Laboratory studies of the influence of cloud droplet size on charge transfer during crystal-graupel collisions (original) (raw)
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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.
Charge transfer during crystal-graupel collisions for two different cloud droplet size distributions
Geophysical Research Letters, 2000
Laboratory experiments of graupel charging during ice crystal collisions reveal a charge sign dependence on temperature, liquid water concentration as well as the size distribution of the cloud droplets used for riming. The measurements were performed with an impact velocity of 8.5 m s-1, the ambient temperature was varied in the range -5 to -30°C and the effective water content up to 4 g m-3. Charge diagrams of the sign of the electric current on the graupel as a function of the ambient temperature (Ta) and the effective liquid water content (EW) for each of the two cloud droplet spectra used are presented. The results indicate that broadening the droplet spectrum leads to negative graupel charging and the implications of these findings to thunderstorm electrification are discussed.
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.
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.
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.
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.
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.
Atmospheric Chemistry and Physics, 2010
In these experiments, the electric charge carried by single particles ejected from the surface of a graupel particle growing by riming was measured. Simulated graupel pellets were grown by accretion of supercooled water drops, at temperatures ranging from −2 to −10 • C in a wind tunnel at air velocities between 5 and 10 m s −1 , with the goal of studying the charging of graupel pellets under conditions of secondary ice crystal production (Hallett-Mossop mechanism). The graupel, and induction rings upstream and downstream of the graupel, were connected to electrometers and analyzing circuits of sufficient sensitivity and speed to measure, correlate and display individual charging events. The results suggest that fewer than 1% of the ejected particles carry a measurable electric charge (>2 fC). Further, it was observed that the graupel pellets acquire a positive charge and the average charge of a single splinter ejected is −14 fC. This mechanism of ejection of charged particles seems adequate to account for a positive charge of around 1 pC that individual precipitation particles of mm-size could acquire in the lower part of the cloud, which in turn could contribute to the lower positive charge region of thunderstorms.
A numerical study of thunderstorm electrification: Model development and case study
Journal of Geophysical Research, 1991
We have developed a numerical model for examining the thunderstorm electrification process in which we assume the electrification is entirely due to noninductive charge transfer between colliding ice crystals and hail. Since this ice-hail charge mechanism is very dependent on particle sizes and distributions, we use an explicit microphysical framework. To maintain simplicity, the electrification model is kinemati• thus the temperature and velocity fields are input into the electrification model. These fields can be either calculated by a background model or retrieved from observations. For this study, we have used the cloud model of Taylor (1989) to generate the temperature and velocity fields to examine the July 19, 1981, CCOPE thundercloud. Using these fields, the electrification model produced time-dependent ice particle concentrations, radar refiectivities, charge and vertical electric field distributions in good general agreement with those observed. The model produced a maximum electric field strength of 1.27 kV cm -•, which is on the order of that needed for lightning initiation, and this maximum occurred very close to the time of the observed discharge (as inferred by the sailplane measuremenu). Thus the ice-hail charge mechanism a•rs to have played an important role in the electrical development of the July 19 cloud. The details of the electrification depended on the liquid water content and the glaciation proceases, and particularly on the ice crystal characteristics. Rapid growth of the crystals to riming sizes (> 400 g) yielded the most efficient charging. The electrification was also sensitive to the ice-ice sticking efficiency but not to the characteristics of the large riming ice. , 1974], and cloud top entrainment of ice nuclei. The relative importance of these contributing glaciation processes depends on environmental conditions, and they produce very different vertical distributions of small ice particles. While hail and supercooled drop concentrations can be significant at temperatures not too far below 0øC, the observed concentrations of small ice particles are often much higher than the ice nucleus concentration at that temperawe. Therefore, ice particles are probably formed at colder temperatures higher in the cloud and transported to warmer temperatures by downdrafts. Finally, the concentrations of ice particles within clouds depend directly and indirectly upon entrainment, which brings dry air and nuclei into the cloud and may modify the development of ice particles Hobbs and Rangno, 1985]. Because these processes are linked,
Journal of the Atmospheric Sciences, 2020
In this two-part paper, influences from environmental factors on lightning in a convective storm are assessed with a model. In Part I, an electrical component is described and applied in the Aerosol-Cloud model (AC). AC treats many types of secondary (e.g., breakup in ice-ice collisions, raindrop-freezing fragmentation, rime splintering) and primary (heterogeneous, homogeneous freezing) ice initiation. AC represents lightning flashes with a statistical treatment of branching from a fractal law constrained by video imagery. The storm simulated is from the Severe Thunderstorm Electrification and Precipitation Study (STEPS; 19/20 June 2000). The simulation was validated microphysically [e.g., ice/droplet concentrations and mean sizes, liquid water content (LWC), reflectivity, surface precipitation] and dynamically (e.g., ascent) in our 2017 paper. Predicted ice concentrations (;10 L 21) agreed-to within a factor of about 2-with aircraft data at flight levels (2108 to 2158C). Here, electrical statistics of the same simulation are compared with observations. Flash rates (to within a factor of 2), triggering altitudes and polarity of flashes, and electric fields, all agree with the coincident STEPS observations. The ''normal'' tripole of charge structure observed during an electrical balloon sounding is reproduced by AC. It is related to reversal of polarity of noninductive charging in ice-ice collisions seen in laboratory experiments when temperature or LWC are varied. Positively charged graupel and negatively charged snow at most midlevels, charged away from the fastest updrafts, is predicted to cause the normal tripole. Total charge separated in the simulated storm is dominated by collisions involving secondary ice from fragmentation in graupel-snow collisions.