Difference Between 2D and 3D Development Characteristics of an Upward Lightning Leader

Tall object not only has a high probability of being struck by downward lightning, but also is easy to trigger upward lightning. Comparing with 2D optical observation, reconstructed 3D observation by dual-station optical observation can better reflect the real characteristics of the lightning channel. Based on the synchronous dual-station optical observations from the Tall-Object Lightning Observatory in Guangzhou (TOLOG), an upward lightning triggered by a nearby positive cloud-to-ground (CG) flash at Guangzhou Tower is investigated, and 2D and 3D development characteristics of the upward leader are compared.The result shows that the length of 3D channel is 5.4 km, which is 1.5 times of 2D channel. 3D channel is developed at a height of more than 4.6 km in the vertical direction and about 800 m in the horizontal direction. 2D speed ranges from 1.8×104 to 4.5×105 m·s-1, with an average of 1.8×105 m·s-1. 3D speed ranges from 3.8×104 to 7.2×105 m·s-1, with an average value of 2.8×105 m·s-1. Within 10 ms after the leader initiation, 3D and 2D speeds show roughly the same trend. After 10 ms, 2D rate decreases significantly and changes little over time, while 3D speed shows an obvious irregular fluctuation with time. The ratio of 3D speed and 2D speed ranges from 1 to 4.7, with an average of 1.5. The ratio remains stable at 1-2 for the first 10 ms after leader initiation and then shows irregular fluctuations with time. In fact, due to the change of horizontal development direction of the channel 10 ms after the leader initiation, the distance between the channel and the station-1 increases rapidly, leading to increasing speed difference between 3D and 2D observations. The cause for the fluctuation of 3D speed and 2D speed ratio is that the angle between the development direction of the channel and the sight direction of the observation station changes greatly. The results of 2D and 3D vary greatly at a certain stage of upward leader development, which further proves the importance of analyzing 3D development characteristics of lightning.