Graupel trajectories and charging: A new numerical approach for cloud electrification studies

In this work the growth, motion and charging of graupel particles are simulated by computer. The cloud fields are calculated by a three-dimensional cloud model. Anothcr program calculates the time evolution, inside these cloud fields, of graupel properties (size, position, charge and others). The electricity enters bolh in the charging of the particles through their interactions with other particles and in the motion itself, since in the equation of motion the electrical force is included. Embryos with 500 p m initial diameter are released at 656 locations uniformly distributed in the updraught centre at four altitudes. The trajectories of all the particles are followed all the way to the 0 C altitude. The graupel is charged by the icc-crystal-graupel non-inductive mechanism. This kind of approach appears to be a powerful tool to study the electric charging of the graupel in greater detail, since a very high spatial resolution (about 1 m) is easily achieved. Some preliminary analysis of the charges on the precipitation particles is carried out and thc rcsults are related to in s i tu observations. For the sake of comparison, trajectories without the electrical interaction are also computed, and it is shown that in some cases the final radius with and without the electrical force can differ by a factor of two to three in either dircction depending on the initial position.