An Altitude triggered Lightning with Multiple Branches and Ground Contacts

Field experiments on artificially rocket triggered lightning are conducted in summer of 2007 in Conghua, Guangdong Province by Lightning Physics and Protection Engineering Laboratory of Chinese Academy of Meteorological Sciences. The lumin osity characteristics of lighting are captured simultaneously by a Redlake Motion Pro HS 4 high speed camera at 5000 frames per second (fps) 630 m away. Field mill, magnetic loop antenna, broadband interferometers, slow antenna and fast an tenna are installed at the experiment site. The time constants of slow and fast antenna are 6 s and 2 ms respectively, and the bandwidth of broadband interferom eters to locate the lightning is from 25 MHz to 290 MHz. One lightning is succes sfully triggered at 13:58:33 30 June 2007. The top of the wire is 335 m height and the bottom of it is 170 m height at that time. The records of the electric field on the land surface show that it is a negative altitude triggered lightning. No return stroke is observed in luminosity, and the initial continue current p rocess of this flash is 178.6 ms. Bi directional leader process occurs be fore mini return stroke and upward stepped leader branches of this altitude triggered lightning occur after that. The upward positive leader appears 4.93 ms earlier before the downward negative leader. The upward stepped leader propagates at an average speed of the order of 10⁴ m/s in its initial stage, and then becomes 1 0⁵ m/s in magnitude. After the downward negative leader appears, the upward po sitive leader channel becomes brighter than before. The downward negative leader has 3 bifurcation points and produces 4 different ground contacts, and two bran ches connects to ground persisted during the initial continue current process. T he average speed of the downward stepped leader is 1.69×1 0⁵ m/s. After mini r eturn stroke, the upward positive leader above the wire channel propagates along several different branch channels into the cloud, and the average speed of it is 1.05×1 0⁵ m/s. In this experiment, the inducing charge provides the charge of the leader to spread, and the environmental electric field may play a decisive role to the multiple branches and multiple ground contacts of this altitude tr iggered lightning.