| According to the lightning data of Chengdu observed by the Sichuan lightning detection system from 2005 to 2015,after correcting the lightning data,this paper analyzes the temporal and spatial features of the cloud-to-ground flashes in the metro traffic area covering the six metro lines in Chengdu and the size of 0.5°×0.5°.Including the temporal distribution characteristics of the frequency and amplitude of CG flashes,the spatial distribution characteristics of the density and amplitude of CG flashes.Based on the lightning detection data,typical positive(104kA)and negative(-14.9kA)return-strokes near a station were selected.The two-dimensional finite-difference time-domain method was used to study the horizontal electric field,vertical electric field and magnetic field in the center of the station hall,platform,left and right entrance tunnels,when the two return-strokes were 260 m,200m,100 m from the station in the models with and without real ground buildings.The results show:(1)From 2005 to 2015,the annual average frequency of CG flashes in the metro traffic area was 14224,and the CG flashes occurred mainly from April to September.The strongest CG flashes activity is usually the in July-September,in the year when the CG flashes activity is strong,CG flashes activity in June may be stronger than August and September.The metro and stations were most affected by lightning activities from 0:00 to 9:00.The convective weather in the metro traffic area was most active in the early morning,and the weakest at11:00-13:00.Therefore,from May to September,0:00-9:00 is the key period for metro and stations to defense lightning strike.The peak intensity of the ground flash current in the metro traffic area is mainly distributed at 0-50 k A,and the maximum amplitude of the CG flashes current in May-September exceeds 400 k A.The CG flashes with current exceeding 400 k A is distributed at 0:00-4:00,14:00-16:00,17:00-18:00 and 21:00-22:00.(2)The annual average CG flashes density of the metro traffic area in 2005-2015 is up to 11 times/(km~2·a).The metro lines and stations located east of 104°E are vulnerable to lightning risks and positive flash disasters.The CG flashes activity of the metro line and stations located in the north of 30.6°N,east of 104°E is generally stronger,and the annual average flashes density can reach up to 7times/(km~2·a).In the year with the strongest flashes activity,the flashes density of metro traffic area can reach up to 40 times/(km~2·a),the area with flashes density exceed 10 times/(km~2·a)is mainly concentrated in 103.9°E-104.2°E,and the metro lines and stations located in the north of 30.6°N,east of 104.1°E are prone to positive flash disasters.(3)The CG flashes activity in the intersection area of metro line 1 to line 4 is strong,and the average amplitude of CG flashes is also large.The transfer station located in 104°E-104.1°E is at greater risk of lightning strikes.North Railway Station,Simaqiao Station,and Luomashi Station,Tianfu Square Station,Sichuan Gymnasium Station,and South Railway Station are the transfer stations with the highest risk of lightning strikes.Among the transfer stations,South Railway Station is most vulnerable to positive flash disaster.(4)The maximum horizontal and vertical electric field peaks always appear in the center of the station hall when there are no ground buildings;in the case of real ground buildings,the horizontal electric field peaks in the center of the station hall are still the largest,but the vertical electric field peaks in the center of the left and right entrance tunnels exceed that in the center of the station hall;the maximum magnetic field peaks in the two models all appear in the center of the station hall or left entrance tunnel.For the two return-strokes,compare with the results in the case of no ground buildings,the peaks of each field component in the station all decrease when the return-strokes are 260 m,200m from the the station with real ground buildings,but the vertical electric field peaks in the center of the left entrance tunnel both increase more than 8% when the two return-strokes are 100 m from the station with real ground buildings. |
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