The Spatial Evolution of Upward Positive Stepped Leaders Initiated From a 356-m-Tall Tower in Southern China

Two initial upward positive leaders (UPLs) in upward negative flashes initiated from a 356-m-tall tower were observed with a high-speed camera, an electric field sensor, and a magnetic field sensor. Although the waveforms of electric fields measured were saturated, both camera images and magnetic fields showed that the two UPLs had obvious stepwise characteristics in their upward moving stage. The magnetic fields associated with the UPLs were characterized by a series of fast-changing bipolar impulses superimposed on a slowly increasing continuous component as the leaders moved upward, which were well corresponding to the light intensity changes of their high-speed camera images. The 2-D step length and step extension speed of the two leaders were estimated in the range of 0.5-3 m and 1.1-9.4 × 10⁵ m/s, respectively. The radius of the leader luminous channel as a function of time and height was also estimated. The channel radius showed first a stable increasing trend in the leader upward moving stage and then 4-5 times of shrinking and expanding processes after the leader connected the cloud. The channel expanding and shrinking speeds were in the range of 2-9 × 10⁴ m/s. Particularly, the channel radius showed an obvious increasing trend with the increase of the height all the time, which was in the range of 2.8-3.5 m when around the tower tip and of 5.6-8.4 m when at 24 m high above the tower tip. Such a feature of the channel radius is in good agreement with the concept of leader corona sheath in literature.