Laboratory studies of the charging of soft‐hail during ice crystal interactions

Abstract

A laboratory investigation of electric charge transfer during the impact of vapour-grown ice crystals and supercooled water droplets upon a simulated soft-hailstone target has shown that the magnitude of the charge transferred to the riming surface when crystals separate from it is a function of temperature, crystal dimension, relative velocity, liquid water content, and impurity content of the water droplets and hence the impurity content of the riming target. The sign of the charge transfer depends on temperature, liquid water content and droplet and rime impurity content.In the absence of crystals, no charge transfer was detected during riming. In the absence of supercooled water droplets, crystals impacting at 10m s-on an evaporating rime target produced a small negative charge on the rime of less than -0.25fC per separating crystal. When the target surface grew by vapour diffusion it gained a small positive charge during such interactions. Much larger charges and completely different charge transfer behaviour was noted during riming. The target became positively charged at high liquid water contents and temperatures above a critical value, but negatively charged at lower temperatures or with lower liquid water contents. The critical sign reversal temperature at a liquid water content of 1 g m -3 was about -20°C. At -10°C with a liquid water content of 2gm-3, a 125pm crystal impacting at 3ms-I charged the target by + lOfC upon separation. The charge transfer increased sharply with impact speed and crystal size. Warming the positively charging rime to cause it to evaporate failed to reverse the sign of the charge transfer. Experiments with impurities showed that the sign reversal temperature increased if the droplets contained contaminants at concentrations found in cloud water.It is suggested that there are two distinct charge transfer processes during crystal interactions with an ice target, the dominant one requiring the presence of supercooled water droplets. Careful control and knowledge of the microphysical properties of the clouds used in these experimental simulations has permitted an examination of charge transfer under many of the conditions used in previous studies. The results provide an understanding of the differences and a reconciliation between some of the previously disparate findings in terms of the two distinct charge transfer regimes.

Observing relationships between lightning and cloud profiles by means of a satellite-borne cloud radar

“…graupel or hail pellets) that collide with ice crystals in a suspension of supercooled water droplets (Reynolds et al, 1957;Takahashi, 1978;Berdeklis and List, 2001;Adamo et al, 2007). Laboratory studies by Takahashi (1978) and Jayaratne et al (1983) also showed that the charge sign is positive or negative as a function of the cloud temperature and liquid water content (LWC). Further laboratory studies have investigated the impact of cloud conditions on electrification (see Saunders et al, 2006), which also underline the importance of mixing solid and liquid hydrometeors.…”

Effects of the Inductive Charging on the Electrification and Lightning Discharges in Thunderstorms

“…The time-height graupel mixing ratio and number concentration (Fig. 4b) show a maximum (Jayaratne et al 1983;Gardiner et al 1985;Ziegler et al 1991). The reversal temperature (for CWC > 0.1 g m -3…”

“…Since the non-inductive charge separated during collisions between ice-phase particles generally depends on temperature, the relative velocity of the collisions, hydrometeor concentration and the supercooled droplet size spectrum (Takahashi 1978;Jayaratne et al 1983;Gardiner et al 1985;Saunders et al 1991;Brooks et al 1997;Saunders and Peck 1998), the contributions of the non-inductive charging process to electrification in these four cases are the same. The time evolution of the non-inductive charging rate is presented in Fig.…”

“…The noninductive charging separation mechanism has been well examined using a host of laboratory experiments and model studies (Takahashi 1978;Jayaratne et al 1983;Gardiner et al 1985;Saunders et al 1991;Ziegler et al 1991;Saunders and Peck 1998). In addition, the comparison of laboratorybased non-inductive charging parameterizations in a full simulation model (with coupled dynamics and microphysics) has revealed significant differences between the results (Helsdon et al 2001;Mansell et al 2005).…”

Atmospheric Electrification in Dusty, Reactive Gases in the Solar System and Beyond

“…In the first stage, macroscopic particles are electrically charged, and in the second stage particles of different polarity are separated by gravitation or other (mechanical) forces; in order to separate particles with different polarities, these forces need to be stronger than the electric attraction between charges of different polarity, since otherwise the electric forces would counteract the growth of the electric field. The possible charging mechanisms at work within normal terrestrial thunderclouds are reviewed, e.g., by Jayaratne et al (1983) and Saunders (2008). An important conclusion of these reviews is that charge is efficiently separated between particles only in direct collisions.…”