The role of dynamic transport in the formation of the inverted charge structure in a simulated hailstorm

The inverted charge structure formation of a hailstorm was investigated using the Advanced Weather Research and Forecasting(WRF-ARW) model coupled with electrification and discharge schemes. Different processes may be responsible for inverted charge structure in different storms and regions. A dynamical-derived mechanism of inverted charge structure formation was confirmed by the numerical model: the inverted structure was formed by strong updraft and downdraft under normal-polarity charging conditions such that the graupel charged negatively in the main charging region in the middle-upper level of the cloud. The simulation results showed the storm presented a normal charge structure before and after hail-fall; while during the hail-fall stage, it showed an inverted charge structure—negative charge region in the upper level of the cloud and a positive charge region in the middle level of the cloud—appearing at the front edge near the strong updraft in the hailstorm. The charging processes between the two particles mainly occurred at the top of the cloud, where the graupel charged negatively and ice crystals positively due to the strong updraft. When the updraft air reached the top of the storm, it would spread to the rear and front. The light ice crystals were transported backward and forward more easily. Meanwhile, the positively charged ice crystals were transported downward by the frontal subsidence, and then a positive charge region formed between the ?10 and ?25°C levels. Subsequently, a negative charge region materialized in the upper level of the cloud, and the inverted charge structure formed.     

随机性与电环境特征对地闪击地点影响的数值模拟

本文利用已有的随机放电参数化方案,结合四次探空资料,进行了12.5 m的高分辨率二维雷暴云数值模拟实验,得到了各种雷暴云电荷结构下的地闪个例,并就地闪击地点与空间电荷、电位分布之间的相互关系进行了分析.结果表明:(1)由空间电荷唯一确定的电位分布决定了先导的传播最大趋势,而闪电传播的随机性所带来的地闪击地点的不确定范围被限制在3 km之内,利用动态聚类法迭代得出的三个击地点位置之间的差为1 km左右.(2)负地闪的初始点与击地点的位置差主要分布在0~6 km范围内,且93%的负地闪分布在0~4 km范围内,正地闪的分布相对较广,0~3 km范围内占48%,3~6 km范围内占34%,6~10 km范围内占18%.(3)正、负地闪主要产生于离地面最近的一对电荷堆之间,其起始高度越高,初始点与击地点位置差分布越广;另外,产生于三级性雷暴云电荷结构下的正地闪,其起始于上部的主正电荷堆与中部主负电荷堆之间,由于下行正先导会绕过底部的次正电荷堆,因此其击地点与初始点的距离基本在6 km以上.     

A comparative study on the growth of cloud drops by condensation using an air parcel model with and without entrainment

During a study of the growth of cloud drops by condensation the evolution of cloud drop size spectra with height above cloud base was determined for maritime aerosols, and for continental aerosols containing aerosol particles of mixed composition. Air parcel models were used in which the parcel was either completely closed to mass and heat transfer (strictly adiabatic models), or open to heat transfer and to partial or complete mass transfer (entrainment models). It was found that adiabatic models and models which consider the entraining of air devoid of aerosol particles predict drop size distributions which are considerably narrower than those observed in non-precipitating cumulus clouds, and have only a single maximum. On the other hand, relative broad drop size distributions and distributions with a double maximum — as they are observed in atmospheric clouds — are predicted if the entrainment of both air and aerosol particles are considered in the condensation model. Our results support the findings ofWarner (1973) which were obtained for a purely maritime aerosol.     

