A laboratory study of static charging by fracture in ice growing by riming (original) (raw)
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Charge transfer during individual collisions in ice growing by riming
The charging of a target growing by riming in a wind tunnel has been studied in the temperature range of (-10, -18'C). For each temperature, charge transfers of both signs are observed and according to the environmental conditions one of them prevails. The charge is more positive as the liquid water concentration is increased at any particular temperature. It is found that even at the low impact velocities used (5 m/s) there is abundant evidence of fragmentation following the collision.
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
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.
Cloud electrification by fracture in ice-ice collisions: a 3D model
Atmospheric Research, 1995
A study to evaluate the electrification caused by only one kind of noninductive mechanism, namely ice-ice collisions with fractures is presented. Its ability to reproduce the most important features of" cloud electrification is discussed. The study is carried out numerically and use is made of recen! laboratory data. The model has the dynamical and electrical developments uncoupled. The particle spectrum is discrete with eight categories and the fragments increase the number of only the smallesl of them. It is shown that high fields (250 kV/m) are produced by fracture charging in times of about 3 rain. It is also found that positive and negative charge distributions overlap considerably and their superposition results in a highly stable stratification forming a vertical tripole structure. The charge: on individual particles so obtained are in good accord with the experimental data reported by airborne: in situ measurements.
Charge transfer measurements during single ice crystal collisions with a target growing by riming
Journal of Geophysical Research, 2002
1] Direct measurements of the electric charge separated from individual collisions between vapor grown ice crystals and an artificial graupel growing by riming have been made in the present laboratory work. The measurements were performed with an impact velocity of 8.5 m s À1 , the ambient temperature was varied in the range À5 to À20°C, the average ice crystal sizes between 20 to 40 mm, with an effective liquid water content up to 1.5 g m À3 . The magnitude of the charge separated per collision is on the order of 10 fC, and the sign of the average charge depends on the ambient temperature for the present EW. We found that the artificial graupel charges positively for temperatures above À12°C and negatively for temperatures below À14°C. The current results are compared with those obtained by other authors that used the multiple crystal collision technique.
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.
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.
Atmospheric Research, 2003
Experimental evidence is presented that shows that substantial electric charge separation can occur by the ejection of charged particles during the riming process. Measurements were performed at temperatures between À 15 and À 30 jC, velocities between 10 and 30 m s À 1 and cloud water contents between 0.2 and 1 g m À 3 . Results show that positively or negatively charged particles were detached from an artificial hailstone, with individual charges ranging between À 0.3 and 0.5 pC. This mechanism could separate approximately 5 pC min À 1 cm À 2 of hail surface area. The charge of the ejected particle depends on the environmental temperature. Charge separation was detected at air velocities above 17 m s À 1 , which indicates that the mechanism could operate in the presence of hailstones larger than 1 cm in diameter. This mechanism is distinct from the usual noninductive mechanism and does not require ice particle collisions to separate electric charge. The physical mechanism has not been elucidated yet, but the results suggest that the ejected particles could be produced by splashing of droplets impinging on the artificial hailstone or by detachment of rime fragments accreted on it. D