Production of space charge at the boundaries of layer clouds

The ionosphere-Earth current density J_z creates space charge at the upper and lower boundaries of layer clouds. This occurs because clouds have an order of magnitude lower conductivity than the clear air at the same altitude, and as the current density flows through the boundaries, it creates a gradient of electric field that must be satisfied by the accumulation of space charge, according to Gauss's law. We have modeled the production of space charge and its partition between charges on droplets, aerosol particles, and ions and performed sensitivity tests for the variation of a number of relevant atmospheric parameters. We find typical droplet charges of 50-100 elementary charges, positive at cloud top and negative at cloud base, consistent with recent observations. The charges are of sufficient magnitude to suggest measurable electrical effects on scavenging of ice-forming nuclei and cloud condensation nuclei. The results are relevant to the modeling of solar or internally forced changes of J_z and space charge on cloud microphysics as a possible cause of small effects on weather and climate.