Laboratory studies of the effect of cloud conditions on graupel/crystal charge transfer in thunderstorm electrification

Abstract

SUMMARYCollisions 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.In order to help resolve the reason for the differences in charge transfer results in various studies, work has continued in the Manchester laboratory with a modified cloud chamber in which the cloud conditions of the crystals and droplets may be controlled independently. Results indicate a profound effect on the charge sign of the particle growth conditions in the two clouds involved. For example, by suitable adjustments to the water contents of the two clouds, graupel is charged negatively by rebounding ice crystal collisions at higher cloud water contents than have been noted previously. It is suggested that the most important influence on charge sign is the relative diffusional growth rate of the two ice surfaces at the moment of impact and that this is affected by an increase in cloud supersaturation experienced by the ice crystals during the cloud mixing process just prior to collision. A range of cloud conditions is used in the present work in order to help determine the reasons for the various results reported previously.Examination of some thunderstorm observations in the context of the present results points to the importance of mixing on the sign of the charge transferred during particle collisions when two cloud regions of different histories mix together.

Compositional and Mineralogical Effects on Ice Nucleation Activity of Volcanic Ash

“…The study of volcanic lightning is still in its infancy, but it is hypothesized that plume lightning may result from triboelectrification due to ice-ash or ice-ice particle collisions [2,5,6]. The generation of electricity in thunderstorms is largely through the noninductive ice-ice charging mechanism (e.g., [59][60][61]), and a similar process is likely occurring in volcanic plumes, where ash results in an overseeding of ice crystals [8]. As ash concentrations in plumes may be up to 1000 cm −3 [37], a higher abundance of smaller ash particles will increase the likelihood of ice nucleation.…”

On the relationships between lightning frequency and thundercloud parameters of regional precipitation systems

“…Electrical charging occurs mainly in the "charging zone" containing graupel/hail, ice crystals, and supercooled liquid droplets. The efficiency of charge transfer depends on the size of the ice crystals and the fall speed of the graupel pellets [Keith and Saunders, 1990;Pereyra et al, 2000;Saunders et al, 2006].…”

Effect of bacterial ice nuclei on the frequency and intensity of lightning activity inferred by the BRAMS model

“…The noninductive charge transfer is well accepted as the main mechanism for cloud electrification and it is reasonably easy to compute numerically, depending only on ice particle collisions, a task already carried by cloud resolving models. However, the amount of charge transferred during each collision is parameterized in a few laboratory studies (Takahashi, 1978;Saunders et al, 1991;Pereyra et al, 2000;Takahashi and Miyawaki, 2002;Saunders et al, 2006) and it is proportional to the size difference between the ice particles, the temperature inside the cloud, and cloud updraft. These laboratory studies show substantial differences in their methodology and results (Saunders et al, 2006) which can lead to different electrical cloud structures and lightning flash rates of simulated thunderstorms (Helsdon Jr. et al, 2001;Mansell et al, 2005;Altaratz et al, 2005;Barthe and Pinty, 2007;.…”

“…Concerning the non-inductive mechanism as the main process responsible for cloud electrification, there are many observational or modelling studies such as Reynolds et al (1957); Williams and Lhermitte (1983); Dye et al (1989); Rutledge et al (1992); Latham et al (2007), among others. The amount of charge transferred depends on the particle sizes and their fall velocity, while the sign depends on the cloud temperature and supercooled liquid water content (e.g., Takahashi, 1978;Saunders et al, 1991Saunders et al, , 2006Pereyra et al, 2000;Takahashi and Miyawaki, 2002). Due to the equilibrium between vertical pressure gradient and gravitational force, the smaller (less dense) particles are carried aloft into higher regions of the cloud by its updrafts, while larger (denser) particles like hail and graupel are carried only to mid levels inside the cloud, creating major electric charged centers inside the cloud of opposite signs.…”