Three-dimensional optical observations of an upward lightning triggered by positive cloud-to-ground lightning

Based on dual-station optical records, electric field and radar data, a detailed analysis of an instance of upward lightning initiated from Canton Tower, in the city of Guangzhou, Guangdong Province, China, is presented. The upward lightning was triggered by a positive cloud-to-ground (PCG) flash nearby. The upward lightning channel and part of the horizontal PCG lightning channel obtained by dual-station optical systems were reconstructed in three dimensions, providing us with spatial channel information on how the PCG lightning process triggered the upward lightning from Canton Tower. The upward lightning occurred in the trailing stratiform cloud area behind a squall line system. The radar echo intensity in this area was relatively weak (30-40 dBZ), and the height of the strongest radar echo center was about 4 km. From the optical records, the two-dimensional length of the horizontal PCG lightning channel was longer than 17.2 km. The length of the three-dimensional reconstructed part of the PCG lightning channel was about 3660 m. Its height increased from 3410 m to 4170 m along the development direction, with an average height of 3640 m. Based on the above result, as well as previous studies on the charge structure of the stratiform region, we infer that the height of the positive charge layer was roughly consistent with the height of the strongest echo center.