Laboratory Experiments on the Microphysics of Electrified Cloud Droplets

The global electric circuit (GEC) is one of the candidates for a coupling of terrestrial climate with solar activity [Friis-Christensen, in Space Sci. Rev. 94(1–2): 411–421, 2000]. It has been suggested that vertical electric currents in the atmosphere can modify cloud microphysics and thereby alter the properties of the earth’s cloud system [Tinsley, in Space Sci. Rev. 98:16889–16891, 2000]. In the framework of the Deutsche Forschungsgemeinschaft (DFG) priority program CAWSES we conducted laboratory experiments which quantify the influence of electric charges on the microphysics of cloud droplets in order to assess the atmospheric relevance of this link between the GEC and the cloud system. More specific, we quantify the influence of charges, electric fields and ionizing radiation on the heterogeneous and homogeneous nucleation in cloud droplets. These experiments were carried out on individual electrified cloud droplets using an electrodynamic balance enclosed by a miniaturized climate chamber to provide realistic atmospheric conditions. Several effects that could link the electrical state of the atmosphere to cloud microphysics have been investigated and quantified. While no direct effect of cloud droplet charge on homogeneous freezing was found, we were able to confirm and quantify the enhanced scavenging of aerosol particles by charged cloud droplets. Together with the first direct measurement of size dependent contact freezing probabilities it is now possible to quantify the role of charges for cloud glaciation in cloud models. Additionally, a substantial effect of cloud droplet and ice charge on the vapor pressure of these cloud elements has been found that has so far not been taken into account in cloud modeling.