Spectral study on CG lightning with a branch developing from node upwards to the cloud

The development of cloud-to-ground (CG) lightning channel is one of the main basis 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, following the grounding of the first return stroke, a branch developed oppositely from a node on the main channel upwards into the cloud. To investigate the propagation properties of this lightning, characteristic parameters such as two-dimensional speed, node temperature and electron density during dart stepped leader and dart leader were analyzed. The physical mechanism of the branch developed oppositely from a node upwards to cloud phenomenon was discussed. Spectral analysis revealed that the main channel temperature and electron density were about 27200~30700K and 0.23~2.03+18Ecm-3. The temperature and electron density increased as the branch approaching the cloud base, similar to the feature of the main channel approaching the ground. The leader circled (gyrated) around the node, the temperature and electron density of the node were higher than other positions of the channel, indicating that massive charges accumulated around the node. The lightning occurred in the rolling Qilian Mountains, the landform is complex, which can easily lead to an uneven distribution of localized electric field in the air, resulting in channel bending, kinking, and even developing in circle. The phenomenon of the branch developed oppositely from the node to the cloud may be related to massive charges accumulation around the node and a strong charge region with opposite polarity in the nearby cloud.