Cloud Electrification Research Articles

A Numerical Simulation of Winter Cumulus Electrification. Part I: Shallow Cloud

Abstract The development of electricity in a shallow wintertime cumulus was studied using an axisymmetric cloud model containing both microphysical and electrical charge separation processes during graupel formation. The charge separation mechanisms considered included ion induction, ion diffusion, polarization and riming electrification. An unexpected result was that polarization did not intensify cloud electrification. Instead, riming electrification appears to be the principal charge separation process acting to intensify cloud electricity. The cloud is electrified during graupel formation, and a relatively large positive potential gradient forms initially near the cloud top along the cloud boundary between negative graupel and positive ions. Later, graupel particles, electrified positively through riming electrification, produce a relatively strong negative potential gradient between the positive space charge and the upper-level negative space charge produced by snow crystals. As these positively char...

Planetary lightning: Earth, Jupiter, and Venus

Terrestrial lightning is usually produced by convective clouds that contain supercooled water, ice, and precipitation. Although convection and a variety of mechanisms based on the principle of electrostatic induction can, in principle, electrify clouds, the available evidence suggests that interaction between riming ice particles and supercooled water, followed by a gravitational separation of charged precipitation elements, is the dominant mechanism for cloud electrification on earth. These mechanisms probably operate on Jupiter in the H2O cloud, and optical images of lightning and whistlers have been detected by Voyager 1. Jupiter has no true surface; therefore the Jovian lightning flashes are cloud discharges. Observations indicate that Jovian lightning emits, on average, 1010 J of optical energy per flash, whereas lightning on earth radiates only about 106 J per flash. There is much uncertainty in the average planetary lightning rate on Jupiter, but estimates range from 3 × 10−3 to 40 km−2 yr−1. Venera probes have reported transient LF and VLF radio emissions in the lower atmosphere of Venus, and an experiment aboard the Pioneer Venus Orbiter has registered electromagnetic energy propagating in the whistler mode. Optical searches for lightning on Venus have been inconclusive, but have set an upper limit to the planetary flashing rate of about 30 km−2 yr−1. The Venus observations have led to widespread speculation that there is lightning in the atmosphere, but application of known charging mechanisms to the Venus environment does not suggest that there should be strong electrification. Since the spacecraft observations do not conclusively show that lightning does occur on Venus, we suggest that alternative explanations for the experimental results be explored.

Ion-Drop Interaction During Drop Evaporation

Abstract As a basic experiment in warm cloud electrification, evaporating large drops were studied as they floated in an ion-rich environment in a vertical wind tunnel. The drops were found to acquire a positive charge during their evaporation, a result in agreement with previous studies by Takahashi (1973).

熱帯における海洋性積雲の電荷構造と電荷発生機構

The electric charge of precipitation particles was measured both at the ground and in the clouds at Ponape, Micronesia. Results were analysed as a function of drop size, surface potential gradient and cloud top height in order to identify the charge separation processes. It was found that the electric structure of a cloud differs greatly with cloud top height, suggesting that charge separation processes in the cloud are primarily determined by microphysical processes.Different charging processes have been proposed to explain the differences between warm and cool cloud electrification. Thunderstorm electrification was explained as an extension of the cool cloud electrification mechanism.

Maxwell currents under thunderstorms

We point out that recent observations of the time variations in thunderstorm electric fields, both aloft and at the ground, can be interpreted in terms of a total Maxwell current density that varies slowly with time in the intervals between lightning discharges. We utilize this quasi‐static behavior to estimate and map the Maxwell current densities under a small Florida thunderstorm using data provided by a large field mill network. An area integral of these current densities gives a total Maxwell current just above the ground of about 0.5 A, a value which is a reasonable lower limit for the total Maxwell current produced by the cloud, and an upper limit for the rate of charge transport to ground between lightning flashes. Using the quasi‐static behavior of the Maxwell current density, we derive an expression for the field‐dependent current density under a thunderstorm during the field recovery following a lightning discharge, and we infer values of air conductivity under the small storm which range from 2 to 6×10 −13 mho/m. Finally, we present data that indicate that the area‐average Maxwell current is not usually affected by lightning, but instead varies slowly throughout the evolution of the storm. Therefore, we suggest that cloud electrification processes probably do not depend on the cloud electric field, which exhibits large and rapid time variations, as much as they do on more slowly varying quantities, such as the meteorological structure of the storm and/or the storm dynamics.

