Numerical Experiments of the Charging Mechanism of Precipitation Particles by the Ion-capture Process below the Cloud Base

[摘要] To investigate the mirror image relation (hereinafter, M.I.R.) observed at the ground surface, a new formulation in which snowflakes and snow particles acquire charges by the selective ion capture process is lead forth and one dimensional numerical experiments are carried out in this paper.The M.I.R. is more clear during snowfalls than during rainfalls and charges on these snow particles are also larger than that of raindrops in which the diameter is equivalent to the melted diameter of snow particles. In their experiments, Asuma and Kikuchi (1983), reported that although snowflakes have numerous pores in themselves, the final charges expected from the selective ion capture process (-12πε0Fa2) are the same as that of a conductive sphere of the same diameter. It is expected therefore that the high porosity of snowflakes leads to the increase of the collection efficiency of ions and it requires a shorter time to attain their final charges. We call this "the penetration effect". The formulation of a charging mechanism for snowflakes including this effect was derived.The modifications of charges on precipitation particles under the cloud base are examined by numerical experiments. Two types of precipitation particles are tested in these experiments, that is, snowflakes and raindrops. In case of snowflakes, the comparison Whipple and Chalmers (1944) formulation with our formulation was discussed. The main results obtained are as follows;(i) There is a tendency that on snowflakes observed on the ground surface hardly keep the electric charges produced in the cloud but raindrops keep the charges produced in the cloud. This tendency would be caused by the differences of their falling speeds.(ii) The penetration effect is more effective near the ground surface where ions are emitted abundantly in the atmosphere by the point discharge.(iii) On the ground surface, the precipitation current and the point discharge current are balanced and this balanced relation brings about the M.I.R.

[学科分类] 大气科学