通道感应电荷对放电活动特征的影响

为探究闪电放电后电荷重置方案中异极性电荷植入法对雷暴云放电效应的影响,利用已有的三维雷暴云起放电模式,结合2011年8月12日发生在南京地区一次典型的雷暴个例,通过控制倍数改变闪电通道感应电荷量进行大量敏感性试验。模拟结果表明:闪电通道感应电荷量对空间电荷结构分布和云闪通道长度有明显影响。通道感应电荷量增加,即空间异极性电荷堆增多,加大空间电荷结构复杂程度;云闪通道在发展过程中难以穿越与自身极性相同的电荷堆,导致短通道云闪频次增加。通道感应电荷累积总量相同,不同闪电通道感应电荷量下云闪频次与通道电荷平均累积量呈负相关,即通道感应电荷平均累积量增大,云闪频次减少。而地闪频次、类型与通道感应电荷量相关性不明显。     

利于上行负地闪始发的电荷区参数数值模拟

在经典偶极性电荷结构下,结合已有的闪电放电参数化方案及中国气象局雷电野外科学试验基地的广州高建筑物雷电观测站(Tall-Object Lightning Observatory in Guangzhou,TOLOG)观测分析结果,不断调整主负电荷区参数进行二维高分辨率闪电模拟试验,讨论自持型上行负地闪与云中闪电之间的相互竞争关系以及有利于自持型上行负地闪始发的云中电荷结构。数值模拟结果表明:自持型上行负地闪始发与电荷结构存在一定关系,在主负电荷区越高的情况下,始发自持型上行负地闪需要的主负区电荷密度与电荷分布范围越大。对于不同类型的闪电始发条件,推测存在自持型上行负地闪始发的主负电荷区高度阈值,当主负电荷区高度高于该值时,随着主负区电荷量的不断累积,会始发起始于云中的闪电而不是自持型上行负地闪,当主负电荷区高度低于该值时,电荷的不断积累会导致自持型上行负地闪始发。     

甘肃中川地区云闪的多站同步观测及雷暴的等效电荷结构

利用甘肃中川地区GPS同步的7个测站闪电慢天线获得的电场变化资料, 通过非线性最小二乘法对2004年8月20日一次雷暴过程中的10次云闪进行了拟合分析, 估算了其所中和的电矩、 取向及空间位置等参量。结果表明, 其中5次云闪是雷暴云中部主负电荷区与其下部正电荷区之间的放电, 另外5次是中部主负电荷区与其上部正电荷区之间的放电, 对应的放电中心的海拔高度分别在3.2~5.6 km和6.8~7.7 km, 中和电矩分别约为4.56~61.0 C·km和 1.06~15.9 C·km。发生在雷暴云上部正电荷区与中部主负电荷区之间的闪电所中和电矩较发生在雷暴云中部主负电荷区与下部正电荷区之间的闪电所中和电矩小。结果证实了中国内陆高原地区雷暴云的上部和下部有两个正电荷区存在, 与闪电放电相联系的雷暴云电荷结构可用简化的三极性来代表。     

湖北地区一次雷暴云电荷结构和放电特征的数值模拟研究

在三维强风暴动力—电耦合数值模式中引入非感应起电参数化方案、感应起电参数化方案以及放电参数化方案,对湖北宜昌2014年6月19日一次闪电过程中雷暴云电荷结构和放电特征进行了模拟分析。模拟结果表明,当云内粒子增多、增大,大部分霰粒子逐渐降落到中低层,上部正电荷区减小,底部正电荷堆范围开始扩大,中部负电荷区和底部正电荷区成为主要的起电区域,这种底部正电荷区较厚的三极性电荷结构不利于地闪的产生。在粒子带电分析中,霰与冰晶粒子携带的电荷量均大于云滴,说明霰与冰晶之间非感应碰撞是云中主要的起电过程。虽然云滴的电荷量较小,但霰与云滴之间感应碰撞的作用不可忽视。结合电荷结构的分布,发现底部正电荷堆的垂直分布高度与霰粒子、云滴的电荷浓度的分布有关,且霰与云滴电荷浓度的累积区与底部正电荷堆相一致。     

