A LABORATORY STUDY OF STATIC CHARGING BY FRACTURE IN ICE GROWING BY RIMMING

Charge transfer by fracture between colliding ice particles is measure d using a wind tunnel inside a cold room. The charges were recorded on e by one and the mechanics of each collision was studied in detail. Th e sign of the temperature gradient at a given point of impact on the s urface influences the sign of the charge transfer. In this work we ext end previous measurements to particles growing by riming. A cylinder g rowing by riming in a wind tunnel was used as target for collisions at 5 m s-, with ice spheres of 100 mum in diameter in a wide range of te mperatures. Charge transfers of either sign were also observed here an d according to the ambient conditions, one of sign prevailed on averag e. The transferences ranged from +/- 50 fC at higher temperatures incr easing in magnitude to -80 fC at lower temperatures. There was ample e vidence of fragmentation following most of the collisions. The influen ce of the growth regime on the charging and the production of fragment s were also studied. A physical model of the microscopical mechanism l eading to the charging of the fragments is presented. The laboratory e vidence, when applied to a thundercloud, suggests that the proposed me chanism could operate in a wide range of conditions and even explain s ome observations in situ related to the abundance of ice crystals. Com parisons are made with other experiments and models on static charging of ice.