青藏高原雷暴云降水与地面电场的观测和数值模拟

在2003年青藏高原那曲地区夏季雷暴云的观测试验中,三次伴有降水的过程中都出现了地面电场(Egnd)随降水的漂移现象,而且固态和液态降水对Egnd的极性改变并不相同。为了模拟再现高原雷暴云降水和Egnd之间的关系,选取了8月13日这次具有高原雷暴代表性特征的过程,模拟了雷暴云移经观测点上空期间,测站固、液态降水与Egnd的变化以及雷暴云下部正电荷区的空间分布。模拟的Egnd及固、液态降水与其的对应关系与观测事实较一致。分析结果表明,固态降水主要携带正电荷,液态降水主要携带负电荷,各类带不同极性和数量电荷的降水粒子常共存在雷暴云中,Egnd主要受携带电荷量占主导地位的降水粒子的影响,地面出现强正电场时正好是云在当顶并且地面出现强固态降水的时间。     

Corona processes and lightning attachment: The effect of wind during thunderstorms

A simple model of a glow corona occurring near the tip of a grounded electrode in a thundercloud electric field that can be enhanced by an approaching downward leader has been studied analytically and numerically with regard to the effect of wind. We obtained an approximate expression for corona current taking into account the (i) removal of space charge from the coronating point due to ion drift and wind and (ii) image of the charge in the ground. As the wind velocity decreases to zero, the expression tends to that obtained previously in the absence of wind. It was shown analytically and numerically that, in a thundercloud electric field, even moderate wind velocities lead to hundreds of percent increase in the corona current. This current decreases with time only slightly in a steady thundercloud electric field, as opposed to the current behavior in the absence of wind. However, even strong wind is not sufficient to affect the properties of a corona intensified in the electric field of an approaching downward leader. The occurrence of wind does not affect the conditions for initiation of an upward connecting leader from grounded objects and consequently the efficiency of lightning rods of ordinary height.     

长春地区对流云起电过程的数值模拟

在三维强风暴动力和电耦合数值模式的基础上,考虑了粒子直径及降落末速差对转移电荷的影响,把非感应起电参数化方案作了进一步改进,对长春地区两次不同强度、不同环境风切变的对流云内电荷累积和电场发展过程进行了模拟分析。结果表明,对流云上升气流达到极大值时,较强和较弱两块云分别具有三极性和反偶极性电荷分布结构。垂直上升气流是表明电场发展强弱的重要参量。对发展强烈的对流云,较强的上升气流使霰和冰晶在云体的中上部维持较长的时间．存在感应和非感应起电的跃增。     

雷暴云下空间电荷层形成的数值研究

利用一个引入了地面尖端电晕放电物理过程的二维轴对称积云起电模式，讨论了雷暴云下空间电荷层的空间分布和随时间的演化规律。在模式计算上采用了"时步分离法"，即在小时步上计算电导及各种反极性离子的复合作用，在大时步上计算平流、对流和湍流扩散项，对于不同的时间步长垂直方向采用不同的格距。通过模拟分析，得到了一些较为合理的结果。最后简单地讨论了本工作在以后需改进的地方。     

Modification of atmospheric DC fields by space charge from high-voltage power lines

Corona ion discharge is responsible for a flux of small ions emanating from an overhead power line capable of modifying the ambient electrical environment. The ensuing space charge can be detected as a change in magnitude of the earth's natural DC electric field at ground level. DC field mill meters were used to measure the vertical component of electric fields upwind and downwind of 132 and 400 kV power lines. Evidence of space charge blowing downwind of power lines was observed in 21 out of 22 cases. Time series measurements recorded in the downwind direction were highly variable with fields of higher magnitude compared to those recorded upwind. Model DC field profiles were used to estimate a lower limit to the space charge density at body height arising from power lines. The average lower limit was 3000 cm⁻³ excess unipolar charges. The result suggests that between 10% and 60% of outdoor aerosols gain excess charge by the attachment of corona ions. Downwind of a 400 kV line in Somerset that was prone to excessive corona discharge, the estimated mean lower limit excess unipolar space charge density was 6000 cm⁻³, suggesting that up to 100% of aerosols gain excess charge by the attachment of corona ions. Investigations into the time variation of DC electric fields around motorways and the natural diurnal variation of the earth's DC field were also undertaken and compared to the power line data. The results show that the power line time series are clearly distinguishable from typical examples of both types of field variation, demonstrating the relatively highly charged atmospheres that generally exist around high-voltage power lines. The results are of potential public health concern, because they suggest a degree of aerosol charging that may result in a non-trivial increase in lung deposition of inhaled pollutant aerosols.     

