Transition in Optical and Radio Features During the Early Development of Negative Intracloud Leader

What happens when the upward in-cloud (IC) breakdown transitions into horizontally expanding extension? Optical observations of this transition in IC lightning have not previously been reported. We identify the radio and optical signatures of this transition in low-frequency (LF) magnetic, very high frequency interferometric and space-borne optical measurements. For initial IC development in stratiform clouds, the ratio of 337/777.4 nm radiance is above unity prior to the transition but is almost always below unity after the transition. In particular, the 337 nm radiance drops significantly while the 777.4 nm radiance remains almost invariant after the transition, suggesting when the dominant illumination process of IC leaders changes from cold streamer discharges to a likely stepped leader. Furthermore, this transition in the LF and optical measurements resembles the reported changes in the cloud-to-ground measurements from initial leader to stepped leader, indicating that the initial leader may be physically different from the stepped leader.