Role of relaxation and contact times in charge separation during collision of precipitation particles with ice targets

The relaxation time of charge transfer and the contact time are two of the parameters controlling charge separation when ice particles collide with one another or with liquid water droplets. For maximum efficiency of charge separation, the relaxation time should be of the same order as the contact time. The relaxation time of charge transport in ice was systematically determined as a function of impurity content and temperature. The contact time for ice/ice collisions was estimated from theory. Depending on the impurity species and their concentration, the relaxation time may be increased or decreased by orders of magnitude with respect to pure ice. Only particles of the order of 10−2 m radius seem to have contact times sufficiently long to give appreciable charge separation by ion transport in collisions between pure or slightly doped (0.1 ppm chloride) ice particles in a dry environment. Charge separation mechanisms involving liquid water and those operating by the transfer of electrons from surface states largely relax these conditions. These results have implications for cloud electrification models involving collision mechanisms.

Possible effect of the cloud electrification on the downdraft in thunderstorms

Changes in the velocity of the raindrops due to electrical forces acting on them will change the drag force imparted by the drops to the air through which they are falling. These changes in the drag force have been argued to influence the initiation and development of the downdraft in thunderstorms. Changes in the development of balance levels and the effect of inclined electric fields have also been discussed in this context.

熱帯における浅い積雲からの温かい雨の電荷発生機構及び降水機構

Electrification and precipitation mechanisms were studied in maritime shallow warm clouds around the island of Hawaii.Ion-drop interaction during drop evaporation is assumed to be the principal charge separation mechanism in warm clouds. During showers a negative potential gradient is observed at the ground, probably caused by negative ions transported from the cloud top to the ground by downdrafts.Giant nuclei do not contribute significantly to the initiation of warm rain. Most drop growth occurs near the cloud top where the updraft speed decreases with height. Raindrop formation near the cloud top is required to initiate rain beneath the cloud base. The rate at which raindrops form depends mainly on the humidity profile in the trade wind layer. The rainwater accumulation rate varies with the cloud type.Unsolved problems related to both cloud electrification and precipitation mechanisms are discussed to encourage future research in the tropics.

Production of Artificial Lightning In An Ordinary Clear Light Bulb

By taking an ordinary clear electric light bulb and subjecting it to rubbing, purplish flashes of minature lightning will be produced inside the light bulb. These flashes are produced by rubbing a plastic (polyethylene) cylindrical food container with cotton cloth and then rubbing the light bulb against the food container. The light bulb can also be placed inside the plastic cylinder and the cylinder rubbed on the outside. In both cases, the base of the light bulb is electrically grounded so the sparks jump from the inner wall of the bulb to the filament. The appearance of the sparks is virtually that of a miniature stroke of forked lightning seen in natural thunderstorms. The sparks also show the intricate branching patterns that are often seen in natural lightning. This technique of making lightning is extremely simple, yet can be a useful tool for the cloud physicist who wishes to investigate cloud electrification, as well as the earth science teacher at the elementary or secondary school level.

Precipitation‐powered mechanisms of cloud electrification

This article reviews those mechanisms of cloud electrification which are based on gravitational separation of precipitation particles and examines at some length the inductive charging mechanism. It is concluded that through the inductive mechanism, glaciated clouds can develop a maximum current density Jmax of 0.1 µA m−2 with a maximum field Emax of about 3.5 × 105 V m−1 (which values are necessary for the occurrence of lightning flashes in them) within a time of 1000 s provided that the precipitation intensity in them is greater than 1.39 × 10−5 m s−1 (50 mm hr−1). For precipitation intensities less than 8.33 × 10−6 m s−1 (30 mm hr−1), Jmax and Emax cannot develop to such high values. By its inherent nature this mechanism does not work in warm clouds.

Numerical study of cloud electrification in an axisymmetric, time‐dependent cloud model

An axisymmetric, time‐dependent, numerical cloud model has been applied to study how an isolated convective cloud can be electrified when rain and cloud particles are allowed to be charged by two particle charging mechanisms: ion attachment and polarization. Charge transports by electrical conduction, air convection, turbulent mixing, and the particle terminal velocities are all simulated. The close tie‐in of the electrical development with the evolutions of the cloud growth is one of the distinctive features of the study. The full dynamical‐microphysical‐electrical interactions are allowed in the simulation. The results of numerical experiments make it appear likely that the polarization charging mechanism is an important one for the electrification of warm clouds. The results also show that with the inclusion of the polarization charging mechanism and depending on the growth stage of the cloud the cloud can be electrified into various charge center structures, which have been observed and reported in the literature for many years. The numerical results show that the cloud cannot be rapidly electrified until rainwater forms and the polarization charging mechanism operates. Initially, the cloud is charged into a ‘positive dipole’ structure with a positive charge center in the upper part of the cloud and a negative one below. As the cloud grows further, another positive charge center forms below the main negative one. This weaker positive center is closely associated with the positively charged rain formed in that region. As the rain falls out of the cloud, this positive center extends below the cloud base. When the cloud top reaches its maximum level, the cloud base starts to rise and the cloud grows weaker thereafter. In the later stage of the development the charge distribution patterns of the rain field dominate the whole electrical structure of the model. The electrical pattern thereafter is greatly affected by the downward movement of the rain. With the fallout of the rain the cloud charge centers exhibit successfully a positive dipole structure and then a ‘negative dipole’ structure consisting of a negative charge center in the upper part of the cloud and a positive one below. The electrical force effects on the terminal velocities of the rain and cloud particles are also included in the study. The results show that the rain particle terminal velocities start to decrease by a few meters per second in a region where the electric field strength grows beyond 2 × 105 or 3 × 105 V/m. The cloud particle terminal velocities reach only several centimeters per second, even in the thunderstorm electrical state. As a result of the levitation effect on rain, the simulated cloud in the present study develops double maxima in the cloud water content, giving it a more irregular appearance than the weakly electrified or nonelectrified cloud.

