A Numerical Simulation of Winter Cumulus Electrification. Part I: Shallow Cloud

Abstract

The development of electricity in a shallow wintertime cumulus was studied using an axisymmetric cloud model containing both microphysical and electrical charge separation processes during graupel formation. The charge separation mechanisms considered included ion induction, ion diffusion, polarization and riming electrification. An unexpected result was that polarization did not intensify cloud electrification. Instead, riming electrification appears to be the principal charge separation process acting to intensify cloud electricity. The cloud is electrified during graupel formation, and a relatively large positive potential gradient forms initially near the cloud top along the cloud boundary between negative graupel and positive ions. Later, graupel particles, electrified positively through riming electrification, produce a relatively strong negative potential gradient between the positive space charge and the upper-level negative space charge produced by snow crystals. As these positively charged graupel particles fall, the positive potential gradient at the ground increases. Due to ion induction, negatively charged snow crystals change sign as they fall through the negative potential gradient field in later stages of the cloud life cycle; thew positively charged snow crystals maintain the positive potential gradient at the ground. When the positive potential gradient is a maximum at the surface, snow crystals become negatively charged near the surface due to ion induction, producing a ?mirror image? between the snow crystal charges and the surface electric potential gradient. Numerical results compare favorably with field observations.