Optical and Current Measurements of Lightning Attachment to the 356-m-High Shenzhen Meteorological Gradient Tower in Southern Coastal Area of China

This paper presents in detail the instrumentation for coordinated optical and current measurements of lightning discharges to a 356-m-high meteorological tower and the observation results. The tower, which is located at Shenzhen, a lower latitude coastal city in south China, was equipped with a non-inductive current shunt at its top and a high-speed camera and electromagnetic field sensors at 440 m away from the tower base. A total of 24 discharges to the tower were well documented in the period of April to June of 2019. Three of them were analyzed in detail with the focus being given to the feature of upward leaders initiated from the tower: Case 1 - a connecting upward positive leader (connecting UPL) induced by a downward negative in a downward negative discharge, Case 2 - an upward positive leader (UPL) in the initial stage of an upward negative discharge, and Case 3 - an upward negative leader (UNL) in the initial stage of an upward positive discharge. All the three leaders had a stepping feature during their initial stages, each step producing an isolated but oscillated current pulse with a time scale of
$1~\mu \text{s}$
. The connecting UPL had 5 steps during its initial 2.6 ms, with a step interval ranging in
$0.1\sim 1.5$
ms and a current pulse peak in 1~5 kA. The UPL had also 5 steps but during its initial
$200~\mu \text{s}$
, with a step interval ranging in
$20\sim 50~\mu \text{s}$
, a step length in 0.8~2.2 m, a current pulse peak in 0.5~2.2 kA, and a leader average speed in
$0.4\sim 1.1\times 10^{5}$
m/s. The UNL had 8 steps during its initial
$104~\mu \text{s}$
, with a step interval ranging in
$13.6\sim 22~\mu \text{s}$
, a step length in 3.9~7.1 m, a current pulse peak in 2.3~12.5 kA, and a leader average speed in
$1.9\sim 6.6\times 10^{5}$
m/s.