Convective electrification of clouds

The analytical, steady state, nonprecipitating convective cloud model of Gutman (1963, 1967) has been applied to the study of cloud convective electrification. A numerical model has been developed for simulating charge transport by convection, conduction, and turbulent diffusion. The formulation includes a description of the dependence of cloud electrical conductivity on liquid water content through micro-physical ion capture processes. The results of numerical experiments make it appear unlikely that organized convection alone can cause strong electrification. These results, subject to certain assumptions concerning the mechanisms involved (discussed at the end of the paper), indicate purely convective electrification generally should cause clouds with usual base elevations to have negatively charged cores and relatively weak upper surface layers of positive charge. There is only a slight dependence of the electrical state on cloud thickness, apparently because greater cloud height results in exposure to higher conductivities, which tends to compensate for greater convective activity and larger conductivity gradients. For clouds with bases only a few tens of meters above ground there is a likelihood of weak charging of polarity opposite to that described above.

Calculations of Electric Field Growth within a Cloud of Finite Dimensions

Abstract A one-dimensional precipitative model of cloud electrification is outlined in which field growth results from the operation of either in inductive or non-inductive mechanism. The cloud is cylindrical, of finite dimensions, and charging is confined to a supercooled zone within which precipitation growth occurs. Account is taken of loss of negative charge arriving at the ground on precipitation and the storage of positive charge carried by the updraft to a level above the charging zone. The most important conclusion is that previous models of cloud electrification, which have often been extremely elaborate, provide gross overestimates of the rate of field growth because they have assumed a cloud of infinite width. They also predict a “top-hat” distribution of field, which is shown to be quite unrealistic. The present calculations cast serious doubt on the capability of an inductive mechanism, by itself, to produce breakdown fields in the available time. These calculations also indicate that 1) the ...

Field Generation and Dissipation Currents in Thunderclouds as a Result of the Movement of Charged Hydrometeors

Abstract The calculations of Gay et al. of the terminal velocities of charged hydrormeteors in the presence of electric fields have formed the basis of computations of the charging current density J flowing through a thunder-cloud as a result of the operation of a precipitative mechanism of cloud electrification. Values of J were calculated for a range of values of field strength E, precipitation rate pO, precipitation content L, cloud water content C, charge distribution, total separated charge, and the fraction of the small particles that have undergone a charging event. It is found that the estimated field required for the initiation of a lightning stroke (σ3.5 kV cm−1 can be achieved only over a narrow range of conditions. The ease with which precipitative mechanisms can produce breakdown fields is considerably increased, however, if account is taken of spatial inhomogenities in the field.

Numerical simulation of atmospheric electricity effects in a cloud model

Results from a numerical model of cloud initiation, growth, and electrification are presented. The model is two-dimensional and time dependent. The first and third equations of motion, a thermodynamic energy equation, and various water conservation equations along with conservation equations for 11 types of ions and neutral particles are solved numerically. The small ion equations (postive and negative ions) model drift velocities, advection, point discharge near the ground, generation due to cosmic radiation, loss due to recombination, and loss or gain due to attachment or detachment of the ions to or from cloud and rain particles. The large ions and neutral particle equations are modeled in the same manner as the small ions but without drift velocities, production due to cosmic radiation, or point discharge. The cloud and rain ions are treated in the same manner as the particles to which they are attached. Positive ions are arbitrarily attached to cloud particles, and negative ions to rain particles to simulate the observations of net positive charge on cloud, net negative charge on rain. Electrical fields and charge concentrations of 70 cm−1 and 10−16 coul cm−3, respectively, are attained with 50–70% free ion attachment to cloud and rain. These fields have a negligible effect on the cloud dynamics. Sedimentation is an important process in establishing the electrical disturbances of the model cloud.

