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云内大粒子对闪电活动影响的个例模拟
Case Study of Big Particles Effect on Lightning Initiation in Clouds Using Model
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摘要: 利用三维云起电放电模式, 通过对2008年9月6日北京地区一次雷暴过程的模拟, 研究了云内雨滴、冰晶、霰和雹4种粒子对云内闪电活动的直接影响。对比几种主要冰相粒子比质量浓度的分布变化与模拟闪电开始发生高度的关系, 并分析几种冰相粒子所带电量变化率的时空变化特征后发现:霰和冰晶是对云内闪电发生作用最为重要的两种粒子; 雹粒子对闪电发生作用有限; 雨滴则对于主电荷区附近闪电发生没有直接影响。模拟结果表明:在实际观测中出现的雷达强回波达到某个高度之上可以预警闪电发生的现象, 主要是由于霰粒子在一定高度之上与冰晶之间发生强烈的电荷分离, 从而使云内电场迅速增强, 并最终引发闪电而导致的。Abstract: A thunderstorm process in Beijing on 20 September 2008 is simulated using a 3-D charging-dischargingcloud model. The effects of big particles consisting of graupels, ices, hails and raindrops on lightning initiation are investigated.Temporal and spatial analysis on the model results, including the mass concentration and the charging velocity, shows that graupels and ices are the most important particles that affect theinitiation of most lightning.Because lightning always initiate in the region with mass distribution of graupels and ices.The charging velocities of graupels and ices also reach large values there.From the analysisof time series, those large charging velocities of graupels and ices appear when lightning initiate intensively.Hails may also be an important factor effecting the lightning initiation except for the early stage oflightning activity.The region of lightning initiation correlates partially with the mass contribution of hails.The period when the charging velocities of hails reach their large values chimes with the lightning activity.But hails meet their large values below the height of lightning initiation.At the early stage of lightning activity, the mass concentration of hails is very small.Raindrops locates beneath the region of lightning initiation from beginning to end.Their charging velocities become prominent after the end of lightning activity.So it is impossible for raindrops to affect the lightning initiation directly.In many cases, it can be the signal of lightning warning that some kind of strong echo reaches athreshold height.Through the effect of particles on lightning initiation above, it can be concluded that thestrong echo should be caused by graupels or hails.When graupels (hails) are brought to the upper level above the threshold height by updraft, graupels (hails) mixed with ices adequately and the strong chargingprocess occurs among them.The first lightning will initiate soon after that.
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图 1 2008年9月6日17:00—9月7日02:00北京雷达范围内40 dBz回波顶高的变化 (a) 和SAFIR3000同时段内观测到的逐小时闪电活动频次变化 (b)
Fig. 1 The 40 dBz echo top evolvement (a) and the lightning frequency observing by SAFIR3000 (b) in Beijing from 17:00 6 Sep 2008 to 02:00 7 Sep 2008
图 2 第12分钟时空间电荷浓度分布剖面图 (单位: C/m3)
Fig. 2 Profile of space charge density distribution at the 12nd minute (unit :C/m3)
图 3 雨滴、冰晶、霰和雹在各高度层上最大比质量浓度 (单位: g/kg) 随时间变化及闪电开始发生的高度
Fig. 3 The maximum specific mass density (unit : g/kg) evolvement of rain drops, ice crystals, graupels and hails at every model levels and the height change of lightning initializations
图 4 每个格点上各类粒子所带电荷浓度随时间变化在高度上的分布
Fig. 4 The height distribution of charge density change rate of rain drops, ice crystals, graupels and hails at every grid
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