Laboratory studies on cloud electrification during ice crystal-graupel collisions: New parametrization scheme

This study investigates charge transfer during collisions between ice crystals and graupel of varying sizes within a simulated super-cooled cloud environment. Experiments were conducted inside a walk-in cold room reaching temperatures as low as −30°C. Graupel sizes of 2, 4, and 6 mm were collided with ice crystals averaging 70 µm and super-cooled cloud droplets averaging 25 µm at impact velocities of 1.2, 1.8, and 2.2 m s^-1, respectively. Cloud temperature ranged from −6.5° to −18.5°C, and cloud water content varied between 0.1 and 1.72 g m^-3. Results indicate that larger graupel particles acquire a greater negative charge upon collision with smaller ice crystals. Unlike some previous studies, a reversal of charge sign at lower temperatures was not observed for 2- and 6-mm graupel targets. The reversal of charge from +ve to −ve at lower temperatures was observed only for experiments with 4-mm graupel target. Parameterization equations for charge transfer to different graupel sizes across temperature and cloud water content ranges have been developed.