闪电放电通道的三维结构特征

通过对闪电VHF辐射源高时空分辨率的三维观测资料的分析发现,无论云闪还是地闪其时间和空间分布特征可表明雷暴中的基本电荷结构。云内闪电放电不仅发生在上部正电荷区与中部主负电荷区之间,也同样会在中部主负电荷区与下部正电荷区之间发生,除极性相反之外,其它特征是一致的。云闪过程在最初的10一20ms内垂直向上(正常极性)或向下(反极性)发展,之后转为水平方向的传输。在正电荷区辐射点较多,闪电通道清晰;在负电荷区辐射点较少,且从闪电的起始位置以一种倒退的方式水平延伸闪电通道。云闪中的K型击穿不仅发生在闪电的后期,而且还发生在活跃期,并不时发展到正电荷区而触发新的闪电分叉。负地闪首次回击之前的梯级先导过程辐射较强,继后回击前的直窜先导的辐射较弱。回击之间闪电在云内水平发展,通道以细小的分叉为主要特征,其间不时有没有到地的企图先导过程发生。正地闪的先导过程基本没有可探测到的辐射点,在回击之前有一段云内过程,回击之后有更长的云内过程发展,其闪电通道不像负地闪那样精细,在回击之后的最初阶段辐射点较少,而在通道的顶端辐射点反而较多。正负地闪的发生发展特征有很大的不同,表明正、负极性的电荷击穿及传输过程的机制存在明显差异。     

空间电荷分布特征对云闪传播行为的影响

在已有的随机放电参数化方案的基础上,建立了雷暴云电荷分布,并进行二维高分辨率闪电放电的模拟试验,定量探讨了雷暴云的空间电荷分布特征对云闪传播行为的影响,结果表明:(1)雷暴云顶部的屏蔽层电荷以及底部正电荷区会限制触发于雷暴云主正、负电荷区的云闪正、负先导的垂直传播范围,使云闪通道在一定垂直范围内传播,并最终趋向于沿水平方向传播;(2)电荷区的电荷密度或分布范围的增大都会导致云闪通道长度增大,但增大方式不同,电荷密度主要是通过增加高密度中心附近的通道分枝数,并且通道长度随电荷密度的增加呈非线性增大,分布范围则是通过扩展通道的水平传播范围,并且通道长度随分布范围的扩大呈线性增大;(3)电荷区电荷密度的增大会增加云闪通道的分枝数,尤其在高密度中心附近,最终使云闪通道的分形维数呈增大趋势,但基本上不会改变云闪通道的双层分枝结构,电荷区分布范围的扩大则不影响云闪通道的分形维数。     

云下部正电荷区与负地闪预击穿过程

三维雷电观测系统LMA(Lightning Mapping Array)是最近发展起来的基于GPS时钟同步的闪电VHF辐射源到达时间差(TOA)定位技术,能以很高的时间分辨率(50 ns)和空间定位精度(50-100 m)展现闪电放电发展过程的三维时空分布,揭示雷暴中电荷结构及其与放电过程的关系.文中利用三维雷电VHF辐射源观测资料分析了负地闪预击穿过程的时空分布特征,讨论了云下部正电荷区对负地闪发生的影响,其结果表明在首次回击之前存在长时间预击穿过程的负地闪中,预击穿过程是云中部负电荷区与下部正电荷区之间的一种云内放电过程,闪电起始于云中部负电荷区,然后向下发展传输,进入正电荷区后闪电通道在云下部正电荷区水平发展,其放电特征与反极性云闪放电一致,云内放电过程最后阶段的K型击穿激发了地闪的梯级先导,梯级先导穿过云下部正电荷区向下发展传输.云下部正电荷的存在是导致负地闪首次回击之前存在长时间云内预击穿过程的主要原因.     

雷暴云内电荷水平分布形式对闪电放电的影响

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。     

一次雹暴的闪电特征和电荷结构演变研究

综合利用SAFIR3000三维闪电定位系统的全闪定位资料与雷达结合对2005年5月31日发生于北京的一次冰雹过程的闪电活动和电荷结构演变特征进行了综合分析.结果表明:该雷暴的闪电活动有两次活跃期,第一个活跃期产生了降雹,降雹结束后,闪电活动突然减少,之后的活跃期产生了更多的闪电,其中一部分处于云砧区.闪电活动峰值超前降雹5 min左右,闪电活动中的地闪仅占6.16%,但正地闪占总地闪的比例达20%,且降雹前的正地闪比例较降雹后要高·降雹发生后,正地闪很少发生.降雹阶段,参与放电的主要电荷区表现为反极性结构,-40℃左右区域为参与放电的主负电荷区,-15℃左右区域为参与放电的主正电荷区,在正电荷区之下,短暂存在一个较弱的负电荷区.降雹结束后,电荷结构经历了持续的快速调整过程,在第2次闪电活跃期,参与放电的主要区域表现为正常的三极性结构,即上正-中负-下正,受西风气流的影响,此三极性结构出现倾斜.动力和微物理过程的分析表明,闪电活动和电荷结构的特征与雷暴云内的动力、微物理过程紧密相关.文中对反极性电荷结构形成的可能机理进行了讨论,并且认为,具有强烈上升气流的灾害性天气可能更易形成反三极性的电荷结构,并在下部两个电衙区的作用下产生较多的正地闪.     

