Cloud Electrification and Lightning Model in the COSMO Numerical Weather Prediction Model

We developed a Cloud Electrification and Lightning Model (CELM) and implemented it in the microphysical scheme of the COSMO non-hydrostatic and convection-permitting 2-moment numerical weather prediction model. CELM explicitly describes the electrification of a thundercloud; it explicitly treats the ion motion including the interaction of ions with six kinds of hydrometeors (vapour, ice, graupel, rain, snow and hail). Charge concentration of the hydrometeors as well as the change of the concentration are both computed by CELM within the cloud microphysical scheme of COSMO. The charging mechanism in CELM is basically due to the non-inductive mechanism, which leads to the charge separation and transfer, though the inductive charging mechanism is also considered in the model. Similar to cloud electrification, the lightning is also explicitly treated in CELM. We use the bidirectional concept of flash leader for modelling the lightning and the dielectric breakdown scheme for probabilistic branching of the leader.

Based on the simulations of an artificially induced thundercloud (warm air bubble), CELM shows the complicated charge structure in the thundercloud. The concentration of the negative charges is mostly connected to high concentrations of graupel, ice and snow, which are the hydrometeors that frequently collide, thereby transferring their charges. The charge distribution is also related to the updraft and downdraft motions. Despite long computation time, CELM successfully simulated the first flashes in the artificial thundercloud.