Study of spatial evolution and electromagnetic properties of positive lightning leaders striking to high-rise objects with experiment-simulation combined approach

Abstract

Lightning can transfer charges from cloud to ground, which is called a cloud-to-ground flash. A cloud-to-ground flash may either start inside the cloud with a so-called downward leader process followed by a so-called upward going return stroke process or start from the top of a high-rise structure with a so-called upward leader process followed by a downward leader- return stroke process. Nowadays, with the rapid development of urban areas in China, there are more and more high-rise structures built, which are more likely to be struck by lightning flashes. According to statistics, although positive lightning flashes initiated with a downward positive leader only make up less than 10 percent of whole cloud-to-ground lightning flashes, positive lightning flashes always have the highest directly measured lightning currents and transfer the largest charges from cloud to ground. On the other hand, almost all negative lightning flashes, which is the major type of cloud-to-ground flash, involve an upward positive leader in its attachment process. For the purpose of effective warning and protection of high-rise structures from lightning striking, this thesis focuses on the physical process of positive lightning leaders attaching to high-objects. The main research outputs are listed as follows: (1) Two initial upward positive leaders (UPL) in upward negative flashes initiated from the 356-m Shenzhen 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 for the first time. The channel radius showed firstly 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 speed was 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. (2) A new approach was presented for retrieving the temporal and spatial evolution of charge density transferred from cloud into leader channel with complicated earth boundary. Combined this approach and simultaneous electric and optical observation data, the evolution of the leader charge density and current of two natural downward positive leaders (DPL1 and DPL2) struck at two different high-rise buildings in Guangzhou were estimated. It is found that the observed electric field with complicated terrain is about 3.4 ~ 3.6 times larger than that with flat ground, which means that the rugged terrain has a great influence on the electrical field observation during lightning discharges. The transferred charge densities estimated for DPL1 and DPL2 along the leader channels varied in a range of 0.4 ~ 8.6 mC/m and 0.4 ~ 15.2 mC/m, respectively. The leader currents flowing along the two DPL channels ranged from 0.7 to 5.4 kA and 0.7 to 4.6 kA, respectively. Furthermore, by using transferred charge density, the profiles of the ambient electric field and space charge density along the leader channels were also calculated. The ambient electric fields of two cases were in a range of -250 to +180 kV/m, and the range of the volume density of space charges in two cases was from -7.8 to +13.9 nC/m3. It was found that the polarity of space charge region had great effects on the horizontal propagation direction of the leader channel. (3) A downward negative flash with nine return strokes struck to the 356-m Shenzhen Tower was analyzed. The high-speed camera captured upward connecting leader channels in the first stroke and two subsequent strokes. Based on the observation results that none of downward dart leaders completely propagated to the tower tip before the occurrence of RS, we infer that the upward connecting leader was initiated from the tower tip and the attachment process occurred in each subsequent RS. The height ranges of all nine attachment processes were different. It is also found that if the downward dart leader propagated faster, the height range of the attachment process was higher. And there is also a linear relationship between the downward dart leader speed and the peak luminous intensity of the return stroke channel.