Numerical simulations of the bi-level and branched structure of intracloud lightning flashes

Intracloud (IC) lightning is used to mean those lightning flashes which channels do not strike the ground. It is an important scientific problem to inves-tigate the IC flash features and the discharge physics.Measurements from the electric field change arrange-ment[1,2] and VHF radiation events[3,4] have provided ample evidence that IC flashes have branches with substantial horizontal extents. The VHF interferomet-662 Science in China: Series D Earth Sciences ric observations[5] also s…     

积云动力和电过程二维模式研究 Ⅱ．计算结果

利用一个二维时变轴对称模式，模拟积云动力和电过程发展，讨论形成雷暴电结构的物理原因．计算结果指出，软雹碰撞冰晶的感应和非感应起电机制是形成雷暴三极性电荷结构和局地产生足以导致空气被击穿的强电场的主要物理过程．雷暴下部的次正电荷区主要由非感应起电机制形成，尤其表现在雷暴发展前期和后期，很依赖于次生冰晶效应．计算得到的次正区和主负区最大电荷浓度约为１０^－８Ｃ／ｍ^３，主正区约低一个量级．     

Occurrence conditions of positive cloud-to-ground flashes in severe thunderstorms

The purpose of this study was to understand the reasons why frequent positive cloud-to-ground (+CG) flashes occur in severe thunderstorms. A three-dimensional dynamics-electrification coupled model was used to simulate a severe thunderstorm to permit analysis of the conditions that might easily cause +CG flashes. The results showed that strong updrafts play an important role in the occurrence of intracloud flashes. However, frequent +CG flashes require not only strong updrafts but also strong downdrafts in the lower cloud region, conditions that correspond to the later phase of the mature stage and the period of the heaviest solid precipitation of a thunderstorm. During this stage, strong updrafts elevated each charge area in the updraft region to a higher level, which resulted in an inverted tripole charge structure. A wide mid-level region of strong positive charge caused largely by positively charged graupel, presented in the middle of the updraft region because of a non-inductive ice-ice collisional charging mechanism. The charge structure in the downdraft region was consistently more complex and revealed several vertically stacked charge regions, alternating in polarity. Much of the graupel/hail outside the updrafts was lowered to cloud-base by strong downdrafts. In this area, the graupel/hail was charged negatively because of the transportation of negatively charged graupel/hail from higher regions of negative charge in the updrafts, and via the inductive charging mechanism of collisions between graupel/hail and cloud droplets at the bottom of the cloud. Consequently, a large region of negative charge formed near the ground. This meant that +CG flashes were initiated more easily in the lower inverted dipole, i.e., the middle region of positive charge and lower region of negative charge. Frequent +CG flashes began almost synchronously with dramatic increases in the storm updrafts, hail volume, and total flash rate. Therefore, the occurrence of +CG flashes appears a good indicator of storm intensification and it could have some use as a predictor of severe weather in the form of hail.     

Die elektrische Ladung eines ausgedehnten,isolierten Cirrus nothus

Zusammenfassung Bis heute vorliegende direkte Messungen von Grösse und Polarität elektrischer Ladungen in verschiedenen Stockwerken von Gewitterwolken mittels Sondierungen verschiedener Art liefern noch kein widerspruchsfreies Bild. Unter gewissen günstigen Umständen erlauben kontinuierliche synoptisch-luftelektrische Registrierungen an Stationen in verschiedenen Höhen wertvolle ergänzende Einblicke. Es wird über Meßergebnisse unter einem weit ausladenden, vom inzwischen abgestorbenen Gewitterherd abdriftenden Amboßschirm (Cirrus nothus) berichtet. Seine Ladung war einheitlich positiv und betrug maximal 2.5·10^–8 Cb/m² (die Wolke als sehr dünne Schicht betrachtet). Sie schwankte jedoch stark als Folge von Turbulenz und der Inhomogenität der Ladungsverteilung. In Übereinstimmung mit anderen, ähnlichen Beobachtungen kann der Amboß einer Gewitterwolke als grundsätzlich positiv geladen gelten. Als Gesamtladung des Cirrus nothus über dem Generator können +150 Cb geschätzt werden.

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Summary The results of direct measurements of the magnitude and sign of electrical charges at different levels in thunderstorm clouds, as derived from soundings of various types, do not, as yet, furnish a consistent picture. Under specific favorable circumstances, valuable additional information may be gained by using continuous, synoptic atmospheric-electrical recordings made at fixed sites at different elevations. — Results of measurements below an extensive anvil cloud (cirrus nothus) are stated. This cloud was drifting away from its source — an extinct thunderstorm cell — at the time the observation was made. The charge of this sheet cloud was consistently positive with a maximum of 2.5·10^–8 Cb/m². — In this study, the cloud is treated as a very thin layer; we must, however, bear in mind that there are marked intracloud variations on account of turbulence and irregular charge distribution. As in other, similar cases, the anvil of a thunderstorm cloud may be regarded as bearing positive space charge. The total charge of the anvil cloud above the generator cell may be estimated to be +150 Cb.

