Electromagnetic Model of K‐Changes

K‐changes are observed as step‐like increases in the thundercloud electric fields. The K‐changes occur in the late part of intra‐cloud lightning or during negative cloud‐to‐ground lightning between return strokes. It has been shown that the processes leading to K‐changes initiate in the decayed part of a positive leader channel and propagate toward the flash origin. They are often accompanied by microsecond‐scale electric field pulses. We introduce a new model to simulate processes leading to the K‐changes in cloud‐to‐ground lightning. Our method is based on the full solution of Maxwell's equations coupled to Poisson's equation for the thundercloud charge structure. To model the K‐changes, we gradually increase the decayed channel conductivity. The modeled current wavefront propagates due to the K‐processes downward along a vertical channel and completely attenuates before reaching the ground. We derive the evolution of the linear charge densities and the scalar electric potential along the channel leading to K‐changes. We model electrostatic step‐like changes in the measured electric field together with the approximate rates and amplitudes of the microsecond scale pulses. Step‐like changes increase their amplitudes with the length of the simulated channel and with a higher conductivity of the channel. The microsecond‐scale pulse waveshapes depend mainly on the propagation velocity of the current wave, and the time scale of the conductivity increase. We show that our modeled waveforms are in a good agreement with observations conducted in Florida. Plain Language Summary K‐changes are the step‐like increases in the measured electric fields in the late part of an intra‐cloud lightning and between return strokes in cloud‐to‐ground lightning. The K‐changes are often accompanied by microsecond‐scale electric field pulses. We have developed a new model to simulate the K‐changes in cloud‐to‐ground lightning. We solve electrodynamic equations to model these processes and we are able to reproduce the observed electric field waveforms recorded in Florida by a flat‐plane antenna. We model how the current wave propagates along the vertical channel and attenuates before reaching the ground. We show how the charges are distributed along the simulated channel and we model why in some cases the breakdown occurs at the positive end of the channel after the recoil leader propagates several hundreds of meters along a decayed channel. Key Points We model for the first time th