A numerical study of the positive cloud-to-ground flash from the forward flank of normal polarity thunderstorm

• The three-dimensional flash was simulated by establishing a three-dimensional lightning model and a charge structure model.
• The conditions that favorable to the +CG flash occurrence from the forward flank of normal polarity storm were investigated.
• The effect of the downshear extension of the charge layers on the maximum electric field and the potential was studied.

This study investigates the electrical conditions favorable to the occurrence of positive cloud-to-ground (+ CG) flashes from the forward flank of normal polarity thunderstorm by building a three-dimensional stochastic lightning model and a normal polarity charge structure model. The lightning model well simulated the bilevel branched structure of lightning flash, which has been observed by previous studies. Simulation results indicate that the downshear extension of the charge layer to the forward flank would decrease the electric field of thunderstorm. When only the upper positive charge layer downshear extended to the forward flank, it could not give rise to the occurrence of + CG flashes. When both the upper positive and midlevel negative charge layers downshear extended to the forward flank, if the charge of the negative charge layer was less than the positive charge layer above it in the forward flank of the thunderstorm, that generated sufficient charge imbalance between these two charge layers, + CG flashes could originate from the forward flank; otherwise, IC flashes occur.