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Physikalisch-Bioklimatische ForschungsstelleGarmisch-Partenkirchen der Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung.     

沿海地区一次多单体雷暴电荷结构时空演变

利用闪电放电辐射源三维时空分布测量,分析了山东低海拔地区一次多单体雷暴过程的电荷结构演变以及与回波强度的关系.结果表明对流云区电荷结构是典型的上正下负电偶极结构,且随着雷暴发展正负电荷层强度增大,高度抬升.负电荷区处在40 dBz以上的强回波区域中,正电荷层处在约40 dBz区域中.层状云区也有类似结构,只是强度弱,高度低.观测到的四层电荷结构是出现在对流区消散阶段,此时,由于云体不同部位的不同消散程度,电荷结构发生断裂,云体前部正负电荷区下沉,云体中部正负电荷区高度变化不大,但负电荷区域变薄,呈现出四层电荷结构.从本例结果说明,雷暴优势起电机制通常能形成电偶极或三极性结构,多极结构可能不是起电形成.本文还分析了一次负地闪传输过程,和宏观电荷结构很好吻合,说明利用三维定位系统观测,可以较好地描述雷暴宏观电荷结构.     

Generation of available buoyant energy by cloud glaciation

The available buoyant energy (ABE, energy from the environment which becomes available to a parcel for buoyant accelerations) arising from glaciation is computed by integrating upward the differences in temperature between a parcel that undergoes instantaneous and isenthalpic freezing followed by an ice-saturation ascent, and one that experiences only a water-saturation ascent from the same initial cloud base conditions. This quantity is computed for three initial cloud base conditions representative of tropical, High Plains summertime, and Great Lakes wintertime cumuli. Substantial increases in parcel updraft speed are realized for all clouds if the ABE arising from glaciation is completely converted to parcel kinetic energy. Variations of the three components of parcel heating involved in the glaciation process (i.e., (1) release of latent heat of fusion from freezing of liquid water, (2) cooling or warming from sublimation or deposition as vapor pressure adjusts from water saturation to ice saturation at the post glaciation temperature, and (3) the additional warming or cooling relative to the intial water-saturation adiabat as the parcel follows an icesaturation ascent to a specified upper reference level) are also determined as functions of glaciation temperature. It is found that sublimation substantially counteracts the parcel warming arising from the freezing of liquid water in the case of warm moist cumuli. In addition, it is found that in some instances ice-saturation ascent following glaciation can produce cooling relative to the initial departure from the water saturation adiabat. This was indicated for Great Lakes wintertime cumuli and also for warm moist cumuli with glaciations at very cold temperatures. The effect upon the buoyancy force, of the change in the mass of condensate during glaciation, is small and can usually be neglected.     

Electric potential gradient changes during explosive activity at Sakurajima volcano,Japan

We report electric potential gradient measurements carried out at Sakurajima volcano in Japan during: (1) explosions which generated ash plumes, (2) steam explosions which produced plumes of condensing gases, and (3) periods of ashfall and plume-induced acid rainfall. Sequential positive and negative deviations occurred during explosions which generated ash plumes. However, no deflections from background were found during steam explosions. During periods of ashfall negative electric potential gradients were observed, while positive potential gradients occurred during fallout of plume-induced acid rain from the same eruption. These results suggest that a dipole arrangement of charge develops within plumes such that positive charges dominate in the volcanic gas-rich top and negative charges in the following ash-rich part of the plume. The charge polarity may be reversed for other volcanoes (Hatakeyama and Uchikawa 1952). We suggest that charge is generated by fracto-emission (Donaldson et al. 1988) processes probably during magma fragmentation within the vent, rather than by frictional effects within the plume.     

