Modeling of thundercloud screening charges: Implications for blue and gigantic jets

A two-dimensional axisymmetric model of charge relaxation in the conducting atmosphere is used in conjunction with a probabilistic lightning model to demonstrate how realistic cloud electrodynamics lead to the development of blue and gigantic jets. The model accounts for time-dependent conduction currents and screening charges formed under the influence of the thundercloud charge sources. Particular attention is given to numerical modeling of the screening charges near the cloud boundaries. The modeling results demonstrate the important role of the screening charges in local enhancement of the electric field and/or reduction of net charge in the upper levels of the thundercloud. The charge relaxation model presented in this work confirms the previous results obtained with a simpler model by Krehbiel et al. (2008), specifically that the accumulation of screening charges near the thundercloud top produces a charge configuration leading to the initiation of blue jets, while the effective mixing of these charges with the upper thundercloud charge may lead to the formation of gigantic jets.