Electrified deep convection and rare lightning events infer rapid
intensification 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. The sustained maximum wind speed increased from
a low-end estimate of 13 m s-1 (0000 UTC 13 September) to 33 m s-1 (0000
UTC 14 September) indicating rapid intensification. Lightning activity,
monitored by the Houston Lightning Mapping Array (HLMA), 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 in the outer rainband’s
stratiform precipitation region. Convection developed and intensified in
the eastern eyewall region by 0130 UTC on 14 September. Several
transient luminous events (TLEs) were observed in the western eyewall
region between 0230-0300 UTC with VHF source points exceeding 40 km
during a decline in lightning activity. The TLEs occurred during a
period of strong cloud top divergence resulting from complex
interactions between southwesterly low-level and westerly deep layer
wind shear. Charge analysis of Nicholas revealed an overall normal
dipole structure, while the megaflash and TLE cases exhibited inverted
charge structures. The upper-level screening and primary charge layer
heights of the TLEs heavily influenced the VHF source altitudes.
Interestingly, a surface wind gust of 42 m2 s-2 was observed near the
time of the first TLE, suggesting a second period of brief
intensification. Future investigations of TC evolution and behavior may
benefit from charge analyses.