Influence of ion attachment on the vertical distribution of the electric field and charge density below a thunderstorm

A numerical model called PICASSO [Production d’Ions Corona Au Sol Sous Orage (French) and Production of Corona Ions at the Ground Beneath Thundercloud (English)], previously designed, is used to describe the evolution of the principal electrical parameters below a thunderstorm, taking into account the major part played by corona ions. In order to improve the model restitution of a real situation, various improvements are performed: an initial vertical distribution of aerosol particles is introduced instead of the previously used uniform concentration; time and space calculation steps are adjusted according to the electric field variation rate; the upper boundary condition is improved; and the coefficients of ion attachment are reconsidered with an exhaustive bibliographic study. The influence of the ion attachment on aerosol particles, on the electric field and charge density aloft, is studied by using three different initial aerosol particle concentrations at ground level and two types of initial vertical distributions: uniform and non-uniform. The comparison between field data and model results leads to adjust the initial aerosol particle concentration over the experimental site at the value of 5000 cm⁻³ which appears to be highly realistic. The evolutions of the electric field and of the charge density at altitude are greatly influenced by the aerosol concentration. On the contrary, the surface intrinsic field, defined as the electric field that would exist underneath a thundercloud if there were no local charges, is weakly affected when the model is forced by the surface field. A good correlation appears between the success in the triggered lightning attempts and this intrinsic field evaluation. Therefore, when only the surface field is available, the model can be used in a triggered lightning experiment.     

青海地区一次雷暴的地闪活动及云内的电场探空观测

2016年夏季在青海大通地区获得一次局地雷暴云内的电场探空资料,结合雷达、地闪定位资料,详细分析了该雷暴的地闪活动特征及云内的电荷结构.结果显示,该雷暴过程的负地闪在时间上呈间歇性发生,在空间分布上表现为不连续,且所有的正地闪都发生于雷暴的成熟阶段.在雷暴成熟阶段与消散阶段过渡期获得云内的垂直电场廓线表明,雷暴内的电荷结构在探空阶段呈四极性,最下部为处于暖云区内负电荷区,往上依次改变极性.最上部的正电荷区由于数据丢失无法判断其上边界外,其余3个电荷区的海拔高度分别为:5.5～5.7km(3.4～2.3℃)、5.7～6.2km(2.3～-0.4℃)和6.2～6.6km(-0.9～-1.7℃),对应的电荷密度为-1.81nC·m^-3、2.47nC·m^-3和-1.76nC·m^-3.其中,下部正电荷区的强度最大,其次为上部的负电荷区.通过分析电荷区分布与正地闪活动的关系,认为暖云区内负电荷区的形成有利于诱发下部正电荷区的对地放电.     

不同非感应起电及感应起电参数化方案对青海东部一次雷暴云电荷结构影响的数值模拟研究

本研究利用加入起电、放电参数化方案的数值模式（Weather Research and Forecasting Model（Version 3.7.1）,WRF3.7.1_ELEC）,通过设计五组不同非感应起电及感应起电参数化方案敏感性试验,对发生在青藏高原东北部青海大通地区的一次雷暴过程进行模拟研究,对比分析了不同非感应起电机制及感应起电机制对雷暴云电荷结构的影响.结果表明：在雷暴云发展旺盛阶段,Saunders（S91）、Riming Rate（RR）、和Saunders和Peck（SP98）三种非感应起电方案模拟的雷暴云最低层均为负电荷区,而混合方案（Brooks and SP98,BSP）模拟的雷暴云最低层为正电荷区,主电荷区自下而上为"+-+-"排列的四层电荷结构.与甚高频辐射源定位法推算的结果对比,BSP方案模拟的本次高原雷暴云电荷结构更接近实际情况;几种不同非感应起电方案模拟的主电荷区外围与主电荷区电荷结构不同,说明在雷暴发展的不同阶段雷暴云的电荷结构是不同的;几种非感应起电方案模拟的电荷结构不尽相同,主要是由于霰、冰和雪粒子在不同高度所带电荷的极性及电量的大小不同,霰粒子的电荷密度对低层的影响较大,冰粒子和雪粒子的电荷密度对中上层的影响较大;加入感应起电机制后,雷暴云电荷结构分布几乎没有变化,但能使雷暴云发展旺盛阶段低层和中层的正负电荷区电荷密度有所加强.     

