Branching Morphology of Negative Leaders with Different Propagation Directions in Natural Lightning

Abstract

Comparing the branching features of negative leaders with different propagation directions could provide insight into the common tendency of development pathways and the formation pattern of branches in natural lightning. This paper reports an upward negative leader (UNL) and a downward negative leader (DNL), and their branching features are analyzed and compared. The UNL is classified into vertical (UNL-V) and horizontal (UNL-H) segments based on propagation directions at different stages. The downward negative leader (DNL) is classified into main (DNL-M) and secondary (DNL-S) channels based on whether the channel is ultimately connected to the upward connecting leader. The vital angle parameters characterizing the branching morphology are investigated. For the strong branch eventually forming a section of the main channel, its deflection angle conforms to the lognormal distribution with a mean range of 22–36°. The included angle between the branches and the deflection angle of weak branches conform to the normal distribution with means close to 40° and 60°, respectively. Moreover, the velocity for four categories of negative leaders decreases noticeably by two or more branching behaviors in a frame interval of about 80 μs. In particular, similarities in branching morphology have been found in UNL-H, UNL-V, and DNL-S, with a semblable distribution in deflection and included angles. Statistical results indicate that branches of DNL-M tend to follow the previous direction of leader development, and the branching behavior has minimal impact on its velocity.

Keywords

• negative leader
• branching features
• propagation direction
• branching morphology
• leader velocity