Global circuit model with clouds

Cloud data from the International Satellite Cloud Climatology Project (ISCCP) database have been introduced into the global circuit model developed by Tinsley and Zhou. Using the cloud-top pressure data and cloud type information, the authors have estimated the cloud thickness for each type of cloud. A treatment of the ion pair concentration in the cloud layer that depends on the radii and concentration of the cloud droplets is used to evaluate the reduction of conductivity in the cloud layer. The conductivities within typical clouds are found to be in the range of 2%-5% of that of cloud-free air at the same altitude, for the range of altitudes for typical low clouds to typical high clouds. The global circuit model was used to determine the increase in columnar resistance of each grid element location for various months in years of high and low volcanic and solar activity, taking into account the observed fractional cloud cover for different cloud types and thickness in each location. For a single 5° × 5° grid element in the Indian Ocean, for example, with the observed fractional cloud cover amounts for low, middle, and high clouds each near 20%, the ionosphereto-surface column resistance increased by about 10%. (For 100%, fraction-that is, uniformly overcast conditions-for each of the cloud types, the increase depends on the cloud height and thickness and is about a factor of 10 for each of the lower-level clouds in this example and a factor of 2 for the cirrus cloud.) It was found that treating clouds, in the fraction of each grid element in which they were present, as having zero conductivity made very little difference to the results. The increase in global total resistance for the global ensemble of columns in the ionosphere-earth return path in the global circuit was about 10%, applicable to the several solar and volcanic activity conditions, but this is probably an upper limit, in light of the unavailability of data on subkilometer breaks in cloud cover.