In-situ measurement of smoke particles in the wintertime polar mesosphere between 80 and 85 km altitude

The MAGIC sounding rocket, launched in January 2005 into the polar mesosphere, carried two detectors for charged aerosol particles. The detectors are graphite patch collectors mounted flush with the skin of the payload and are connected to sensitive electrometers. The measured signal is the net current deposited on the detectors by heavy aerosol particles. The collection of electrons and ions is prevented by magnetic shielding and a small positive bias, respectively. Both instruments detected a layer of heavy aerosol particles between 80 and 85 km with a number density approximately 10³ cm⁻³. Aerodynamic flow simulations imply that the collected particles are larger than ∼1 nm in radius. The particles are detected as a net positive charge deposited on the graphite collectors. It is suggested that the measured positive polarity is due to the electrification of the smoke particles upon impact on the graphite collectors.     

The granulometrical characteristics of a slowly-moving dust cloud

A torn-up road offered the opportunity to sample a dust cloud continuously at different heights during a time interval of 51 hours. In particular the granulometrical characteristics of the dust caught were investigated. It appears that if the air layer in the vicinity of a dust cloud is not too turbulent, the cloud will be clearly granulometrically stratified. The coarse material moves chiefly at the bottom of the cloud, whereas the fine material will occur both at the bottom and at the top. The experiment also shows that the higher the dust in the cloud, the worse it is sorted. Above a critical height, however, the degree of sorting increases again. The variation of the mean dust diameter d₅₀, as a function of the height z, can be expressed in a simple semi-logarithmic equation. The variation of the dust quantity G, as a function of the height z, can also be expressed in a semi-logarithmic form but the correlation is higher when a power equation is used. The ratio of fine silt to coarse silt seems to vary parabolically with height. One can only speak of an optimal transport height in the case of particles > 16 μm. For finer particles, the turbulence of a normal air stream is usually high enough to carry the particles anywhere in the dust cloud, so that one can no longer speak of an optimal transport height. The granulometrical border between aeolian transport in saltation and aeolian transport in suspension lies, according to the results of the experiment, at a diameter of about 63 μm.     

Noctilucent clouds seen at 60°N: origin and development

A noctilucent cloud is seen at a particular time from a specified place. The journey of the cloud particles from nucleation to observation can be calculated by using a simple model of growth and taking account of the fall speed of the cloud particles. Cloud particles can be backtracked by bringing together growth and fall speed equations and a model of mesospheric winds to find where the particles of a cloud seen at a particular time and place have originated. The wind model that is used here suggests that there is a distinct outer edge to the summertime polar circulation pattern in which water vapour is being carried up from the lower mesosphere to the mesopause. The change in latitude of this outer edge during the summer season may well account for the observed seasonal change in occurrence of mesospheric clouds. Polar mesospheric clouds cause a drying of the upper mesosphere. It is suggested here that diffusion of water vapour dumped at the level of polar mesospheric clouds will take an appreciable time to carry water vapour back up to the mesopause. In consequence, there will be a significant separation between the observed location of a noctilucent cloud and its precursor polar mesospheric cloud.     

Some recent investigations on the nature and properties of natural and artificial freezing nuclei

