Enhancement of cloud formation by droplet charging

Abstract

Charge affects the activation of cloud droplets by reducing the minimum supersaturation at which haze droplets begin to grow. Although the droplet charge required to enhance activation is substantial, we show that sufficient charging occurs at the edges of layer clouds because the fair-weather current in the global atmospheric electrical circuit flows through a discontinuity in conductivity. Our theory predicts that droplet neutralization will cause a transient cooling of cloud base. This hypothesis was tested during a period of extreme solar activity, when we detected transient current bursts at the surface beneath a layer of cloud. We attribute these to bursts of ion production, which would cause transient droplet neutralization in the cloud and an associated increase in droplet critical supersaturation. We observed transient decreases in downward long-wave radiation measurements coincident with the transient current bursts. As the vertical current density passing through stratiform clouds is a global phenomenon, there are many regions in which a charge enhancement effect on cloud formation can potentially occur; we find that the effect of charge-enhanced activation on surface radiation in the present-day climate could be as large as 0.1 W m −2 .