On the generation of charge layers in MCS stratiform regions

We suggest a quantitative one-dimensional model treating the formation of charge layers near the 0 °C isotherm in stratiform regions of mesoscale convective systems. A number of factors principal for the field generation have been taken into account: both non-inductive and inductive melting charging, light ions, a complicated profile of the vertical air velocity near the 0 °С isotherm, the boundary conditions proper for the horizontally extended systems in the global electric circuit. Non-inductive collisional charging near the 0 °C isotherm was not considered. It was found that both non-inductive and inductive melting mechanisms can contribute; the inductive melting charging of ice aggregates was found more preferable, while the contribution of non-inductive mechanisms might be significant depending on particular conditions. The role of light ions in the formation of the positive charge layer near the 0 °C isotherm may be important. If the advection from the convective region ensures charge inflow to the upper charged layers, the melting charging mechanisms are able to explain an observable electric field structure in the whole stratiform region. It is important that the mutual position of the zero point on the vertical air velocity profile and the point of maximum melting-charge-transfer determines the fine structure of the electric field in the vicinity of the 0 °C isotherm.     

冰雹过程中闪电演变和雷达回波特征的综合分析

应用闪电定位网的资料和多普勒雷达资料,分析西北地区的降雹过程,详细讨论了2004年5月15日发生在兰州皋兰的一次降雹雷雨云的闪电演变、雷达回波特征和云特定参数之间的相互关系,对此雷雨云的闪电机制和电结构进行了分析推断。在皋兰产生降雹的个例分析表明,云中液态水含量是否出现高值区,是识别降雹开始的重要因子。在雷雨云移动路径上,总闪和云闪为线状分布,地闪集中分布在降雹区,云闪最多,地闪中绝大多数为负闪。综合计算降雹时段云参数分析表明,云中降水率和含水量对产生闪电相对敏感,推断这次雷雨云的电结构可能属于上正下负的偶极子型。     

Studying the effects of severe aerosol pollution of the atmosphere on the dynamics of cumulonimbus cloud charge structure by numerical modeling

Studied are the effects of severe aerosol pollution of the atmosphere on the parameters of a cumulonimbus cloud (including its charge structure) and on precipitation. Considered is the example of the cloud that developed on May 11, 2009 near the town of Kharagpur (India) under conditions of severe aerosol pollution of the atmosphere due to dust transport from the desert. The in situ observations of the evolution of the cumulonimbus cloud of large vertical and horizontal extent and of its electric conditions were carried out on that day. It is found that the distribution of electric charges in the cloud was characterized by inverted polarity (i.e., the main positive charge is in the bottom of the cloud and the negative one is in the upper part of the cloud that contradicts usually observed cases). Using the small-dimension numerical model conducted are numerical experiments on the simulation of aerosol effects produced on the evolution of dynamic, microphysical, and electric structure of the cloud under study, namely, the cloud development under background conditions and in case of high aerosol concentration. It is assumed that aerosol particles possess ice-forming properties. It is obtained that the dynamic, microphysical, and electric structures of the cloud are significantly transformed under the influence of high aerosol concentrations; precipitation generation also significantly intensifies; polarity in the distribution of electric charge varies that agrees with the data of in situ observations.     

A possible new molecular mechanism of thundercloud electrification

Thunderclouds are electrified when charge is transferred between small and large ice particles colliding in a cloud that contains strong updrafts. The small ice particles rise with one type of charge and the large ice particles fall and carry with them downward the other type of charge, which is most often negative, so that normally lightning lowers negative charge from cloud to the ground. While the collisional mechanism of thundercloud charging is well established, the nature of the charge transfer between the colliding ice particles is not very well understood on the atomic level, and no present theory can explain it in full detail. Here we propose a new charge separation mechanism that is based on molecular dynamics simulations of particle surfaces and collisions, keeping track of the individual charges as they move in the form of salt ions from one ice particle to another. Under normal conditions, when sulfates dominate as cloud condensation nuclei, this ionic mechanism is consistent with the prevailing negative charging of graupels in thunderclouds. Moreover, with dearth of sulfate anions, the present mechanism predicts a shift towards positive charging. This fits well to a large range of observations of enhanced positive lightning, connected with smoke rich in chlorides and nitrates, that could not be explained satisfactorily previously.     

