Abstract
Abstract. In these experiments, the electric charge carried by single particles ejected from the surface of a graupel particle growing by riming was measured. Simulated graupel pellets were grown by accretion of supercooled water drops, at temperatures ranging from −2 to −10 °C in a wind tunnel at air velocities between 5 and 10 m s−1, with the goal of studying the charging of graupel pellets under conditions of secondary ice crystal production (Hallett-Mossop mechanism). The graupel, and induction rings upstream and downstream of the graupel, were connected to electrometers and analyzing circuits of sufficient sensitivity and speed to measure, correlate and display individual charging events. The results suggest that fewer than 1% of the ejected particles carry a measurable electric charge (>2 fC). Further, it was observed that the graupel pellets acquire a positive charge and the average charge of a single splinter ejected is −14 fC. This mechanism of ejection of charged particles seems adequate to account for a positive charge of around 1 pC that individual precipitation particles of mm-size could acquire in the lower part of the cloud, which in turn could contribute to the lower positive charge region of thunderstorms.
Highlights
Hallett and Mossop (1974) and Mossop and Hallett (1974) observed that ice splinters were ejected during the formation of rime by the accretion of supercooled droplets on a cylinder of diameter 2.4 mm at 3 m s−1, providing that the air temperature is between −3 and −8 ◦C
Dong and Hallett (1989) point out that the spreading is effective at temperatures above −3 ◦C because the liquid like layer on the ice surface is thicker at higher temperatures
While for the low temperature cut-off at −8 ◦C, Griggs and Choularton (1983) say that at temperatures below −9 ◦C several ice dendrites cross the liquid droplet from the substrate side to the outside which initiates freezing at several points on the outer surface of the droplet so that the freezing front moves inwards, making the shell very strong, so it does not fragment; Dong and Hallett (1989) suggest smaller and more concentrated air bubbles will be formed at lower temperatures because the solubility of air in water increases at lower temperatures
Summary
Hallett and Mossop (1974) and Mossop and Hallett (1974) observed that ice splinters were ejected during the formation of rime by the accretion of supercooled droplets on a cylinder of diameter 2.4 mm at 3 m s−1 , providing that the air temperature is between −3 and −8 ◦C. Hallett and Saunders (1979) 2 Experimental determined in an indirect way the electric charge of the ejected fragments by measuring the charging current to a riming rod moving through a cloud of supercooled water droplets under conditions of secondary ice crystal production.
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