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Description
In the context of lightning physics, upward lightning represents a rare and distinct class of electrical discharges that originate from ground-based elevated structures and propagate toward storm clouds. In this study, a Phantom v310 high-speed camera located in Johannesburg, South Africa, served as the primary instrumentation to capture the detailed development of upward positive leaders. Analysis of 54 video recordings, encompassing 71 distinct lightning cases, revealed that the majority exhibited recoil leaders (76%) and branching (89%), frequently co-occurring (75%), with recoil leaders retracing decaying branch channels in several instances. Average speeds were on the order of $10^4\text{ m/s}$, predominantly concentrated below $5.0 \times 10^4\text{ m/s}$ and largely associated with branching events, whereas the two cases with the highest propagation speeds exhibited no branching. Along their upward trajectory, leaders propagated primarily vertically during initiation, showing a decrease in the vertical velocity component and an increase in the horizontal component as they approached the cloud base in stratiform storm regions. Regarding the correlation between channel luminosity and leader speed, a wide dispersion of correlation coefficients was observed. Nevertheless, 51% of the cases demonstrated a moderate to very strong positive correlation, partially supporting the proposed hypothesis. The complexity of lightning events, influenced by multiple physical and atmospheric parameters, contributes to the observed dispersion in the analyzed characteristics. Future research should incorporate additional parameters and investigate storm cloud structures to deepen the understanding of the phenomenology associated with upward positive leaders.