对流强度对雷暴云微物理发展和电荷结构影响的数值模拟

为了探讨对流强度大小对雷暴云内微物理发展和起电过程的影响,基于已有的二维积云起、放电模式,改变其扰动温度进行敏感性试验。试验结果表明:对流强度对雷暴云内微物理过程、起电率及后续电荷结构的产生均有一定程度的影响:1）对流强度较小时,冰晶粒子极大值在高温区（高于-13.8℃）出现,对流强度较大时,上升风明显增强,将更多的水汽带入高空,气溶胶活化过程明显增强,使得云滴粒子明显增多,冰晶粒子较早产生,冰晶粒子极大值在低温区（低于-13.8℃）出现,发展过程更为剧烈;同时,较高的对流强度也使得降雨量增多,霰粒子数目也在对流发展旺盛时期显著增多。2）非感应起电率主要和冰晶-霰的碰并分离过程有关,对流强度较大时,非感应起电率较大,电荷结构持续时间较长,过程明显,感应起电率也较强。3）对流强度较大时,电荷结构更为复杂,雷暴云发展初期基本呈现为三极性,发展旺盛时期底部正电荷区域嵌入一个较小的负电荷区,呈现四极性电荷结构,雷暴云发展末期基本呈现偶极性电荷结构;对流强度较小时,发展初期、旺盛时期均呈现三极性电荷结构,发展末期呈现偶极性电荷结构。     

Investigations of the development of thunderstorm with hail. Part 3. Numerical simulation of cloud evolution

The three-dimensional nonstationary model of a convective cloud is used for investigating a thunderstorm with hail which developed over Pyatigorsk on May 29, 2012 and produced a severe hailstorm. The values of cloud characteristics (liquid water content, ice content, vertical velocity, etc.) are obtained. The importance ofconsidering wind shear is noted. The simulation results are used to analyze the transformation of precipitation field and the electric charge structure of the analyzed cloud during its development.     

Electrification of thundercloud by an entrainment mechanism

Summary A time dependent model for the growth of electric field by an entrainment mechanism has been developed, in which charge separation takes place at the interface of the descending blob due to discontinuity in the conductivity either from the onset of ice process or from the evaporative depletion of small ice or water particles in cloud parcels diluted by entrainment of dry air at the cloud top. Charge generation by the entrainment mechanism may be self limiting in high electric fields. We examine the extent to which the leakage current produced by the electrical forces acting on the descending thermals influences the net charge separation caused by the entrainment mechanism. When the effect of electrical force on the descending blob is negligibly small and the fractional change in conductivity of the blob is 0.75, high electric field of the order of a few hundred kV/m can be generated within the duration of 800s. The electrical force acting on the blods, however, does set an upper limit to the growth of an electric field, and the considered mechanism can generate electric field of the order of ten kV/m within the duration of 400 to 800s, depending on the meteorological and electrical conditions. We conclude that an entrainment mechanism can account for the observed electrification of thunderstorm at the initial stages, but that it is inadequate to generate an electric field high enough to initiate lightning.With 8 Figures     

雷暴云宏观特性对其电过程影响的数值模拟研究

利用一维雷暴云起电放电模式,初步模拟了STEPS(夏季雷暴降水与闪电研究计划)一次雷暴个例的一些基本电学特性。模拟的电荷结构为:雷暴云上部为正电荷,下部为负电荷,即符合典型雷暴云的偶极性电场分布特征,放电层电场随时间变化规律呈锯齿状分布。在此基础上,对云底高度和温度垂直递减率进行了敏感性试验,研究这两种因子对雷暴电过程的影响。结果表明:当云底高度降低时,放电高度升高,闪电频数增加,首次放电时间随之提前,即放电过程变强。温度垂直递减率增大,闪电频数降低,首次放电时间随之延后,正负电荷的分布范围减小。     

2014年青藏高原云和降水多种雷达综合观测试验及云特征初步分析结果

加密外场试验可提供云降水物理过程新的数据。2014年7月1日—8月31日,第3次青藏高原大气科学试验项目组在那曲开展了水汽、云和降水的综合观测,使用了中国最先进的Ka波段毫米波云雷达、Ku波段微降水雷达、C波段连续波雷达和激光雷达,并配以微波辐射计、雨滴谱仪等设备,获取了高时空分辨率的云和降水宏微观垂直结构特征数据;利用C波段双线偏振雷达与新一代天气雷达配对,进行双多普勒雷达观测,获取青藏高原对流云三维风场和降水粒子相态的结构和演变数据。文中简单介绍了本次试验的情况,并利用这次观测的云雷达数据对那曲地区夏季云的云顶和云底高度、云厚、云量、云层数等特征的日变化进行了初步统计分析,对不同类型云的宏观特征进行了讨论。结果表明:本次外场试验首次成功获取到了多种雷达的云观测数据。那曲地区夏季云主要集中在6 km(距地面高度,下同)以上和4 km以下;总云量、高云的云顶、云量和云厚等云的统计参数有明显的日变化,10时(北京时)为云发展最弱的时段,20时云发展最为旺盛;初生的积云和层云常常出现在3 km高度上,这一高度上常常存在明显的上升气流;深对流系统高度可达16.5 km,同时存在上升气流和下沉气流,对流中可能存在过冷水。这些数据和初步结果为进一步开展高原云和降水机理、云和降水物理过程参数化方案研究及卫星反演结果的订正提供了基础。     

