Occurrence of rare lightning events during Hurricane Nicholas (2021)

Abstract

Hurricane Nicholas was classified as a Category 1 tropical cyclone (TC) at 0000 UTC on 14 September 2021 and made landfall along the upper Texas Gulf Coast at 0530 UTC with maximum sustained winds of 33 m s-1. Much of the electrical activity during Nicholas was monitored by the Houston Lightning Mapping Array (HLMA) network. Deep convection developed in the rainband at 1700 UTC on 13 September, diminished by 2030 UTC, and re-intensified after 2200 UTC. At 2004 UTC (13 September), a curved megaflash (~220 km) was observed by the HLMA in the stratiform precipitation region of the outer rainband. By 0130 UTC on 14 September 2021, vigorous deep convective cells developed in the eastern eyewall region and propagated cyclonically to the western eyewall region. Several “jet-like” transient luminous events (TLEs) were observed by the HLMA emanating from a deep convective cell in the western eyewall region between 0230-0300 UTC with VHF source points ranging from 30-45 km in altitude. Moreover, the TLEs occurred within a region of strong wind shear, upper-level graupel-ice crystal collisions (~15 km), and strong cloud top divergence. Charge analysis of the deep convection during Nicholas revealed an overall normal dipole structure, while the megaflash and TLE deep convective cases exhibited inverted dipole charge structures. Dissipation of the upper-level screening charge layer resulting from cloud top divergence likely played a role in the observed TLE VHF sources escaping to altitudes exceeding 30 km.