雷暴电荷分布对正极性云闪放电特征影响的数值模拟

本研究将云闪随机放电参数化方案植入到偶极电荷结构中,固定主负电荷区的参数和位置,通过不断调整上部正电荷区的参数（电荷浓度和水平范围）和位置以此来模拟不同高度处起始的正极性云闪,进而探讨正云闪放电特征与雷暴电荷分布之间的关系。模拟结果表明在偶极电荷结构中,抬升上部正电荷区的高度,能够产生如观测所示的高海拔正云闪,不同于普通正云闪起始于向上传播的负先导及水平或稍向下延伸的正先导,高海拔处起始的正云闪以大范围向下传播的正先导及水平或轻微向上延伸的负先导为主要特征。随着上部正电荷区位置的抬升,正云闪起始高度也随之升高,当上部正电荷区抬升到一定高度后（本研究中当上部正电荷区下边界超过12 km）,云闪通常起始于主正电荷区内,且上部正电荷区的浓度以及水平半径对于云闪的起始高度没有显著影响。此外,云闪正、负先导通道的长度与电荷区的浓度、水平半径以及起始点和负、正电荷区之间的距离存在显著的正相关关系。     

Fine-resolution simulation of cloud-to-ground lightning and thundercloud charge transfer

2-D 12.5 m-resolution simulations of cloud-to-ground (CG) lightning discharge processes have been performed using an improved stochastic lightning model for different types of cloud charge distributions, such as dipole, tripole, bi-dipole and multi-layer charge structures produced from the numerical simulation of thundercloud electrification. The modelling produced the fine branched channel structure of CG lightning and the results illustrate the relations between CG lightning channel propagation and cloud charge distribution. The simulated features of CG lightning are associated with the observed results. The simulation studies are essential in our understanding of complex charge transfer processes caused by CG lightning discharges in thunderclouds. The induced charges of opposite polarity are deposited or embedded in the local volumes where the bidirectional leaders passed during a CG lightning discharge. Although the embedding affects charge structure only in a pair of significant positive and negative charge regions closest to the ground, the electric field strength acutely weakens and electrostatic energy in thunderclouds is significantly consumed when the discharge terminates. In addition to simulating the upward and downward breakdown of the initial leader to ground and the ensuing return stroke (RS), the simulation assumes that current continues to flow in the channel to ground and determines the upward breakdown until the end of the discharge. For the subsequent discharge sub-process, the upward leader channel tends to transfer the charges with the same polarity as the RS, while the downward leader channel favors transfer of opposite charge to ground. In the sub-processes of a few CG flash simulations, the magnitude of the opposite charges from the downward leader exceeds that of charges with the same polarity from the upward leader so that the net charges transferred to the ground have a reversed polarity to the RS. The simulation presents similar features of CG lightning as those observed in realistic bipolar CG lightning.     

雷暴云电结构与闪电关系初探

利用雷暴云偶极性电结构理论,结合卫星和雷达资料分析了不同荷电结构下地面电场。结果表明:当正电荷中心高度越高、负电荷中心高度越低,形成的地面负电场越大,越有利于地闪的形成;负地闪发生后或云砧下方,地面电场成正极性,易引发正地闪;地闪分布与雷达回波、卫星云图中雷暴云位置基本一致,结合雷达和卫星资料可初步判断正/负地闪发生位置。     

Description and first results of an explicit electrical scheme in a 3D cloud resolving model

The three-dimensional non-hydrostatic mesoscale model MésoNH of the French community offers the numerical environment to develop a cloud electrification scheme in a consistent way with the original mixed phase microphysical scheme. The charge separation mechanisms are entirely due to non-inductive processes and result from elastic ice–snow, ice–graupel and snow–graupel collisions. The electric charges carried by each of the five hydrometeor categories are transported along the airflow and are exchanged according to the various microphysical mass transfer rates but assuming a power law distribution of the individual charges as a function of the particle size. The electric field is diagnosed at each time step after integrating the electric potential induced by a net charge density in the Poisson equation. Finally, a lightning ash is triggered when the electric field locally steps over a given threshold. It propagates in two opposite directions until the magnitude of the electric field falls below a prescribed value. A fractal branching algorithm is then activated to extend lightning streamers away from the main channel and toward cloudy regions where substantial charge densities are present. Charges are neutralized along the tortuous lightning path with a simple procedure that preserves total charge conservation.The complete electrification scheme tested for an ideal case of vigorous supercellular storm shows an intense electrical activity all along its lifecycle. We show that the model is able to produce a direct tripolar structure of the charges as the result of a temperature charge reversal of − 10 °C and of the different sedimentation rates of the hydrometeors.     