Electrical Balance in the Lower Atmosphere

The omnipresent, fluctuating, vertical, negative electric field of fine weather ranges in intensity from 100 or 200 V 1m at the earth's surface to about 5 V 1m at the tropopause. Because the atmosphere is a conductor, this field causes an ionic conduction current to flow of about 10-12 A/m2 or 103 A over the entire globe. This flow of positive charge from the atmosphere to the earth is balanced by various electrical processes that transport positive charge from the earth into the atmosphere at an equal rate. Blowing dust, sand, and snow, volcanic activity and the spray from the ocean play a part, but the primary cause of this atmospheric electrification is the thunderstorm. For reasons that are not understood, large convective clouds produce extensive regions of space charge, electric fields in excess of 105 Vim, and lightning sparks many kilometers in length. Not only the causes, but also the effects of electrification of clouds are still far from being understood. Some scientists believe the electrification is inconsequential, others that it may have important effects, such as accelerating the formation of precipitation.

Experimental study of the electrification produced by dispersion of dust into the air

Some laboratory experiments have been performed to study the electrification of dust clouds created by blowing different types of dusts into a dust chamber. The polarity and magnitude of the space charge in such dust clouds have been found to be sensitive to the mineral constituents of the dust. Even a single dust cloud, if allowed to settle under gravity in a field-free space with no charge added to it, can have opposite polarities of space charge at different times of its sedimentation. The space charge produced increases with an increase in the length of the surface over which the dust is blown. It also increases with an increase in the temperature and velocity and a decrease in the relative humidity of the blowing air. External electric fields of up to a few hundred V/cm, applied to the surface from which the dust is blown, have little effect on the generated space charge. Size distributions of positively and negatively charged particles show a greater abundance of smaller (∼ 3 μ) particles compared to those of small neutral particles.

Some combined electrification processes in convective clouds

A quantitative evaluation of the electrification processes due to thermal gradients in ice and inductive charging processes is made for the purpose of identifying those processes and regions in convective clouds where electrification is occurring the most strongly. When the thermal-gradient charging process is possible, due to the presence of solid particles in the clouds, it is found that initially the electrification proceeds more rapidly. This electrification stage is followed quickly by the enhancement of the existing field through induction charging processes. These processes work together to increase the electrification in clouds most rapidly and with the greatest intensity when solid precipitation at temperatures near 0°C are present in cloud air with ice particles that are several degrees colder so that the thermal gradient mechanisms can be present of the right sign and the conductivity of the ice high enough for fast charge transfer between colliding particles. Incorporation of the effects of the electric field on the relative velocities of the charged particles after collision limits the strength of the maximum field but provides for a more natural shape to the electric field growth curve. When this restriction is too severe, relative motion of charged volumes by large scale convection may be required to push the electric field to its discharge intensity.

Dependence of Ice Melting Electrification on Earlier Freezing Rate

MANY explanations have been offered for the electrification of clouds and precipitation involving effects such as the change of state of water between phases, selective ion capture, a thermo-electric effect in ice, the rupture of water films, induction charging when polarized particles collide, concentration cells and ion diffusion. These processes are probably not all independent, but often only one may have a dominant role1. Dinger and Gunn2 have reported that melting ice containing air acquires a positive charge when ventilated by an air stream which in turn carries away negative charge released as the trapped bubbles burst. Chalmers1 suggests that melting is often particularly important and may account for the positive charge sometimes detected near the base of thunderclouds and also for that usually found on continuous rain. Drake3 observed that when small ice particles melt in a wind tunnel the separation of electric charge is highly dependent on convection currents which develop in the meltwater and rapidly transfer bubbles to the surface where they burst. He showed that the onset of strong convection and the most vigorous electrification occurred only when the heat flow rate to the melting particle was above a certain level. According to Iribarne and Mason4 this charge is produced by the rupture of the electrical layer at the air/water interface. Our experimental results show a new aspect of this melting electrification.

Electric Surface Potential of Growing Ice Crystals

The surface electric potential of a growing ice crystal was studied for cases of growth by condensation of water vapor. Positive potential was found during the first stage of condensation. It was found that this positive potential was related to the thin liquid water layer which was produced on the ice surface by the condensation process. Also, it was found that this condensation process was necessary for the production of this positive potential. It was shown that H2O molecules arranged themselves so that protons were directed outward from the liquid surface at condensation. It was also seen that negative potential observed at the later condensation stage corresponded to the growth of ice crystals, this negative potential being produced by the negatively electrified dislocations on the ice crystal surface. It is suggested that these observations are very relevant to electrification in natural clouds.