An Advanced Model of Lightning M‐Component

Abstract

An advanced nonlinear and nonuniform distributed circuit (RLCG) model of lightning M‐component has been developed. The model accounts for the variation of the series resistance R of M‐component channel due to its heating by the transient current and its subsequent cooling, longitudinal voltage drop along the channel due to the background continuing current, ohmic losses in the channel corona sheath (represented by shunt conductance G), and variation of series inductance L and shunt capacitance C of the channel with height above ground. The model was tested against the channel‐base current and corresponding close electric fields measured for seven M‐components in negative lightning triggered using the rocket‐and‐wire technique. Detailed sensitivity analysis was performed for one M‐component. The influences of height‐varying series inductance and shunt capacitance and the length of in‐cloud channel (representing the excitation source) on the computed current and field waveforms were found to be relatively insignificant, while the influences of ohmic losses in the channel corona sheath and voltage drop along the grounded channel were significant. The effects of background continuing current level and grounding resistance were significant for M‐field, but not for M‐current. Model‐predicted overall power and current profiles below the cloud base are consistent with the observed M‐component luminosity profiles and are drastically different from the observed downward leader/upward return stroke profiles. The characteristic feature of M‐components, the time shift between the current onset and close electric field peak (essentially absent for leader/return stroke sequences), was well reproduced by our model.

A New Engineering Model of Lightning M Component That Reproduces Its Electric Field Waveforms at Both Close and Far Distances

“…• The neutralization in Branch A drains positive charges from the main channel. As a result, a negative current wave moves downward in the main, current-carrying channel and undergoing attenuation and dispersion in its path, as expected for M component waves (Tran & Rakov, 2019). This downward wave gets reflected off the top of the tall object, and the reflection moves upward (guided-wave mechanism).…”

“…The exact mechanism of charge neutralization and redistribution upon the attachment of Branch A to the main channel is not known. In addition, the electrical properties of both the main channel and Branch A are possibly time varying and nonlinear (see Tran & Rakov, ). As a result, i c ( t ) cannot be retrieved from the measured channel‐base current due to the unknown attachment mechanism, dispersion, and attenuation along the main channel.…”

“…If that is also the case for the higher junction points associated with M components, then it is possible that the slow M component current waveshapes observed at the base of the channel stem from the effects of propagation along the main channel to the ground. Tran and Rakov (), who modeled M components using a nonlinear distributed circuit model, reported significant differences in the current (and power) waveform frequency content near the ground level and aloft. The higher‐frequency components present near the junction point were largely “filtered out” within a few kilometers of that point.…”

“…Warner et al (2016) recorded high-speed video images of a bidirectional leader whose negative end connected to a luminous upward positive leader channel and, upon the connection, observed a fast pulse in the associated electric field recorded at 18 km. Tran and Rakov (2019) recently developed an advanced (distributed-circuit) model of M components that takes into account transient processes both in the grounded channel and in the in-cloud channel making connection to it but tested their model only against the field measured at close distances. He, Azadifar, Li, et al (2018).…”

Measurement and Modeling of Both Distant and Close Electric Fields of an M‐Component in Rocket‐Triggered Lightning

“…The proposed modification is aimed at taking into account the effect of channel losses on the M‐component current propagation. Losses have already been considered in the model recently proposed by Tran and Rakov (2019) in which the M‐component channel was represented as a nonlinear and nonuniform RLCG distributed circuit. The model of Tran and Rakov is more physics‐based compared to the model proposed in this paper in which the dispersion effects are ignored.…”