Upward Leaders Initiated From Instrumented Lightning Rods During the Approach of a Downward Leader in a Cloud‐To‐Ground Flash

Abstract

In this paper we analyze electric‐field and current measurements of upward leaders induced by a downward negative lightning flash that struck a residential building. The attachment process was recorded by two high‐speed cameras running at 37,800 and 70,000 images per second and the current measured in two lightning rods. Differently from previous works, here we show, for the first time, current measurements of multiple upward leaders that after initiation propagate to connect the negative downward moving leader. At the beginning of the propagation of the leaders that initiate on the instrumented lightning rods, current pulses appear superimposed to a steadily increasing DC current. The upward leader current pulses increase with the approach of the downward leader and are not synchronized but present an alternating pattern. All 2D leader speeds are approximately constant. The upward leaders are slower than the downward leader speed. The average time interval between current pulses in upward leaders is close to the interstep time interval found by optical or electric field sensors for negative cloud‐to‐ground stepped leaders. The upward leaders respond to different downward propagating branches and, as the branches alternate in propagation and intensity, so do the leaders accordingly. Right before the attachment process the alternating pattern of the leaders ceases, all downward leader branches intensify, and consequently upward leaders synchronize and pulse together. The average linear densities for upward leaders (49 and 82 μC/m) were obtained for the first time for natural lightning.

The Attachment Process of Negative Connecting Leader to the Lateral Surface of Downward Positive Leader in a Positive Cloud‐To‐Ground Lightning Flash

“…The 2-D speed of NCL increases from 2.2 × 10 5 to 12.8 × 10 5 m/s (by a factor of 5.8), with an average of 7.3 × 10 5 m/s, and the 3-D speed of NCL increases from 2.2 × 10 5 to 15.3 × 10 5 m/s (by a factor of 7.0), with an average of 8.5 × 10 5 m/s (see Figure S4 in Supporting Information S1). That is consistent with previous reports on UCLs initiated from tall objects with its speed being increased with time (e.g., et al, 2014;Lu et al, 2013Lu et al, , 2015Qi et al, 2021), while it is contrary to UCLs observed on common buildings with the speed being approximately constant (e.g., Saba et al, 2017Saba et al, , 2022Saba et al, , 2023.…”

“…Saba et al. (2023) reported that a DNL had a constant 2‐D speed (2.85 × 10 5 m/s) and twice faster than the speed of UCL, which is consistent with the results reported by Visacro et al. (2017a, 2017b) from four natural DNLs before attachment.…”

“…In recent years, with the development of state‐of‐the‐art technology, video cameras of high temporal and spatial resolutions have been applied to observations and studies. Several different properties of the lightning attachment process, especially those during negative cloud‐to‐ground (CG) flashes were revealed, such as, the leader propagating speed before connection (e.g., Campos et al., 2014; Gao et al., 2014; Lu et al., 2010, 2015; Qi et al., 2019; Saba et al., 2017, 2023; Srivastava et al., 2019; Visacro et al., 2017a; Wang et al., 2015; Warner, 2010), leader connecting behavior (e.g., Lu et al., 2013, 2016), striking distance characteristics (e.g., Qi et al., 2021; Visacro et al., 2017b), and streamer zone structure (e.g., Jiang et al., 2021; Plaisir et al., 2023; Saba et al., 2022; Tran & Rakov, 2017).…”

Research on the Initiation of Multiple Upward Leaders From an Isolated Building Based on an Improved Lightning Attachment Model

“…It is worth noting that the lack of branching in the UL is by design. Although the overall UL channel tends to be toward the DL because of the attraction between positive and negative leaders (Saba et al, 2023), the propagation direction of UL may change several times in a flash event (Figures 2b-2d), which is similar to the development of leaders recorded by cameras (Qi et al, 2019;Saba et al, 2017;Warner, 2010). When the final jump condition is reached, LAMM can reproduce two basic leader connection scenarios documented in the optical data, namely, the tip of DL to the tip of upward connecting leader (Figures 2a-2d) and the DL's tip to the lateral surface of UL (Figure 2b) (Lu et al, 2013(Lu et al, , 2016.…”

Evaluation of the Direct-Stroke Shielding Performance of Substations Through Stochastic Modeling of Lightning Attachment

“…A velocity ratio was adopted in this work and not absolute velocity values as an engineering approach; it is noted that most lightning attachment models from literature consider a constant speed ratio. However, recent high-speed video records [52], [53], [54], [55], [56] have shown a dynamic variation of downward and upward leader velocities; actually, velocity ratios even lower than unity were recorded [52], [54], [55]. In this work, a dynamic velocity ratio formula is employed based on the expression proposed in [12] and modified in [49] to account for recent field observations on downward and upward leader velocity ratios (allowing also for values lower than unity) and consider the effect of atmospheric conditions.…”