Laboratory studies of the influence of the rime accretion rate on charge transfer during crystal/graupel collisions

Abstract

Abstract. The process of thunderstorm electrification by charge transfers between ice crystals and riming graupel pellets (the noninductive process) has been the subject of extensive study in the laboratory in Manchester. Quantitative dependencies of the sign and magnitude of charge transfer have previously been determined as functions of ice crystal size, graupel/crystal relative velocity, temperature, and the effective liquid water content (EW) in the cloud experienced by the riming graupel pellets. We now present results of laboratory studies of thunderstorm charging in terms of the rime accretion rate (RAR = EW x V), which combines into one variable the velocity and EW dependence of the sign of graupel charging on temperature. The magnitude of the charge transfer can be determined from its dependence on the crystal size and graupel velocity, while the sign of the timer charging can now be determined from a new figure showing the dependence of the charge sign on RAR and temperature. This figure may be used to compare charge transfer results from other laboratories obtained over a range of graupel/crystal velocities. These new experiments extend the temperature range of the previous studies and indicate that negative charging of graupel can occur at temperatures as high as -2øC in conditions of low RAR, while at temperatures below-30øC, more positive graupel charging is noted than in the earlier work.

Role of Water Vapor Content in the Effects of Aerosol on the Electrification of Thunderstorms: A Numerical Study

“…The polarity of charges carried by ice particles after the rebounding collision depends on the difference between the RAR and the RARC [34]. If RAR > RARC, then the graupel particles carry positive charge and the rebounding ice crystals or snow particles carry negative charge.…”

“…Under conditions with a low water vapor content, increasing aerosol concentrations did not lead to a reversal of charge carried by the ice particles, resulting in similar charge structures between clean and polluted simulations. The polarity of charges carried by ice particles after the rebounding collision depends on the difference between the RAR and the RAR C [34]. If RAR > RAR C , then the graupel particles carry positive charge and the rebounding ice crystals or snow particles carry negative charge.…”

“…The mean flash rate was increased by about 16.7% as the initial CCN concentration changed from 300 to 1500 cm -3 . The electrification processes in thunderstorms are caused by rebounding collisions between ice-phase particles [34]. As the thunderstorm develops, the charges carried by ice-phase particles accumulate and this will lead to an increase in the electric field in the cloud.…”

A laboratory study of graupel charging

“…In their study, in the temperature range - Rime accretion can heat the target sufficiently for it to sublimate; equation (3) relates the rime surface temperature to A mechanism proposed by Baker et al [1987] and developed by Jayaratne [1993] relies on the relative growth rates of the interacting particles; in their hypothesis the particle whose surface grows fastest by vapor diffusion charges positively. This mechanism has been used by Saunders and Peck [1998] to account for their results in which the rimer charges negatively at low EW and positively at high EW. For low EW, the heat from rime accretion ensures that the rimer grows by diffusion slower than the crystals, and so the rimer charges negatively.…”

Theory of charge and mass transfer in ice‐ice collisions

“…Larger collisional losses will, of course, produce greater depths. 1997; Saunders and Peck, 1998;Jayaratne, 1996Jayaratne, , 1998], and functional equations have been fitted to the data over limited regimes. Keith and Saunders [1990] showed that the charge transfer to the target depended on crystal size and speed as power laws, with exponents varying over particular ranges of size and temperature in the positive and negative charging regimes.…”