Abstract: In recent years, with the advancement of lightning observation technologies, particularly the progress of high-speed photography and optical observation techniques, it has become possible to capture 2-dimensional morphological characteristics of lightning channels with much higher precision. Observations show that downward negative leaders typically develop in a stepwise manner, with channels often showing complex fractal structures characterized by dense branching and tortuous paths. In contrast, observations on positive leaders show that they frequently propagate in a continuous manner, and their channels tend to display simpler and straighter morphologies. To some extent, morphological features of positive and negative cloud-to-ground (CG) lightning channels influence the differences observed in their strike distributions. Nevertheless, systematic comparative studies of 2-dimensional optical morphology of lightning channels with different polarities remain relatively scarce, particularly those that integrate high-resolution observational data with theoretical modeling. To overcome limitations of purely observational research, numerical simulation has become an essential method for studying the development of lightning channels. However, existing models of downward CG lightning have predominantly focused on negative lightning, with limited attention given to the developmental characteristics of positive leaders.
To better align simulated morphological characteristics of downward positive CG lightning channels with observations, improvements are made to an existing 2-dimensional stochastic discharge parameterization scheme. In particular, the method used to determine the development direction of the leader channel is optimized to produce smoother leader paths. Additionally, both the velocity ratio between positive and negative leaders and their connection schemes are modified. Results indicate that the new model significantly reduces the number of channel turns, resulting in smoother channel development that aligns more closely with observations. Based on the improved model, 500 simulations are conducted, and morphological parameters of downward positive lightning channels are derived through statistical analysis. These parameters include the fractal dimension of the channels, the inclination angle, and the number of turns in the return stroke channels. It shows that simulated positive lightning channels have an average fractal dimension of 1.11, an average inclination angle of 20.98°, and an average number of turns of 4.73. All of these values are in strong agreement with those derived from observations, providing further confirmation that the improved model offers a reasonable and effective representation of the morphological development of downward positive lightning channels.