Optical and Electric Field Observations of Two Concurrent Upward Flashes Triggered by a Positive Cloud-to-ground Flash

Tall objects are easy to trigger upward flashes. They also provide more opportunities for observation of upward flashes and a suitable platform for analyzing the intiation and development of upward flashes. Upward flashes are usually divided into two types, one is self-initiated and the other is triggered by environment, and the latter type is often related to the activity of positive cloud-to-ground (CG) flash. At present, development characteristics and triggering mechanism of the upward lightning channel are mainly studied based on the image of the extracloud lighting channel, the ground electric field change data and radar echo data. Few examples of mutiple upward flashes triggered by the same positive CG flash on different tall objects have been observed, and detailed characteristics of upward flashes are not well understood.On 16 June 2017, a positive CG flash (single stroke, peak current +141 kA) is recorded by the lightning photoelectric synchronous observation system of Tall Object Lightning Observatory in Guangzhou (TOLOG), triggering upward flashes of two nearby super-high-rise objects. Results show that within about 0.8 ms after the return stroke of the positive CG flash, two upward negative flashes intiate from the Canton Tower (600 m high) and the East Tower (530 m high), which are about 3.9 km and 4.1 km away from the positive CG flash, respectively. The initiation of two upward flashes could be caused by combined effects of the return stroke of positive CG flash, its associated continuing current, and the negative leader in the cloud approching to towers. A total of 7 strokes occur in about 353 ms, 6 strokes on the Canton Tower and 1 stroke on the East Tower, and the leader/return stroke sequence on the Canton Tower occur after the leader/return stroke sequence and the attempt leader on the East Tower, i.e., there is no overlapping between strokes of two upward flashes. The average peak current (-21.4 kA) of return strokes of upward flash initiating from the Canton Tower is about 3 times of the peak current (-7.3 kA) from the East Tower. It's supposed that the negative charge region in the upper cloud of the Canton Tower is wider than that in the upper cloud of the East Tower, and the charge amount is larger. The 2D velocity range of the positive leader of two upward flashes is 9.4×10⁴ to 1.8×10⁶ m·s^-1, with an average of 6.9×10⁵ m·s^-1.