Abstract: Proportions of positive and negative cloud-to-ground (CG) natural lightning are approximately 10% and 90%, respectively. Although the frequency of positive CG lightning is relatively low, these events are more likely to produce high current peaks compared to negative CG lightning. Additionally, positive CG flashes have longer continuous current durations and release larger amounts of charge, which can easily result in more severe disasters and accidents. Besides differences in leader polarity and occurrence frequency, there are also differences in morphological characteristics of channel development between positive and negative CG lightning. Moreover, morphological differences between positive and negative CG flashes can result in varying probabilities of side-flashes on tall buildings. However, because of the infrequent occurrence of positive CG flashes, current research on the morphological characteristics of lightning channel development remains quite limited. There remains a lack of clarity concerning the morphological characteristics associated with the development of positive CG lightning channels, as well as the morphological differences between positive and negative CG lightning channels.
Based on a collection of digital images of 50 CG lightning events (25 positive and 25 negative), the optical two-dimensional morphological characteristics of lightning channels with different polarities are investigated and compared. Results indicate that there are significant differences in the number of bifurcations between positive and negative CG lightning channels. Negative CG lightning channels display a greater number of bifurcations, ranging from 7 to 85, with an average of 35. In contrast, 76% (19/25) of positive CG lightning cases only observed the return stroke channel, without any additional bifurcations. Except for one instance involving 42 bifurcations, the number of bifurcations in other positive CG lightning events does not exceed six. This difference in bifurcation number also leads to differences in fractal dimension, with the fractal dimension of positive CG lightning ranging from 1.03 to 1.30 (average of 1.11), while that of negative CG lightning has an average of 1.21, ranging from 1.12 to 1.30, with its minimum value exceeding the average of positive CG lightning. Moreover, there are significant differences in the tilt angles and directional turning counts of the return stroke channels between positive and negative CG lightning. Compared to negative CG lightning, the return stroke channels of positive CG lightning tend to develop more inclinedly with fewer directional changes.There is no significant difference in relative length (the ratio of the straight-line distance from the cloud exit point of the return stroke channel to the strike point to the total length of the return stroke channel) of return stroke channels between downward positive and negative cloud-to-ground lightning.