Charge sign reversal in irregular ice particle-graupel collisions (original) (raw)
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Quarterly Journal of the Royal Meteorological Society, 2004
Laboratory studies of a thunderstorm charging mechanism involving rebounding collisions between ice crystals and riming graupel pellets, have shown the importance of the growth conditions of the interacting ice particles on the sign of the charge transferred. The present study shows a new result: if an ice crystal is not in thermal equilibrium with the environment (immediately following the mixing of two clouds at different saturations) the crystal surface may experience an enhanced growth rate that can influence the sign of the charge transfer and promote negative rimer charging. Furthermore, when an ice crystal in ice saturation conditions is introduced to a cloud at water saturation, leading to transient growth and heating, the period of thermal nonequilibrium is shown to be sufficiently brief that the enhanced negative rimer charging is short lived. These results suggest that the earlier conclusions of Berdeklis and List-that the cloud saturation conditions around a growing ice crystal impart to the crystal surface a property that is carried with it and that influences the sign of subsequent charge transfer-are unfounded. The discrepancy is because in their laboratory simulations of thunderstorm conditions there is adequate time for the growing ice crystal surface to come to equilibrium with its environment. The established concept of the relative diffusional growth rate of the interacting surfaces controlling the sign of charge transfer, such that the faster growing surface charges positively, is consistent with the observations.
Quarterly Journal of the Royal Meteorological Society, 2006
Collisions between vapour-grown ice crystals and a riming target, representing a graupel pellet falling in a thunderstorm, were shown by Reynolds, Brook and Gourley to transfer substantial charge, which they showed to be adequate to account for the development of charge centres leading to lightning in thunderstorms. Related experiments by Takahashi and Jayaratne et al. determined that the sign of charge transferred is dependent on the cloud liquid water content and on cloud temperature. There are marked differences between the results of Takahashi and Jayaratne in the details of the dependence they noted of the sign of graupel charging on cloud water and temperature. More recently, Pereyra et al. have shown that results somewhat similar in form to those of Takahashi are obtained by modifying the experimental technique used to prepare the clouds of ice crystals and supercooled water droplets used in the experiments.
Atmospheric Research, 2001
Laboratory experiments, in which vapour grown ice crystals interact with riming graupel targets, simulate charging processes in thunderstorms. The introduction of cooled, moist, laboratory air into a supercooled droplet and ice crystal cloud enhances charge transfer and, when the air-stream is directed at the riming target, can reverse its charge sign. The suggestion is that the extra water vapour introduced increases the supersaturation and influences particle diffusional growth. The results have been considered in terms of the Relative Growth Rate Hypothesis, which states that the interacting ice surface growing fastest by vapour diffusion charges positively. A corollary to this was noted, when dry air is introduced into a cloud of ice crystals so that both the crystals and target surface sublimate, the ice surface that sublimates fastest charges negatively.
Quarterly Journal of the Royal Meteorological Society, 1999
Experiments were conducted with a wind tunnel in a cold room, in order to investigate the influence of the cloud-droplet spectrum on the charges transferred when individual ice spheres collided with a fixed artificial graupel pellet growing by riming. The experiments were carried out with ice spheres of about 100 pm in diameter, impact velocities around 4 m s-', temperatures between -10 "C and -30 "C and effective water contents representative of real clouds. Two different cloud-droplet spectra were used. One had more than 30% of the droplets with sizes greater than 13 pm, and the other had more than 50% of the droplets greater than that. The new results show that the size distribution of the droplets is very important to the sign of electric charge transferred. The target graupel charged positively over all the temperature range covered when the smaller-droplet spectrum was used, but negatively at temperatures below -18 "C for the larger-droplet spectrum. These results show the importance of droplet sizes to thunderstorm charging.
Journal of Geophysical Research, 1996
Measurements of charge transfer between ice particles are reported. The ambient temperature and the average graupel temperature are presented as an alternative pair of variables describing the charge transferred between ice particles during a collision. These variables are alternative to the generally used air temperature and liquid water content. A new charging diagram is also presented here. The results suggest that other variables would be necessary to better describe the phenomena. Some of the current hypotheses on charge transfer are also discussed. The data gathered in the present experiment correspond to collisions made in a wind tunnel between a cylinder growing by riming and ice spheres of 100 /•m in diameter; the velocity was between 5 and 6 m s -1. The ambient temperature was between -8 ø and -24øC. The target temperature was adjusted with a Peltier element keeping the target always equal or above than the ambient temperature. The implications of these results to the thunderstorm electrification are discussed. lnf. rc•c] 11Pflc•n Particle interactions are usually considered as responsible for the electrification of clouds. Laboratory experiments have shown that the separation of electric charge during these kind of interactions is influenced by the particles themselves as well as the environment in which the interactions take place. The magnitude and sign of the separated charge depends, among other things, on the impact velocity, 1)article sizes, ambient temperature, and liquid water concentration, etc. [Takahash, i, 1978; Jayaratne et al., 1983; Gaskell and Illingworth, 1980]. In particular, for ice-ice collisions and using different techniques, Gaskell and Illingworth [1980] and Keith and Saunders [1989] analyzed the dependence of the charge transferred on the impact velocity and particle sizes. The former used individual collisions, and the latter used multiple collisions. Both works show that the magnitude of the separated charge is proportional to the impact velocity and to the sizes of the particles, while the sign appeared to be unrelated to these parameters. Takat, ast, i [1978] and Jayaratne et al. [1983] carried out multiple collisions experiments to measure the charge transferred between a simulated graupel and ice crystals. In both works the simulated graupel was a rotating metal target inside a mixed cloud of ice crystals and water droplets. The charge transferred to the Copyright 1996 by the American Geophysical Union. Paper number 96JD01614. 0148-0227/96/96JD-01614509.00 qim,,lnto,1 o:rauuel was analyzed in terms of the ambient temperature and liquid water content, (LSVC). Although the techniques were similar, the results were not. For 29,609
