Contribution of the ground corona ions to the convective charging mechanism

The convective charging mechanism of thunderclouds is based on the vertical transport of space charge generated by corona from ground irregularities under the influence of the surface electric field. The present work estimates the amount of charge which is expected to reach cloud base by conduction and convection processes during the Lifetime of a thunderstorm. This estimate is made using the numerical model PICASSO, previously designed to characterize the evolution of this corona space charge between ground level and cloud base. Experimentally determined values of surface electric field are introduced into the model in order to initiate the computation. These values are based on six events documented during four different field experiments carried out in Florida and in France. As an upper Limit, the convective transport is uniformly applied as a linearly increasing vertical air speed profile and competes with the conductive transport. The fraction of the positive charge generated at the surface by corona which finally reaches the upper limit of the layer varies between 26 and 86%, essentially depending on the electric field evolution at altitude. Assuming that the vertical transport conditions remain the same over an area of 10 km X 10 km, the overall charge amount can be roughly estimated. It ranges between about 63 and over 300 C. Because the present assumptions probably lead to an overestimate of these amounts, such a range suggests that the convective charging mechanism is unlikely to be able to account for the major electrification process of the thundercloud. However, it could be considered as a relevant mechanism contributing to the lower positive charge center of the thundercloud, often observed close to cloud base. (C) 1999 Elsevier Science B.V. All rights reserved.