The ice crystal-graupel collision charging mechanism of thunderstorm electrification

Abstract

The ice crystal-graupel collision charging mechanism, which is considered important in thunderstorm electrification, was studied using the newly developed Triple Interaction Facility that allows independent control of the solid, liquid, and vapor phases of a simulated cloud. The advanced experiment led to the discovery of a new, dominant effect on charge transfer: the effect of relative humidity at which the ice crystals grow. It exceeds the impact of temperature and liquid water content (LWC). Higher relative humidity (close to water saturation) always promoted stronger negative charging, while lower humidity (close to ice saturation) led to weaker negative or stronger positive charging. The effect was greatest at temperatures of around -15°C± 3°C. Newly established was also a velocity dependence on charging with a maximum at relative graupel-air speed of ∼5 m s-1. The reversal temperature, previously considered to be unique at a given LWC, was found to be also a function of the relative humidity (RH). Now, changes in RH can explain quantitatively the differences between once controversial observations of previous investigators. A parameterization of the results is presented for use in numerical thunderstorm models, and a new conceptual model of thunderstorm electrification is suggested.