Charge separation in low-temperature ice cloud regions
Journal of Geophysical Research, 2011
1] New laboratory measurements of graupel pellet charging due to collision with ice particles at low temperature are presented. The experiments were carried out in the temperature range −37°C to −47°C, with an impact velocity of 7 m s −1 and in the absence of supercooled liquid water. The graupel pellet was simulated with a previously rimed brass cylinder of 4 mm diameter, and the small ice particles were formed by natural freezing of supercooled water droplets at low temperature without seeding the cloud. The effective ice water content used in the experiments was between 0.25 and 0.33 g m −3 . Cloud particle samples show small ice particles with diameters up to 24 mm. The results show that the sign of the charging current acquired by the graupel is predominantly negative, and its magnitude ranges from 0 to −150 pA; although there is a significant dispersion of data, a marked dependence on temperature is not observed. It is estimated that the magnitude of the charge transfer per collision is between 0.01 and 0.1 fC; the charging rate of a graupel pellet of 4 mm diameter then would be about 300 pC min −1 . Based on this experimental evidence, we suggest that the charging mechanism associated with graupel-frozen droplet collision and separation may be relevant in clouds whose internal temperatures are substantially lower than −37°C and could be the main generator for high-altitude lightning.
Journal of Geophysical Research, 1998
Further laboratory measurements of charge transfer between ice crystals and riming graupel pellets, which are thought to be associated with the electrification processes within thunderstorms, have been carried out in the University of Manchester Institute of Science and Technology cloud chamber. In experiments with clouds in the temperature range -6 øC to -26 øC, the supercooled droplet spectrum has been extended to larger droplet sizes, above 60/xm maximum diameter, representative of the broadest spectrum observed in some thunderstorm cloud charging regions. The results indicate that at temperatures from -6 øC to -18øC, broadening the droplet spectrum leads to negative graupel charging at higher values of cloud effective liquid water content than has been reported in previous laboratory studies. The significance of the result is that in order to ensure that laboratory experiments simulate as closely as possible the thunderstorm cloud microphysical environment, attention must be paid to the spectrum of droplets used. Two mechanisms of charge transfer that may account for this behavior are discussed, the relative growth rate theory and the surface splinter theory, and both are found to be compatible with the results on the assumption that the larger droplets lead to a reduction in the rate of vapor deposition to the timing surface. Analysis of the implications of these results to thunderstorm electrification requires more details of the evolution of droplet spectra in thunderclouds, their spatial and temporal development and location relative to observed regions of electrification.
Charge separation in thunderstorm conditions
Journal of Geophysical Research, 2008
1] A laboratory investigation of the electric charge transfer in collisions between vaporgrown ice crystals and a riming target is presented in this work. A series of experiments were conducted for ambient temperatures between À8°C and À29°C, air velocity of 8 m s À1 , and effective liquid water content from 0.5 to 10 g m À3 , with the goal of studying the performance of the noninductive mechanism under a wide range of temperature and liquid water content. At low temperatures (below À19°C), the results revealed no dependence of the charge separated per collision upon variations of the liquid water content. While at temperatures above À19°C, the efficiency of the graupel charging could decrease as the liquid water content increases, as a consequence of the decrease of the probability that the ice crystals impact and rebound from the graupel surface in the dry growth regime. We found that the dominant sign of the graupel charging was negative for temperatures below À15°C and positive at higher temperatures. A simple functional representation of our laboratory results is given so that they can be incorporated in cloud electrification models.
Charge separation in updraft of convective regions of thunderstorm
Geophysical Research Letters, 2006
1] The experiments described in this work are concerned with ice-crystal graupel interactions. The influence of the impact velocity on charge separation during collisions is analyzed for three different velocities: 6, 8 and 11 m s À1 . The ambient temperature was varied in the range À5 to À30°C and the effective water content between 0 to 2 g m À3 . Charge diagrams of the sign of the electric current on the graupel as a function of the ambient temperature and the effective liquid water content for each velocity are presented. The results indicate that increasing the velocity leads to negative particle charging during riming at higher velocity and the implications of these findings to non-severe thunderstorm are discussed.
Measurements of initial potential gradient and particle charges in a Montana summer thunderstorm
Journal of Geophysical Research, 1985
An Aerocommander aircraft made three passes through a small isolated thunderstorm on July 19, 1981. These passes were made at a temperature of-5øC and were in conjunction with other aircraft at different altitudes. The Aerocommander was equipped to measure the vertical component of the ambient potential gradient, particle charge, particle size and character, cloud liquid water content, temperature, and vertical velocity. Evidence is presented for the association of charge with ice particles. The development of the potential gradient in relation to movement of these ice particles is discussed. In general, the highest charge densities, of order-0.5 C km-3, were coincident with regions of high graupel concentration and the highest potential gradients, of order-15 kV m-x, were measured below regions with the highest radar reflectivities. Only a small fraction (< 10%) of ice particles were found to be charged significantly (> + 5 pC), but these were found even at the earliest stages of electrification. The results are compared to concepts derived from recent laboratory experiments on the charge produced during ice particle collisions.