| Using the advantages of triggered lightning flash(TLF)in time prediction,position determination and direct measurement of current,the thunderstorm environment conditions associated with the TLFs were studied in this study,which provides a unique perspective for understanding the relationship between spatiotemporal-proximity lightning flashes and the relationship between lightning discharge characteristics and thunderstorm structure.In this paper,three-dimensional lightning location data(the observation of the Low-Frequency E-field Detection Array),thunderstorm radar data(observation of S-band dual-polarization radar in Guangzhou),TLF channel base current data and so on that obtained by Chinese Academy of Meteorological Sciences during the TLF experiment in Guangdong were comprehensively analyzed to study the temporal and spatial correlation between TLFs and adjacent natural lightning flashes(NLFs),the thunderstorm structure associated with the TLFs,and the relationship between TLF discharge characteristics and thunderstorm structure.Some main conclusions are as follows:(1)The spatiotemporal relationship between TLFs and their adjacent NLFs was clarified.It was found that 5s and 5km are suitable threshold parameters to characterize the isolation of TLF.For approximately 93% of TLFs conducted in South China,there was no NLF of which the channel entered in to a range 5 km from the flash-triggered position within 5 s before and after TLFs.For all TLFs,there was no NLF initiated within a 5 km distance from the flash-triggered position within 5 s before and after the TLFs.For some NLFs which occurred before or after the TLFs with a small time interval,and were initiated in or or extended their channels into the vicinity of the triggered-flash position,the charge regions associated with their discharges were either not at the same level as the TLF-related charge regions,or at the same level as the TLF-related charge regions but at the position horizontally separate from the TLF-related charge regions.Most NLFs corresponding to the sharp variations of the ground atmospheric average electric field(AAEF)which occurred just before or after the sharp variations of AAEF associated with TLFs were not contributed by the TLF-related charge region.Based on the 3-D lightning location data within 10 minutes before and after TLFs,the average time interval between the LTFs and NLFs which were contributed by the TLF-related charge region was about145 s.(2)The structural characteristics of the thunderstorms corresponding to the TLFs were revealed..The radar reflectivity(ZH)of the triggering-flash area(TFR)tended to be significantly smaller than that of the convective core area(CCR)in the TLF thunderstorms.Below 5km the ZH of TFR was mainly valued around 30 d BZ,and the ZDR mainly ranged from 0 to 1 d B.The ZH in CCR was mainly valued approximately50 d BZ below 5 km;while the ZDR was mainly distributed between 1 and 1.5 d B below 4 km,and rapidly decreased to 0–0.2 d B at 4–6 km.The TFR was dominated by relatively small liquid raindrops below 0℃ level,and was mainly composed of dry snow,ice crystals and graupel particles at the levels between 0 and-20 ℃.The proportion of regions dominated by ice crystals gradually increased with the increasing height,and there were nearly all ice crystals above-40 ℃ layer.Below0 ℃,the CCR was mainly composed of large drops,rain,heavy rain,hail.The predominant hydrometeors between 0 and-20 ℃ particles in CCR was consistent with those in TFR,while the proportion of the region dominated by graupel particles was higher,and the graupel particles could occur at higher levels.In CCR,ice crystals also dominated the levels above –40 ℃.In TFR,the TLF-related charge region were mainly located between 3.5 and 6.5 km,with a average height of 5.2 km,corresponding to the ambient temperature between 12.5 and –12.5,and –1.7 ℃,respectively.The average area of the effective negative charge region(means that it could support the propagation of the TLF channels)could be equivalent to a circle with a diameter of about 10.3 km.there was a positive charge region above this TLF-related negative region.The CCR tended to feature a tripolar charge structure,with the lower small positive charge region being located near and below 0℃(approximately below 5.5 km),the middle negative charge region being located between 0 and-20 ℃(corresponding to the height approximately between 6 and 11km),and the upper positive charge region being located above the-40 ℃ layer(approximately above 12 km).On the whole,the regions linked with the TLFs show the characteristics associated with typical stratiform region.(3)The current characteristics of TLFs were clarified and their relationship with the thunderstorm structure was explained.For the TLFs,the average duration was539.0 ms,charge transfer was 64.4 C,action integral was 50.1×103 A2 s,and peak return current was 18.9 k A.Compared with the TLF of which the charge transfer was less than or equal to 50 C,the TLF of which the charge transfer was greater than 50 C tended to correspond to smaller average value of maximum ZH in the regions around the flash-triggered position and at the heights where the intracloud channels of TLFs mainly occurred.Referring to the ZH in the grid boxes where the TLF channels propagated the TLFs with charge transfer greater than 50 C tended to be associated with smaller values overall,but the larger maximum ZH.The average horizontal channel extension length and convex hull area of TLFs with charge transfer greater than 50 C were approximately 1.71 and 2.70 times those of TLFs with charge transfer less than or equal to 50 C,respectively.The results indicate that the TLFs with relatively large charge transfer tends to be associated with the thunderstorm region with weak dynamic and microphysical processes,meanwhile,this region tended to has a relative large horizontally-extended charge region.On the other hand,the channel extension size of TLFs has relatively good linear fitting relationships with their duration and charge transfer. |
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