Combined Optical and Radio-Frequency Perspectives on Hybrid
Cloud-to-Ground Lightning Observed by the FORTE Satellite
Abstract
We use the coincident optical and radio-frequency measurements taken by
the FORTE satellite to shed light on common optical signatures recorded
by NASA and NOAA lightning imagers during Cloud-to-Ground (CG)
lightning. We build flash cluster data for FORTE using the same
clustering techniques as GLM and document the optical / RF evolution of
an oceanic hybrid -CG flash over its 656 ms duration. The flash began
with strong VHF emission from a Narrow Bipolar Event (NBE) that
initiated a period of normal bilevel intracloud (IC) activity in two
vertical layers (8 km and 12 km) that lasted for 490 ms. VHF waveforms
show step leader activity ahead of seawater attachment in the return
stroke. All impulsive VHF sources after the stroke come from the lower
(8 km layer) only. K-changes are noted following the return stroke, but
no subsequent strokes are detected. The optical flash began 136 ms after
the NBE RF pulse. 22 of the 30 optical groups were dim and occurred
during the in-cloud phase of the flash. This activity included both
isolated pulses and sustained periods of illumination over tens of
milliseconds. Initial cloud pulses accounted for 23% of the total
optical radiance from the flash. Illumination during the return stroke
contributed a further 58% of the total radiance, and the K-changes and
cloud pulses after the stroke supplied the remaining 19%. These results
highlight the benefit of having RF alongside optical lightning
measurements for clarifying signatures in the optical data and providing
information on their physical origins.