Abstract
Laboratory investigations have been carried out to find the sign and magnitude of charge transfer during the collision between ice crystals and graupel in the presence of supercooled water droplets inside a cylindrical steel chamber kept inside the walk-in cold room, which can reach a temperature of -30C. The experiments were performed with pure water (Milli-Q, 18.2 Megohms-cm) and with a trace amount of ammonium sulfate and sodium chloride at different concentrations at low rime accretion rate (<0.1 g m-2 s-1), similar to stratiform clouds in the temperature range -6C to -26C. The cloud was composed of supercooled water and vapor was produced by boiling premixed solutions containing trace amount of impurities. The impurities are carried into the cloud by the bubble-breaking mechanism. This way of introducing impurities is different from the earlier experiments. The experiments were carried out at a constant impact velocity of 2.2 m/s. At higher temperatures, the sign of the graupel charging during the ice crystal-graupel collision for pure water obeys that of earlier studies. With a trace amount of ammonium sulfate and sodium chloride, the sign of charge transfer is different from that of pure water. The effect of cloud droplet spectra on the sign of charge transfer for pure water were in agreement with earlier studies but differed for sodium chloride.