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.