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This study compares waveforms recorded by “broadband” very low frequency/low‐frequency/medium‐frequency (VLF/LF/MF) electric field change sensors (bandwidth ~0–2.5 MHz) and very high frequency (VHF) sensors (bandwidth 186–192 MHz) during the initiation of 20 negative cloud‐to‐ground lightning flashes. In the first 2 ms of each flash, initial breakdown (IB) pulses are detected with the VLF/LF/MF sensors. Comparison shows that all classical IB pulses are accompanied by VHF pulses, where classical IB pulses are defined herein as bipolar with duration >10 μs and amplitude >25% of the largest IB pulse amplitude in the flash. There are on average 47% of IB pulses (of all amplitudes and durations) that are accompanied by VHF pulses within ±1 μs. There are also many VHF pulses with no associated IB pulses. These observations indicate that the initial in‐cloud lightning channel extension process (es) occurs very fast and at multiple length scales, since substantial electromagnetic radiation is emitted in the VLF/LF/MF and VHF bands.

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This study describes results from video observations of five intracloud flashes located ≤ 20 km from the camera and recorded with 6.1 µs exposure time and 6.66 µs frame intervals. Video data are supported with electric field change (E-change) and VHF measurements, with emphasis on the flash initiating event (IE) and initial breakdown (IB) stage. In four of the five flashes, the IE is accompanied by weak luminosity, ≤ 5% above background, lasting for 300–500 µs. Two of these four IEs were positive Narrow Bipolar Events (NBEs) with VHF powers of 43 and 990 W; these are the first (known) data showing visible light detected with a positive NBE. Two other IEs with weak luminosity had powers of 0.5 and 1 W, and the IE with no detected luminosity had a VHF power of 3 W. A typical IB cluster consists of several narrow pulses and one classic pulse in E-change data (along with many VHF pulses), and each example flash has 2–10 IB clusters in the first 5–50 ms. The luminosity of IB clusters was substantially greater than IE luminosity, ranging from 10 to 40% above background in four examples, while for one flash with 10 IB clusters, the luminosity range was 35–360% above background (average 190%). Luminosity durations of IB clusters were 520–1750 µs with average 1210 µs. For both IEs and IB clusters, increases in the detected luminosity were closely timed with substantial VHF emissions and decreased when VHF emissions weakened.

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