New revelations on lightning initiation and evolution using a newly developed array of wideband electric field sensors.

摘要

Lightning is fundamentally a multi-scale discharge in atmosphere, spanning scales from less than a meter to many kilometers. This research introduces a new instrument array, named the Huntsville Alabama Marx Meter Array (HAMMA), capable of making measurements of the radiated electric field in the wideband frequency regime of ∼ 1 Hz – 500 kHz. The combination of sampling frequency (1 MHz) and baselines (∼ 15 km) of HAMMA sensors has never been attempted in prior published research.;This study chronicles the development and initial applications of HAMMA. Using time of arrival techniques, HAMMA can determine the spacetime position of the wideband radiation from a lightning flash. Measurements from the National Lightning Detection Network (NLDN) are used to quantify the spatial resolution of HAMMA sources; in particular, the average difference in locations of return strokes between HAMMA and NLDN is 250 m. Further, the spacetime positions of wideband sources map out the same general electrical extent of a lightning flash as VHF sources, a result that was previously thought unlikely.;The wideband waveforms measured by HAMMA are compared to space-based optical observations by the Lightning Imaging Sensor (LIS). Analysis shows when an optical emission is detected by the LIS, the wideband waveform also detects a distinct discharge. However, no unique signature is present in the VHF sources. LIS also detects return strokes at a far higher frequency than previous published research using VHF sources shows. Finally, the peak electric field radiated by return strokes is used to suggest there exists a threshold below which not enough optical emission is produced to be detected by LIS.;For the first time, wideband sources associated with initiation are found. These are compared with VHF sources and are found to differ by ∼310 m (horizontally) and ∼ 640 m (vertically). Further, the time difference between wideband and VHF sources are shown to be generally incompatible with the cosmic ray runaway breakdown theory of initiation. Finally, initiation locations are compared to radar data to show that lightning is preferentially initiated near dry snow and small hail/graupel, suggesting that these particular hydrometeors play a non-trivial role in initiation.