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.

Combined with the existing stochastic lightning parameterization scheme, a classic tripole charge structure in thunderstorms is assumed in the paper, and then 2-dimensional fine-resolution lighting discharge simulations are performed to quantitatively investigate the effect of lower positive charge (LPC) on different types of lightning. The results show: (1) The LPC plays a key role in generating negative cloud-to-ground (CG) flashes and inverted intra-cloud (IC) lightning, and with the increase of charge density or distribution range of LPC region, lightning type changes from positive polarity IC lightning to negative CG flashes and then to inverted IC lightning; (2) Relative to distribution range of charge regions, the magnitude of charge density of the LPC region plays a dominant role in lightning type. Only when the maximal charge density value of LPC region is within a certain range, can negative CG flashes occur, and the occurrence probability is relatively fixed.