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Effects of Horizontal Charge Distribution in Thunderstorm Clouds on Lightning Discharge
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摘要: 为了定量探究雷暴云内电荷水平分布形式对闪电类型和先导传播行为的影响，建立了典型雷暴云电荷结构模型，引入控制电荷水平分布的参数，利用改进的随机放电参数化方案，开展二维高分辨率模拟试验。结果表明：主正电荷区电荷水平分布不均匀且向中心聚集时，产生的闪电类型多为正地闪和正极性云闪，随着电荷水平分布趋于均匀，闪电类型转变为负地闪；主负电荷区电荷水平分布趋于均匀时，闪电类型由负地闪向正极性云闪再向正地闪转变；闪电先导传播特征有较大差异，电荷分布密集不均匀时，先导被束缚在电荷高密度中心，主要在电荷区内发展，当电荷分布单一均匀时，先导能穿出电荷区并水平延伸十几至二十多千米。分析两个电荷区之间的电位分布发现，电荷区电荷水平分布趋于均匀时，位势线向电荷密度中心集中，整个位势阱水平延展，闪电触发点的初始电位值有较大差异，有利于闪电类型和先导传播行为的改变。Abstract: The charge structure of thunderstorm and its internal charge distribution is an important subject in the field of atmospheric electricity research, because it has a direct impact on characteristics of lightning discharge. Influences of different charge structures in thunderstorm on lightning discharge are studied in many aspects, such as lightning types, polarity, and scales, however, but there are few quantitative investigations on effects of the horizontal distribution. Therefore, based on the existing stochastic lightning parameterization scheme, a thundercloud model is set up based upon the work of Stolzenburg et al.(1998) revealing charge structure with four charge regions within convective updrafts in thunderstorms and a negative screening layer usually exists at the top of storms. Parameters that control the horizontal distribution of charge is introduced, and then 2-dimensional fine-resolution lighting discharge simulations are performed. Results show that horizontal distribution forms of charge in upper positive region play a key role in lightning discharge, and with forms of charge changes from dense unevenness into single uniform, lightning type changes from positive cloud-to-ground flashes to positive intra-cloud flashes, then into negative cloud-to-ground flashes, and finally into positive intra-cloud flashes in limited cases. When the distribution of charge levels in the main negative charge region tends to be uniform, the type of lightning changes from negative cloud-to-ground flashes to positive intra-cloud flashes, then to positive cloud-to-ground flashes and finally to positive intra-cloud flashes in limited cases. The horizontal distribution of space charge has a significant effect on the propagation of lightning leader. If it is dense uneven, the leader propagates in the center of the charge density, otherwise, the leader can extend more than 10 to 20 km in the horizontal direction. As the horizontal distribution of charge in the charge region tends to be uniform, potential lines between two charge regions are concentrated towards the charge density center, and the potential well extends horizontally, causing the different initial potential values of the lightning trigger points, which result in the generation of different types of lightning and far-spreading lighting leader in the horizontal direction.
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图 2 主正电荷区不同分布参数λ取值下的空间电荷分布(等值线，单位：nC·m-3)与闪电通道结构(黑色实心菱形表示闪电启动点，红色实线和蓝色实线分别表示正、负先导通道，紫色实线表示闪电接地后的后续放电通道)
(a)λ为0.550，(b)λ为0.650，(c)λ为0.825，(d)λ为1.200，(e)λ为1.800，(f)λ为1.900
Fig. 2 Space charge distribution from different λ of upper positive region(the contour, unit: nC·m-3) and lightning channel structure (black diamond is for initiation point, red and blue lines are for positive and negative leaders, purple line is for follow-up discharge path after lightning grounded)
(a)λ is 0.550, (b)λ is 0.650, (c)λ is 0.825, (d)λ is 1.200, (e)λ is 1.800, (f)λ is 1.900
图 3 主负电荷区不同分布参数λ取值下的空间电荷分布(等值线，单位：nC·m-3)与闪电通道结构(黑色实心菱形代表闪电的启动点，红色实线和蓝色实线分别代表正、负先导通道，紫色实线表示闪电接地后的后续放电通道)
(a)λ为0.625，(b)λ为0.700，(c)λ为0.800，(d)λ为0.950，(e)λ为1.500，(f)λ为1.850
Fig. 3 Space charge distribution from different λ of main negative region(the contour, unit: nC·m-3) and lightning channel structure(black diamond is for initiation point, red and blue lines are for positive and negative leaders, purple line is for follow-up discharge path after lightning grounded)
(a)λ is 0.625, (b)λ is 0.700, (c)λ is 0.800, (d)λ is 0.950, (e)λ is 1.500, (f)λ is 1.850
图 4 不同水平分布形式下的电位分布(等值线，单位：MV)和闪电通道结构(黑色实心菱形代表闪电的启动点，红色实线和蓝色实线分别代表正、负先导通道，紫色实线示闪电接地后的后续放电通道)
(a)P区λ为0.550，(b)N区λ为0.625，(c)P区λ为0.825，(d)N区λ为0.950，(e)P区λ为1.800，(f)N区λ为1.500，(g)P区λ为1.900，(h)N区λ为1.850
Fig. 4 Potential(the contour, unit:MV) and lightning channel distribution from different λ(black diamond is for initiation point, red and blue lines are for positive and negative leaders, purple line is for follow-up discharge path after lightning grounded)
(a)λ of upper positive region is 0.550, (b)λ of main negative region is 0.625, (c)λ of upper positive region is 0.825, (d)λ of main negative region is 0.950, (e)λ of upper positive region is 1.800, (f)λ of main negative region is 1.500, (g)λ of upper positive region is 1.900, (h)λ of main negative region is 1.850
图 5 主正和主负电荷区不同分布参数取值与闪电触发点初始电场和电位的关系
(a)主正电荷区λ与初始电场和电位的关系，(b)主负电荷区λ与初始电场和电位的关系
Fig. 5 The relationship between different λ and the initial electric field and potential at initiation point of lightning in the upper prositive regions and main negative regions
(a)the relationship between λ of upper positive region and the initial electric field with potential, (b)the relationship between λ of the main negative region and the initial electric field with potential
表 1 雷暴云电荷结构各电荷区的相关参数设置
Table 1 Geometrical and electrical parameters of thundercloud charge regions
电荷区 ρ0/(nC·m-3) x0/km z0/km rx/km rz/km S区 -1.00 38 12.25 4 1.0 P区 1.00~8.24 38 9.75 6 1.5 N区 -1.00~8.24 38 6.75 6 1.5 LP区 1.00 38 4.25 2 1.0 
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