Physics Department, UMIST - U.K.
Laboratory studies over many years in the cloud chamber in UMIST have shown that charge transfer to small hailstones (graupel pellets), during rebounding collisions with ice crystals may be a viable thunderstorm charging process. Charges of the order of tens of femto-coulombs are transferred during the brief contact between an ice crystal and a graupel pellet. A requirement of this process is the presence of a supersaturation within the cloud usually provided in real clouds by the presence of supercooled water droplets that may be collected by the graupel to form a surface layer of rime ice.
Extensive studies have shown that the sign of the charge transferred depends on the details of the cloud in the laboratory cloud chamber. In general, at high rates of rime accretion the graupel charges positively and at lower riming rates it charges negatively. The charge sign observed has been attributed to the relative diffusional growth rates of the interacting particle surfaces such that the ice surface growing faster by diffusion charges positively. So, rime heating (during the freezing of the accreted supercooled water droplets) reduces the diffusional growth rate of the rimer below that of the cooler colliding ice crystals causing the riming graupel to charge negatively; the crystals carry away the equal and opposite charge. At higher rime accretion rates, despite the resulting higher rime temperature, the rimer charges positively because its surface is bathed in vapour from the many accreted droplets freezing on its surface. During freezing the droplets have a temperature of 0 °C while the rimer surface is at a lower temperature so that vapour diffuses from the droplets to the rimer causing it to grow by diffusion faster than the colliding colder, slower growing, ice crystals.
Studies of the effects of temperature on the charge transfer show that higher cloud temperatures favour positive rimer charging; this may be because ice crystals grow more slowly at higher temperatures. Studies of the effect of droplet sizes show several effects: a cloud of very small droplets, say 6 µm diameter, are not accreted by the rimer as efficiently as larger droplets so, for the same cloud liquid water content, the rate of rime accretion is lower favouring rimer negative charging. However, the use of large droplets, over 20 µm diameter, has shown that the rimer is heated more efficiently than by the accretion of smaller droplets for the same rime accretion rate. This result is attributed to the smoother rime surface retaining heat because of reduced ventilation in the passing air stream, the heat reduces the rimer diffusional growth rate so that it charges more negatively than with smaller accreted droplets for the same rime accretion rate. On the other hand, there is some evidence from our studies that smaller droplets favour negative charging – if confirmed, this result would be consistent with a higher ice crystal growth rate in a finely divided cloud.
Recent laboratory evidence has shown that the charging processes are even more sensitive to the cloud properties than the above results would suggest so far. New cloud particle measuring technology is allowing us to study the laboratory cloud microphysics in greater detail in order to resolve the outstanding issues in the field of thunderstorm charging. New results from these ongoing studies will be presented at the conference.