Investigation Of Influence Of Tall Structures On Lightning Activities And Characteristics

With the development of metropolises, more and more high buildings are growing. The influence of structures on lightning activities and characteristics and also the lightning protection for tall structures has caused extensive concern. To study the influence of tall structures on lightning activities and characteristics, A Tall-Object Lightning Observatory in Guangzhou (TOLOG) was set up to get the simultaneous optical and electromagnetic observation data with high resolusion for lightning occurring on high structures. The observation indicated that the existence of a tall structure may lead to increase of the stroke frequency near the tall structure, upward leader initiated from the top of tall structure will extend with a length more than 1 km, the amplitude of LEMP and the rise time of the lightning current in the return stroke channel would be enlarged too. Based on the observations, a return stroke model containing the effect of long upward connecting leader for lightning striking high structures and a leader connection process model were established to simulate and analyze the tall structure-effects on the characteristics of lightning occurring on tall structures. The main conclusions are as follows:(1) A Tall-Object Lightning Observatory was set up, the Lightning Attachment Process Observation System (LAPOS), high speed cameras, electromagnetic observation system and the thunder acoustic imaging system were integrated, all of them can be triggered simultaneously and operated at a fully-automatic mode. A ground of simultaneouse acoustic, optical and electromagnetic observations with high resolusion for natural lightning flashes were obtained in Guangzhou. Based on these observations of lightning flashes to tall structures and also the observation of triggered lightning flashes obtained in Conghua, Guangzhou, performance evaluation for Lightning Location System (LLS) of Guangdong Power Grid was conducted. The results show that the flash detection efficiency was 94%, the arithmetic mean location error was estimated to be about 710 m and the absolute percentage errors of peak current estimation was 16.3%.(2) Negative lightning location records occurring within 1 km radius centered by Canton Tower with a height of 600 m, farmland and hill were statistically analyzed. The results indicated that after the Canton Tower was completed (2010-2012), the annual mean number of negative strokes and the mean peak current within the zone around Canton Tower was 2.5 and 2.2 times respectively of those during the period when Canton Tower was not established(1999-2005), while 1.9 (1.5) and 1.0 (1.1) times for the farmland zone (hill zone), which indicated that the existence of tall structure may lead to increase of the stroke frequency near the tall structure and the amplitude of LEMP.(3) Simultaneous optical and electromagnetic observations for downward flashes striking on structures with different heights were comparely analyzed. It was found that for the downward negative lightning flashes occurring on the structures lower than 200 m, stroke number containing in one flash ranged from 1～14(with AM value of 3.7), the frequency of single-stroke flash was 51%, and the mean inter-stroke intervals ranged from 4～392 ms(with AM value of 95 ms), while 1～8(with AM value of 2.6),55%,11～446 ms(with AM value of 94 ms) for those occurring on the structures higher than 200 m. In general, the height of the structures did not show apparent influence on the strokes number and the inter-stroke intervals of the downward negative lightning flashes.(4) The AM value of the peak currents of the first strokes and the subsequent strokes occurring on structures taller than 200 m were 1.8 and 1.5 times of those occurring on structures lower than 200 m respectively. Furthermore The AM value of the 10-90% rise times of the first stroke optical pulses, the widths from the 10% wave front to the 50% wave tail of the first stroke optical pulses, the 10-90% rise times of the subsequent stroke optical pulses, the widths from the 10% wave front to the 50% wave tail of the subsequent stroke optical pulses for the flashes occurring on structures taller than 200 m were 7.4,3.1,4.3 and 2.4 times of those occurring on structures lower than 200 m respectively too. The results show that taller structure has greater effect on the the amplitude of LEMP and also the wave parameters of the lightning current, further more the effect on the first strokes is greater than that on the subsequent strokes.(5) A return stroke model containing the effect of long upward connecting leader for lightning striking high structures was established and validated by the magnetic field measurement for a first stroke occurring on tall structure with long upward connecting leader. The lightning current within the return stroke channel above the top of tall structure and the magnetic field at a distance of 100 km for structures with different heights were simulated and compared with the actual observation. It is found that the computations derived from the model containing the effect of long upward connecting leader were more consistent with the observation than that derived from traditional return stroke model. It can be inferred that the longer upward connecting leader on taller structure induced by down ward negative leader may prolong the process that the current wave was propagated to the ground and then reflected back to the lightning channel, resulting a bigger rise time for the current in the lightning channel. Further more, the current sources distributed along the upward connecting leader will be activated and inject more current inside the lightning current during the return stroke process, resulting a bigger amplitude of LEMP. That is why the difference between the characteristics of return strokes occurring on high structures and lower structures is more apparent for the first strokes than that for the subsequent ones.(6) Connecting processes for 10 downward negative flashes occurring on structures taller than 350 m were comprehensively analyzed. It was found that the inception times prior to return first strokes ranged from 0.6～1.8 ms (with AM value of 1.0 ms), the two dimension (2D) length ranged from 187～750 m (with AM value of 455 m) and the 2D propagation velocities ranged from 0.6～21.4 ×105 m/s (with AM value of 4.8×105 m/s) for the upward connecting leaders (UCLs). The mean radio between the 2D velocity of the primary branch of downward leaders and the UCLs was about 0.8 after the UCLs initiated from tall structure. A simplified model for the connection process of natural lightning occurring on tall structure was build and the initiation and development for UCLs on structures with different heights were simulated and compared to the observation. The results showed that the taller structure may lead to earlier initiation and longer length of UCL, while the height of structure did not show obvious influence on the mean propagation velocity of UCL.