[1] Intracloud lightning discharges are simulated in 12.5 m (fine) and 250 m (coarse) resolution for 2-dimensional domain by using an improved Stochastic Lightning Model. Simulation results indicate that the bi-level branched channel structure, horizontal extending ranges and maximum vertical electric field changes obtained from fine resolution lightning model are in better agreement with previously observed data than those from coarser model. The IC flash channels from fine lightning model have the fractal feature with fractal dimension of 1.43, and propagation tendency is dependent on the potential distribution. In addition, fine resolution lightning modeling shows that after IC flash initiation at the boundary between positive and negative potential zones, potential wells attract the leaders of opposite polarity into the central area and prevent their outward expansion. Leaders can propagate throughout regions of net charge of the opposite polarity, but they avoid the isolated charge areas of the same polarity.

The favorable factors that affect the initiation of intracloud (IC) lightning and cloud-to-ground (CG) flashes are investigated by using a fine resolution two-dimensional lightning model. Simulation results indicate that potential at initiation point is a key to decide whether downward leader reaches ground. The absolute values of initiation potential of CG flashes are greater than 30 MV, while the absolute values of initiation potential of IC lightning are basically less than 30 MV. Since potential field is determined by space charge distributions, polarities and types of lightning discharges are also dependent on relative locations and magnitudes of positive and negative charge zones near initiation points. In general, dipoles (positive charge above negative) initiate normal IC lightning and positive CG flashes and inverted dipoles are associated with inverted IC lightning and negative CG flashes. The magnitude of upper charge near initiation point is generally larger than that of lower charge when CG flashes occur, and the two are comparable when IC lightning occur. For CG flashes, the magnitude of lower charge near initiation points should be enough for initiation breakdown, but not so strong that the lower potential well prevents propagation to ground. In addition, CG flashes are initiated within various special charge distributions, only if the reference potentials at initiation point are far away from 0 MV.

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This paper extends the use of Stochastic Lightning Model in a simulation experiment, in which 12.5 and 250 m resolution simulations of intra-cloud (IC) lightning discharges within a two dimensional numerical thunderstorm model have been performed, and gives the comparing analyses of the simulated results with the observed data. The results show: (1) During an IC flash discharge, more complex distribution of the space charges in thundercloud is caused by the induced charges along the channels, leading to a sudden decrease in the maxima of electrostatic potential and potential gradient as well as consume of electrostatic energy of thundercloud. The electrostatic potential and field strength drop down respectively to ±30 MV and ±20 kV·m −1 where the flash channels passed, and consumed electrostatic energy of thunderstorm for a positive or negative IC flash is about 10 7 ∼ 10 10 J. (2) As contrasted with transient IC discharges, the charge neutralization in thundercloud is a slow process with a relaxation time of about 14∼40 secs. The relaxation time is confirmed by turbulence exchanging, advection transporting and gravitational sedimentation, during which the induced charge along leader channels drops down to 50%. There are some excess of induced channel charges to be not neutralized, which contributes to rebuilding new distribution of space charges and electrostatic potential.