Summary Working on the hypothesis that atmospheric ice-forming nuclei are largely of terrestrial origin, the nucleating ability of various types of soil particles and mineral dusts has been investigated. Of the thirty substances tested, twenty-one, mainly silicate minerals of the clay and mica groups, were found to produce ice crystals in supercooled clouds and also on supercooled soap films at temperatures of – 18° C, or above, and of these, ten were active above – 12° C. The most abundant of these is kaolinite with a threshold temperature of – 9° C. Ten natural substances, again mainly silicates, were found to become more efficient ice nuclei having once been involved in ice-crystal formation, i.e. they could be pre-activated or «trained». Thus, ice crystals grown on kaolinite nuclei, which are initially active at –9° C, when evaporated and warmed to near 0° C in a dry atmosphere, leave behind nuclei which are thereafter effective at – 4° C. Particles of montmorillonite, another important constituent of some clays, and which are initially inactive even at –25° C, may be pre-activated to serve as ice nuclei at temperatures as high as –10° C. It is suggested that although such particles can initially form ice crystals only at cirrus levels, when the ice crystals evaporate they will leave behind some «trained» nuclei which may later seed lower clouds at temperatures only a few degrees below 0° C. On this hypothesis, the fact that efficient nuclei are occasionally more abundant at higher levels would not necessarily imply that they originate from outer space. Indeed, in view of our tests on products of stony meteorites, produced both by grinding and vaporization, which show them to be ineffective at temperatures above – 17° C, it seems likely that atmospheric ice nuclei are produced mainly at the earth's surface, the clay minerals, particularly kaolinite, being a major source.Although a good deal of work has been carried out in different laboratories on the ice-nucleating ability of a wide variety of inorganic compounds, there has been little agreement in the results. Careful tests carried out in our laboratory have revealed a number of reasons for this. Spurious results may be obtained because of the presence, in the air or the chemicals, of small traces of silver or free iodine, leading to the formation of silver iodide: if all such trace impurities are removed, many of the substances that have been claimed to provide efficient ice nuclei are found to be quite ineffective. It is dangerous to infer that all twinkling particles in a water cloud are ice crystals since particles of some seeding agents glitter even at positive temperatures. The threshold temperature of a nucleant will depend upon the criterion adopted for the onset of nucleation, i.e. upon the fraction of the total number of particles of seeding agent which are activated; this, in turn, will depend upon the fraction of particles which happen to possess suitable crystallographic faces for nucleation. Much may also depend upon the manner in which the test is performed. Since some nucleating materials produce ice crystals only after a delay of 30 seconds or more, they may appear to be ineffective if tested in the transient cloud of an expansion chamber but highly effective if allowed to remain in an ice-supersaturated atmosphere for a minute or more. Again, we have found that the efficiency of some nuclei is governed by the supersaturation as well as the temperature of the environment, and the supersaturation regimes in expansion, diffusion, and mixing-cloud chamber may be widely different. Highly soluble particles, although able to act as «sublimation» nuclei in atmospheres super-saturated relative to ice but sub-saturated relative to water, on entering a water cloud go quickly into solution and lose their nucleating ability.Inorganic substances which definitely nucleate a supercooled water cloud in a mixing-cloud chamber at temperatures of –15° C and above are: AgI (–4° C), PbI₂ (–6° C), CuS (–6° C), Ag₂S (–8° C), Ag₂O (–9° C), HgI₂ (–8° C), V₂O₅ (–14° C), Cu₂I₂ (–15° C), the figures in brackets indicating the threshold temperatures at which about one particle in 10⁴ becomes active as an ice nucleus. Cadmium iodide (–12° C), ammonium fluoride (–9° C) and iodine (–14° C) are examples of salts which will act as sublimation nuclei in an ice-supersaturated atmosphere and will nucleate a supercooled soap film, but which are ineffective in a water cloud because of their solubility.Although the most efficient nucleating agents tend to be hexagonal in structure, there are some striking exceptions e.g. Ag₂S, Ag₂O, HgI₂, but in most cases, we have been able to find a low-index crystal surface on which the ice lattice could grow with a misfit of only a few per cent.In an attempt to investigate the nucleation mechanism in more detail, we have studied the growth of ice on single crystals of various nucleating agents. Perfect orientation of ice crystals has so far been observed on the basal faces of silver iodide, lead iodide, cupric sulphide, cadmium iodide, and freshly-cleaved mica, on the (001) plane of iodine, and on the (010) plane of mercuric iodide.     

