Stratiform cloud electrification: comparison of theory with multiple in-cloud measurements

Stratiform clouds constitute similar to 40% of global cloud cover and play a key role in determining the planetary radiation budget. Electrification remains one of the least understood effects on their microphysical processes. Droplet charging at the top and bottom edges of stratiform clouds arises from vertical current flow through clouds driven by the Global atmospheric Electric Circuit. In-cloud charge data are central in assessing the role of charge in droplet growth processes, which influence droplet size distributions and associated cloud radiative properties and precipitation. This study presents the first high vertical resolution electrical measurements made in multiple layer clouds. Of the 22 clouds sampled, all were charged at their edges, demonstrating unequivocally that all stratiform clouds can be expected to contain charge at their upper and lower boundaries to varying extent. Cloud base and cloud top are shown to charge asymmetrically, with mean cloud-top space charge +32 pC m(-3) and base space charge -24 pC m(-3). The larger cloud-top charges are associated with strong temperature inversions and large vertical electrical conductivity gradients at the upper cloud boundary. Greater charging was observed in low altitude (< 2 km) clouds (20.2 pC m(-3)), compared to higher altitude (> 2 km) cloud layers (7.0 pC m(-3)), consistent with the smaller air conductivity at lower altitudes associated with reduced cosmic ray ionisation. Taken together, these measurements show that the greatest cloud droplet charges in extensive stratiform clouds occur at cloud tops for low altitude (< 2 km) clouds, when vertical mixing is suppressed by appreciable temperature inversions, confirming theoretical expectations. The influence of cloud dynamics on layer cloud edge charging reported here should inform modelling studies of cloud droplet charging effects on cloud microphysics.