1998年7月21日武汉暴雨小尺度动力特征的数值模拟研究

利用三维完全弹性积云数值模式,模拟了1998年7月21日武汉暴雨期间强降水积云的发生、发展过程.着重分析了积云动力学特征以及近地层散度场、水汽通量等对积云降水的发展、维持的贡献.结果表明,特殊的温、湿层结配置是这次强降水对流云发生的主要原因,而近地层高温、高湿气体源源不断地向云体输送,是强降水积云长时间维持的能量来源.合适的上下层风切变,特别是近地层逆向云体风速的存在,使得云体移动前方下层高温、高湿气体向云内输送,云后部伴随降雨而出现的干冷出流迅速流出,上层高空急流的存在,为积云顶部出流在更大范围扩散提供了条件.散度场、水汽通量场的分布及演变,也进一步证明了上述结论.     

豫北一次局地雹暴天气的预警特征和触发机制

利用常规气象观测、多普勒雷达、卫星资料和区域自动站观测资料及NCEP再分析资料,对2011年6月11日豫北局地强对流天气的预报预警特征和触发机制进行分析。结果表明：局地强对流天气是在东北冷涡背景下产生的,高低层中尺度影响系统(槽、切变线、大风速轴)交汇处右侧是强对流发生潜势区。局地强对流天气发生前,CAPE较大,0-6 km垂直风切变达到中等偏强,有利于超级单体的形成和发展。高空冷平流南侵、低层暖平流北上,有利于大气对流不稳定度进一步加大。中-β尺度强对流云团在东北冷涡槽底后部形成,其发展演变对局地强对流天气预报预警有参考意义。强对流回波经历了细胞状、带状发展期和块状减弱期。回波带南侧形成的超级单体造成了局地强风雹天气,冰雹发生时伴有“三体散射”现象。冷空气和地面辐合线是强对流天气的主要触发机制;地面辐合线对强对流天气还有提示作用。     

雷暴下的电流密度测量和准稳态特性讨论

利用电场和麦克斯韦电流密度的同时测量，对雷暴下闪电间的麦克斯韦电流密度的准稳态特性进行了验证，并利用实验观测和模式计算相结合的方法，对传导电流、位移电流和电晕电流的相对重要性作了估计。     

一次南方春季强对流过程中影响对流发展的环境场特征分析

利用地面自动站资料、多普勒雷达资料、FY-2E卫星云图及NCEP FNL 1°×1°逐6h分析场资料,分析了2010年5月5-7日我国南方春季大范围强对流天气过程中尺度对流系统(MesoscaleConvective System,MCS)的发生、发展特征,重点探讨了环境条件差异及其对MCS的影响。结果表明,由于环境场三维动力结构、水汽条件和热力不稳定条件配置的差异,造成对流发展的多样化特征。利用相对风暴螺旋度分析了环境场动力特征对MCS组织结构的影响。重庆上空中高层较干且具有较强的垂直切变,环境场气旋式旋转相对深厚,随着锋面强迫抬升克服对流抑制作用后,局地激发出相对孤立的类似超级单体的强对流风暴,造成冰雹和雷暴大风等天气;而贵州湿层相对深厚,高低空急流的耦合机制更明显,贵州北部的多单体对流风暴组织程度较高,MCS尺度相对较大,局地短时强降水较明显;广东具有最强的垂直切变和深厚湿层,在浅薄冷空气的触发机制下,发展出深厚湿对流形式的中尺度对流复合体,出现了高度组织化的线状对流带,MCS尺度大、持续时间长,造成较强的降水。     

Charge structure of a summer thunderstorm in North China: Simulation using a Regional Atmospheric Model System

Electrification and simple discharge schemes are coupled into a 3D Regional Atmospheric Model System （RAMS） as microphysical parameterizations, in accordance with electrical experiment results. The dynamics, microphysics, and electrifi- cation components are fully integrated into the RAMS model, and the inductive and non-inductive electrification mechanisms are considered in the charging process. The results indicate that the thunderstorm mainly had a normal tripole charge structure. The simulated charge structure and lightning frequency are basically consistent with observations of the lightning radiation source distribution. The non-inductive charging mechanism contributed to the electrification during the whole lifetime of the thunderstorm, while the inductive electrification mechanism played a significant role in the development period and the mature stage when the electric field reached a large value. The charge structure in the convective region and the rearward region are analyzed, showing that the charge density in the convective region was double that in the rearward region.