Measurements of electric charge distribution in volcanic plumes at Sakurajima Volcano,Japan

Measurements of perturbations in the atmospheric potential gradient around volcanic plumes at multiple (from two to five) sites, and measurements of the charge-mass ratio of ash particles falling from volcanic plumes, were carried out at Sakurajima Volcano, Japan. Results from 28 and 29 October 1995, show that the nature of the perturbations depends on the intensity of plume activity. Although plume activity was vigorous on 28 October, negative perturbations were predominant. As plume activity peaked, the magnitude of negative perturbations decreased just below the plume and increased at an off-axis site. During the peak period, positively charged ash particles fell out from the plume. This suggests that the active plume dominantly contained negatively charged materials, and that positively and negatively charged materials were added to the lower and upper parts of the plume, respectively, during the peak period. On the other hand, as plume activity became less vigorous on 29 October, the perturbations were characterized by a positive anomaly followed by a negative anomaly. Because wind velocity increased with altitude that day, we infer that positive and negative charges were distributed in the upper and lower parts of the plume, respectively. The differences in perturbations observed on 28 and 29 October suggest that volcanic plumes are generally composed of three parts: an upper part with positively charged gas and aerosol, a middle part with negatively charged fine ash particles, and a lower part with positively charged coarse ash particles. The compilation of present and previous results from Sakurajima and other volcanoes indicates that the effect of the negative charge in the middle part was predominant in most cases, although positive perturbations caused by the upper part were observed around some weak plumes. The effect of positively charged particles in the lower part was observable only when plume activity was sufficiently strong because positively charged coarse particles tended to fall out near the vent.     

Spatiotemporal characteristics of positive cloud-to-ground lightning discharges and bidirectional leader of the lightning

The negative CG lightning discharges neutralizing negative charges in cloud usually dominate for most of thunderstorms. However, a lot of positive CG light-ning discharges often occur in the disappearing stage of thunderstorms, in the stratiform region of mesoscale convective systems and some supercells producing hail and tornado. Because the positive CG lightning discharges produce larger current of the return stroke and neutralize more charges due to the continuing currents with longer las…     

Observations of condensation growth determined by entity type mixing

It has been speculated for many years that the development of the droplet spectra in cloud is probably influenced by mixing processes. Various theoretical attempts to broaden the droplet spectra by mixing parcels with different velocity histories has shown that that particular effect is small. Similarly, very simpleuniform entrainment procedures did not lead to cloud drop size spectra which were broad enough, although by producing cloud drop size distributions with a double mode these models did substantially improve the drop size spectra of earlier adiabatic models which only exhibited a single mode.Recently a model based on entraining entities representing moving parcels of cloud air within the cloud was detailed byTelford andChai (1980). This study showed that the mixing in of dry air at cumulus turrets could lead to vertical cycling of diluted parcels, and that this cycling, with continual entrainment across the parcel boundaries, will produce much larger drops, as well as smaller drops of all sizes, in the droplet spectra. The entity entrainment concept studied there appears to apply to the observations of stratus cloud discussed in this paper.This paper presents data taken in marine stratus off the California coast which give a particularly clear example of how such droplet spectra modification occurs in practice. Both large drops, and the spread of the spectra to smaller sizes, occur in relation to other variables in such a way as to be consistent with an entity entrainment explanation, with no other obvious possibility.In a marine stratus cloud just over 200 m thick and many tens of miles in extent we find clear evidence that dry air is mixing in at cloud tops. Strong vertical motion is to be found in the cloud, large sized drops are found in cloud parcels where the mixing gives lower droplet concentrations, and there is evidence that newly formed cloud parcels are warmer and contain many more smaller droplets.The observations show that immediately following entrainment of dry air drop diameters are not reduced appreciably, but, in the same parcels, drop concentrations have been reduced by a factor of ten or more. Further down in the cloud big drops, able to start growth by coalescence, are found associated with low total droplet concentrations.Overall, it seems likely from the consideration of these observations that the formation of the large drops which lead to precipitation processes in clouds depends critically on the mixing in of dry air at cloud tops, and very little on the size of the small drops resulting from the condensation nucleus counts. As a conclusion it appears reasonable to state that if entrainment occurs at cloud tops, then big drops will be formed!