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Recent Results in Thunderstorm Electrification

by Dr. Christopher Emersic

Fig. 1. Sketch of electrified thunderstorms. Storms of different sizes occur around the world throughout the year, yet they share a common charge structure that is dependent on temperature and not physical height. Adapted from Krehbiel (1986).

An overriding historical field of research making extensive use of the cold room and cloud chamber facilities in Manchester has been the study of the electrification of thunderstorms (Fig. 1). A highly contentious scientific field, Manchester has provided decades of leading research into study of the now widely accepted main charging mechanism of most thunderstorms. This mechanism involves collisions between riming graupel particles and ice crystals, most commonly (but not exclusively) in the presence of supercooled water droplets, and is described by the Relative Growth Rate theory.In the chamber, we simulate a riming graupel particle with a small metal rod which rapidly becomes rimed through accretion of supercooled water droplets. Collision and subsequent separation of ice crystals with this rimed surface leads to charge transfer which is measured by a sensitive charge amplifier for later comparisons with the chamber conditions.

Our experiments have involved the use of both ‘one-cloud’ and ‘two-cloud’ experimental setups. In a single-cloud setup, the riming rod rotates in the same cloud containing ice crystals and supercooled water droplets. In a two-cloud experiment, separate clouds of ice crystals and supercooled water droplets are generated, which are then mixed together briefly through a carefully designed tubing system before interacting with the stationary rod (Fig 2.). These two types of experimental setups have been shown to give very different resulting charge transfers, as discussed in more detail later.

Fig. 2. One-cloud experiments operate differently to two-cloud experiments. In a one-cloud experiment, the target rod rotates in a cloud of supercooled water droplets and ice crystals. In a two-cloud experiment, separate clouds of droplets and crystals are grown and are then mixed together briefly before interacting with the target. We found this affects the surface properties of the ice crystals prior to interaction with the target in accordance with the Relative Growth Rate theory. Results from one-cloud experiments are therefore fundamentally different from and unresolvable with those from two-cloud experiments, owing to the different microphysics involved.

The two main parameters that are most easily altered in the lab studies are the cloud liquid water content and the cloud temperature. It is consistently observed that the polarity of graupel charging tends be to positive at higher temperatures and negative at lower temperatures, with lower temperatures required for polarity reversal as cloud liquid water content increases (Fig. 3).

The focus in recent years of Manchester research has been on examining the Relative Growth Rate theory—the most successful explanation of the charge transfer between ice crystals and riming graupel independently of an ambient electric field. The Relative Growth Rate theory reads as “the particle growing fastest from vapour diffusion at the instant of collision will charge positively on separation”. This hypothesis was originally empirically observed and has stood the test of time in subsequent decades of experiments.

Fig. 3. The resulting reversal lines achieved are highly dependent on experimental technique, which in turn affects the microphysics of the interacting particles’ surfaces, and to which the results are very sensitive. Different researchers over the years have achieved different reversal lines and this led to contention in the literature over who was right. At the time, it was not clear that the microphysics played such an important roll in the outcome, nor that the choice of experimental arrangement had such a critical influence. Recently we have gained an understanding into how the microphysics determines results (based around the Relative Growth Rate theory) such as to be able to reproduce the results of others, supporting our suspicions

In examining the Relative Growth Rate mechanism, several key findings have been made. One particular recent important discovery has related to the charging effects of changes in type of experimental setup and cloud considerations discussed above. Historically, many different researchers around the world have performed similar cloud chamber experiments looking at charge transfer between graupel and ice crystals. All researchers appeared to have the same environmental conditions and variables, but their results were often strikingly different (Fig. 3). This led to infamous arguments in the literature over which research was correct, and this debate was further fuelled when numerical models of storms used the different researchers’ parameterisations from the studies to simulate observed storms. Understanding through examination of the Relative Growth Rate theory has led us to realise that it is the surface conditions of both interacting particles at the instant of collision that governs the resulting charge transfer, and these are, in turn, affected very sensitively by the environmental conditions. We have identified subtleties in the precise experimental conditions of our and others’ experimental studies that were unknown previously, that have likely accounted for the discrepancies in results. We have recently been able to reproduce the results of others to verify our hypotheses, and furthermore, gained a much deeper understanding of why those differences arose thanks to the Relative Growth Rate theory. In particular, there has been a long standing discrepancy between the results of studies in Japan in the late 70s and of the Manchester group in the early 80s—a source of much contention in the literature. Our most recent published work has likely identified the causes of this discrepancy by identifying counterintuitive conditions likely in the Japanese experiments leading to the results that were observed. Despite using a one-cloud setup in the Japanese studies, the chamber dynamics likely were such as to produce the two-cloud–like reversal line that was observed.

Another area of interest has been associated with the so-called “anomalous zones” of charging in EW-T plots. An EW-T plot (effective-water content–cloud temperature plot) (Fig. 3) represents the cloud temperature (on the horizontal axis) and the target accreted liquid water content during a given accretion time (on the vertical axis) at which the polarity of graupel charging reverses. We use effective water content (which is the product of the rimer (target) collection efficiency and the cloud liquid water content) rather than simply cloud liquid water content because this is a measure of that portion of the cloud liquid water that was accreted by the rimer, and thus directly involved in the charge transfer process. Under certain conditions, there are regions in EW-T parameter space at low EW in which the polarity of charging can be reversed from that of its surroundings and what is expected. These are dubbed the ‘anomalous zones’ (Fig. 4). Furthermore, under similar special conditions, the magnitude of charging in these regions has been observed to be up to an order of magnitude greater than that of the surroundings. This caused problems with numerical model simulations of thunderstorms, and calls for their rejection were proposed. Recent experimental work in the chamber however has confirmed their existence by reproducing them, supporting them as a real phenomenon.

Fig. 4. The anomalous zones in EW-T parameter space represent regions of charging at low values of EW that are the opposite to their surroundings. They do not always appear, and if they do, can sometimes be associated with an order of magnitude more charge transfer than normal. These zones were originally reported by Saunders et al. (1991) but were not reproducible by others, and caused sufficient contention in the literature that calls for their dismissal were made. They have recently been re-observed in Manchester and confirmed to be a real phenomenon.

There is scope for further electrification research in the new cold room facility, with the tall tube allowing for the growth of large crystals which are known to affect the resulting charging.

References.

Emersic, C., 2006: Investigations into thunderstorm electrification processes. Ph.D., Physics and Astronomy, The University of Manchester.

Krehbiel, P. R., 1986: The electrical structure of thunderstorms. The Earth's Electrical Environment, eds. E. P. Krider and R. G. Roble, Washington DC: National Academy Press., 90–113.

Pereyra, R. G., E. E. Avila, N. E. Castellano, and C. P. R. Saunders 2000: A laboratory study of graupel charging. J. Geophys. Res., 105, 20803–20812.

Saunders, C. P. R. and S. L. Peck 1998: Laboratory studies of the influence of the rime accretion rate on charge transfer during crystal/graupel collisions. J. Geophys. Res., 103, 13949–13956.

Saunders, C. P. R., H. Bax-Norman, C. Emersic, E. E. Avila, and N. E. Castellano 2006: Laboratory studies of the effect of cloud conditions on graupel/crystal charge transfer in thunderstorm electrification. Quart. J. Roy. Meteor. Soc., 132, 2653–2673.

Takahashi, T. 1978: Riming electrification as a charge generation mechanism in thunderstorms. J. Atmos. Sci., 35, 1536–1548.