Spectral study of rare upward developing, circling, and branching cloud-to-ground lightning

The development of cloud-to-ground (CG) lightning channel is one of the main bases reflecting the discharge characteristics. Time-resolved spectra of CG lightning with eight return strokes were recorded by a slit-less high-speed spectroscope. We found an unusual phenomenon in which a branch developing upward from a node into the cloud exhibited a circling structure after the first return stroke. To investigate the propagation properties of this lightning, characteristic parameters, such as the two-dimensional speed of the leader, node temperature, and electron density, are analyzed. The physical mechanism of the branch is discussed. Spectral analysis reveals that the main channel temperature and electron density were approximately 2.72–3.07 × 104 K and 0.23–2.03 × 1018 cm-3, respectively. The temperature and electron density increase as the branch approaches the cloud base, similar to the feature of the main channel approaching the ground. The temperature and electron density of the node are higher than at other locations in the circular channel, indicating that massive charge accumulated around the node. Lightning occurred in the Qilian Mountains in a complex landscape, which easily yielded an uneven distribution in the local electric field of the air, resulting in channel bending, kinking, and even circle development. The phenomenon of the branch developing opposite from the node to the cloud may be related to massive charge accumulation around the node and a strong charge region with opposite polarity in the adjacent cloud.