C. P. R. Saunders

The effect of liquid water on thunderstorm charging

Laboratory studies have shown that thunderstorm charging caused by the interactions of ice crystals and graupel pellets is affected in sign and magnitude by temperature and cloud liquid water content; the presence of water droplets is a requirement for substantial charge transfer. Relationships showing the dependence of charge transfer on ice crystal size and velocity have previously been reported and now, in a continuation of the laboratory studies, the effect of liquid water content on the charge transfer has been investigated. The experiments show that positive graupel charging occurs at temperatures above a “charge sign reversal temperature” with negative charging at lower temperatures. The reversal temperature moves to lower temperatures when the liquid water content is increased. However, at low values of liquid water content, the sign of the graupel charging is inverted being positive at low temperatures and negative at higher temperatures. A discussion is presented of the various charge transfer theories. The results are consistent with the idea of two competing mechanisms whose relative success depends on the temperature and liquid water content. Positive graupel charging occurs when the graupel surface grows from the vapor and the crystals interact with a negative surface charge caused by a temperature gradient across the rime ice surface layer. Negative graupel charging occurs when the surface growth effect is swamped by freezing droplets which create either a pseudo contact potential with which the crystals interact, or a positive surface charge, due to dislocations in the rime ice, which is removed during glancing crystal interactions. Relationships between charge transfer, liquid water content, temperature, ice crystal size, and velocity have been determined and the equations may be used in numerical models of the development of thunderstorm electric fields. A one-dimensional model indicates that the charge separation rates noted here are adequate to account for thunderstorm electrification. Use of the equations with cloud parameter values obtained in a thunderstorm research flight, leads to a predicted charge reversal level around −13°C, which is in agreement with the analysis of the electric field in the thunderstorm studied.

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

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 × 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 rimer 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 −2°C in conditions of low RAR, while at temperatures below −30°C, more positive graupel charging is noted than in the earlier work.

Further laboratory studies of the charging of graupel during ice crystal interactions

Abstract Laboratory simulations of thunderstorm electrification by ice crystal collisions with graupel pellets have been extended by the availability of larger crystals than used hitherto. A new cold-room facility permits crystals up to 800 μm to be grown, and these have been interacted with ice targets in the presence of supercooled water droplets leading to substantial charge transfer. The results show that the previously reported strong dependence of charge transfer on crystal size for small crystals is not applicable to large crystals. A velocity-dependence of the charge transfer has been determined, and a function, Q = AdaVb, describing the charge transfer Q, is given where d is the crystal size, V the speed and A takes certain values for particular ranges of ice crystal sizes. This equation can be used with the appropriate values of A, a and b determined from these experiments, to calculate charging rates in thunderstorms. A new theory of charge transfer due to charged dislocations on the interacting ice surfaces is outlined.

The effect on thunderstorm charging of the rate of rime accretion by graupel

Abstract Analysis of laboratory data concerning the charging of small graupel pellets in thunderstorms has shown that the charge transferred to a riming target during collisions with ice crystals is affected by the rate of rime accretion. Earlier, we found that the magnitude of the charges transferred depend on velocity; however, the velocity also controls the rate of target time accretion which affects the sign of the charge transfer with higher accretion rates favoring positive timers during crystal collisions. Experiments have now been conducted to confirm this effect and a set of equations are presented describing the charge transfer as a function of ice crystal size, velocity, temperature and rime accretion rate. As a first step in determining the importance of the new formulations on thunderstorm electrification, they have been tested in a one-dimensional numerical model. The model results indicate that the normal thunderstorm positive dipole can be generated in the observed time by means of ice crystal/graupel interactions.

Charge Separation Associated with Secondary Ice Crystal Production

Abstract Laboratory studies of rime growth on a moving rod under conditions of secondary ice crystal production show that the rod acquires a positive charge, equivalent to charge associated with each ejected particle of 5 × 10−4C. Ice crystals produced by seeding also impart a positive charge to the rime, equivalent to a charge per particle of 5 × 10−16C. As the water vapor supply is cut off, the charge sign reverses. The results suggest that the sign of the charge transfer depends on the physical state of the rime surface and its vapor pressure excess or deficit relative to the environment. Charge separation in convective clouds is critically dependent on the changing proportion of graupel and small secondary ice crystals.

An experimental investigation of the inductive mechanism of thunderstorm electrification

A laboratory study of the inductive charging mechanism has been carried out, in which conducting spheres are allowed to fall through a region of high uniform field in a cloud of supercooled water droplets. The mean charge transfer was measured and found to be of the same order as the theoretical value for the same conditions.