Electric Charge Life Cycle in Warm Clouds

Abstract

Warm cloud electricity was extensively studied using a system comprised of raindrop charge radiosondes, raindrop charge-size radiosondes, cloud droplet charge radiosondes, space charge radiosondes, and electric potential.gradient radiosondes. More than 90 radiosondes were launched by balloon, mainly over the land, and also dropped from a helicopter. Special efforts were made to identify visually the cloud life stage at the sounding. The data were analyzed and arranged to present a composite picture of the electric charge profile with respect to the cloud life cycle. It was found that warm clouds produce excess negative charge in-cloud throughout their life cycle. Two charge separation processes appear to occur: one results in the negative charging of cloud droplets and raindrops in-cloud, which occurs mainly in the developing stage. The other produces the positive charging of raindrops near the cloud top, occurring mainly in the mature stage. The raindrop charge distribution in clouds with cloud top height lower than 3 km differs strongly from the distribution in clouds higher than 3 km. This difference parallels the difference in rainfall mechanisms in shallow and deep warm clouds. In the shallow cloud, large raindrops are formed in-cloud and fall against the updraft. In the deep cloud, most drops are carried to the cloud top where large raindrops grow because of the higher updraft. A model of warm cloud electricity has been completed, and the charge separation processes are discussed in this paper. When the cloud is shallow, negatively charged raindrops and cloud droplets generated in-cloud fall, while small droplets evaporate and form a negatively charged layer near the ground. Positively charged raindrops originating from the upper part of the cloud fall through this negative charge layer, and their positive charge is almost cancelled out by that of the negatively charged group. However, when the cloud in deep, large positive raindrops are formed near the cloud top and fall through the regions dominated by negative raindrops and cloud droplets. The heavy rainshower produces a strong downdraft, and the negatively charged group at the cloud top is carried down toward the ground and produces a large negative potential gradient at the ground. The positive raindrops are dispersed over a wide region in space and time, during which negatively charged particles are carded by downdraft for a brief period.