Charge source of cloud-to-ground lightning and charge structure of a typical thunderstorm in the Chinese Inland Plateau

GPS-synchronized measurements of electric (E) field changes induced by lightning flashes were recorded at six stations in the northeastern verge of the Tibetan Plateau. The height and magnitude of charge neutralized by 65 return strokes, including 16 negative cloud-to-ground (CG) flashes and 2 positive CG flashes, have been fitted with the nonlinear least-square method based on the E field changes of CG flashes observed in a typical thunderstorm with larger-than-usual lower positive charge center (LPCC). Results show that the height of the charge region neutralized by negative CG flashes ranges from 3 km to 5 km above the ground, corresponding to an ambient temperature between − 2 °C and − 15 °C. For the two positive CG flashes, the neutralized charge regions are located at a height of about 5.5 km and the ambient temperature is about − 18 °C, indicating the existence of upper positive charge in the thunderstorm.     

中国内陆高原地区典型雷暴过程的地闪特征及电荷结构反演

利用2002年夏季在青海省大通县进行的雷暴及闪电综合观测实验中所获取的6站GPS同步闪电电场变化资料,对8月4日一次下部具有大范围正电荷区的典型性雷暴过程的雷电特征进行详细研究。利用点电荷模式对16次负地闪和2次正地闪所包含的共65次回击所中和电荷源进行的非线性最小二乘法拟合研究发现,负地闪所中和的负电荷距离地面的相对高度在3~5 km,而两次正地闪所中和的雷暴云电荷在5~6 km的高度,表明该雷暴云呈三极性电荷结构。负地闪单次回击所中和电荷量平均为1.48 C,而两次正地闪都为单次回击且中和的电荷量分别为1.37 C和2.68 C。     

强风暴中反极性电荷结构研究进展

反极性电荷结构是强风暴系统中一种常见的电荷结构配置,它是强风暴中正地闪大量发生的重要原因之一,同时也往往与灾害性天气相联系。人们对云内电荷结构的认识随电场探空、多种地面观测手段的发展而深入。反极性电荷结构并非在强风暴的起始阶段就出现,而存在一个演变过程,出现在风暴发展的特定阶段。宽广强烈的上升气流被认为在反极性电荷结构的形成中起关键作用,它使得上升气流区液态水含量等微物理条件发生改变,进而影响大小粒子碰撞的起电过程,使风暴内主要起电区霰粒子荷正电,冰晶等粒子荷负电,从而形成反极性电荷结构。强风暴中气流的动力输送、风切变等也被认为是反极性电荷结构形成的可能原因。利用数值模式,在真实的气象背景场下再现强风暴的反极性电荷结构演变特征和闪电活动特征,也是研究反极性结构形成的有效途径之一。针对正地闪大量发生的强风暴开展大规模外场观测试验,并将观测结果与数值模拟相结合,将有利于理解强风暴中反极性电荷结构的形成及其与闪电活动特征的关系。     

Interaction between adjacent lightning discharges in clouds

Using a 3D lightning radiation source locating system (LLS), three pairs of associated lightning discharges (two or more adjacent lightning discharges following an arbitrary rule that their space-gap was less than 10 km and their time-gap was less than 800 ms) were observed, and the interaction between associated lightning discharges was analyzed. All these three pairs of associated lightning discharges were found to involve three or more charge regions (the ground was considered as a special charge region). Moreover, at least one charge region involved two lightning discharges per pair of associated lightning discharges. Identified from electric field changes, the subsequent lightning discharges were suppressed by the prior lightning discharges. However, it is possible that the prior lightning discharge provided a remaining discharge channel to facilitate the subsequent lightning discharge. The third case provided evidence of this possibility. Together, the results suggested that, if the charges in the main negative charge region can be consumed using artificial lightning above the main negative charge regions, lightning accidents on the ground could be greatly reduced, on the condition that the height of the main negative charge region and the charge intensity of the lower positive charge region are suitable.