Advances of Observation and Study on Tall-object Lightning in Guangzhou over the Last Decade

Abstract

Comparing to the ground or low-object, tall-object is more likely to reach the threshold for the initiation of leader on its top due to the distortion and enhancement of electric field, therefore tall-object is not only easier to get struck by downward lightning, but also can initiate upward lightning. A field experiment, mainly focusing on the observation of lightning flashes terminating on tall structures, has been conducted since 2009 at the Tall-Object Lightning Observatory in Guangzhou (TOLOG), which is the important part of the Field Experiment Base on Lightning Sciences, China Meteorological Administration (CMA_FEBLS). Hundreds of tall-object lightning flashes have been captured during 2009-2018.For the lightning attachment process, tall-object will play the role of "magnifier":The TOLOG high-speed optical observation discovers the connection of the downward negative leader to the lateral surface of the upward connecting leader for the first time, and shows two basic types of the leader connection behaviors during the attachment process in negative cloud-to-ground lightning; the fine structure of negative stepped leader in natural lightning at close distance is revealed by using high-speed video records; the 2D/3D propagation characteristics of downward and upward leaders are analyzed; and the striking distances of lightning flashes to tall-object with different heights are also estimated.Tall-object plays an "amplifier" role on lightning electromagnetic field:Statistical analysis of the TOLOG data show that the magnetic field peak values induced by the first return stroke of lightning flashes to objects higher than 200 m is 2.4 times of that of lightning flashes to objects lower than 200 m; the higher the tall-object is, the larger the lightning location system inferred peak current of strokes are recorded in the vicinity of the tall-object; and the numberical simulation of the tall-object on electromagnetic field of lightning return stroke also show that the height of tall-object has significant enhancing effects.Tall-object is the "hot spot" of downward and upward lightning:Attraction effects of tall-object on downward lightning can protect other objects near tall-object from lightning strikes; the upward negative lightning from the tall-object can be triggered by the return stroke, the continuing current or the discharging process in cloud of positive cloud-to-ground lightning; and in upward lightning, abrupt extension is found at the positive end of the recoil leader which propagates bidirectionally.Using data of the TOLOG, the detection efficiency, the location error and the systematic bias of lightning location systems in Guangdong are evaluated, showing that the observation area of the TOLOG can be used as a "calibration field" for ground-based or space-based lightning monitoring system.

Keywords

• tall-object
• downward lightning
• upward lightning
• leader
• attachment process