data and satellite based optical emission. Using the Huntsville Alabama Marx Meter Array (HAMMA), the North Alabama Lightning Mapping Array (NALMA) and the Lightning Imaging Sensor (LIS) this study consists of a spatio-temporal analysis of radiation field pulses from 19 IC flashes. Each flash is analyzed from initiation to bi-level breakdown and optical emission is present during the leader process of each flash. While the most common IC pulses are small amplitude and narrow (initial half cycle (IHC) width), pulses temporally matched with optical emission are often large amplitude (>1.0 V/m) and have large pulse widths (>20 microseconds). Additionally, the active stage is specfically associated with a period of vertical leader propagation, corresponding to the IC leader extending between two idealized charge layers in a thunderstorm. Features unique to this vertical extent include an increase in VLF source frequency and optical emission is first detected by LIS. |
