Figure 5 contains observed data from an intracloud triggering flash which initiated an UPL from a single tower. The flash initiated in the convective portion of a multicellular MCS with a small trailing stratiform precipitation area over the towers. The negative leader development initiated at 7 km above ground level (AGL) based on LMA data (right panels) and then traveled down to 4 km before passing over the towers. Visual in- cloud brightening, as recorded by a high-speed camera at the West Cameras site (upper left panels), traveled from right to left (south to north) over the towers prior to UPL initiation and correlated with the positive field change (lower left panels) and LMA-indicated VHF source points associated with negative leaders. Vertical red line in fast antenna and LMA data displays corresponds to time of high-speed video image.

### A. Intracloud Triggering Flash with Negative Leaders as Triggering Component (28 June 2014 – 23:47:15 UT)

The first example will be an intracloud triggering flash in which the negative end of the bidirectional leader development passes over the towers. Based on analysis of LMA data, the initiation of this type of triggering flash typically occurs between the upper positive charge region and main negative charge region near the convective area of the storm. The negative leader development then follows the positive charge region as it slopes downward into the stratiform precipitation area and over the towers. For this scenario, the negative leaders simply pass over or near the towers and the increased positive electric field between the negative leaders and towers (which already enhances the electric field due to the towers’ shape) exceeds a threshold for the initiation and development of an upward propagating positive leader (see Figure 4). The rapid quasielectrostatic field change due to the approaching negative leaders and their associated negative charge likely exceeds the increasing screening effect due to the production of positive corona space charge production.

### B. +CG Triggering Flash with Post Return Stroke Negative Leader Growth as Triggering Component (22 June 2014 – 02:24:08 UT)

We next address how negative leader growth following a +CG return stroke can travel near a tall object and trigger an UPL. The visual model shown in Figure 6 begins with bidirectional leader development prior to the return stroke (Figure 6A). The return stroke, as shown in Figure 6B-D travels through the leader network formed prior to the return stroke. After the return stroke reaches the outer extent of the leader network (Figure 6D), new negative leader growth results in negative leaders passing near the towers and triggering UPLs as shown in Figure 6E-F. Figure 7 contains an example of a +CG triggering flash in which post return stroke negative leader growth triggered an UPL from a single tower. The high-speed video recording from the East Camera location (upper left panels) had a view looking west toward the towers and triggering flash. The flash originated 66 km northwest of the towers at 10 km AGL with the negative leaders descending to 7 km after traveling 30 km to the southeast. The negative leaders remained at 7 km AGL while extending another 50 km southeast, remaining west of the towers by a minimum of 14 km. This was followed by a +CG return stroke 21 km west of the towers and back along the path traveled by the negative leaders. The high-speed video recording of the +CG return stroke showed in-cloud brightening traversing the path previously taken by the negative leaders (see Figure 7A upper left panel and light blue south-pointed arrow in LMA display). This is followed by new negative leader growth from near the +CG return stroke location eastward toward the towers as shown in Figure 7B. As the negative leader growth passed near the towers, an UPL initiated as shown in high-speed video images in Figure 7B and 7C). The West electric field change sensor recorded the positive field change associated with the +CG return stroke (red line location in Figure 7A lower left panel), however it was the field change associated with the new negative leader growth that passed over the towers (red line location in Figure 7B lower left panel) that initiated the UPL. The delay time between the +CG return stroke and UPL initiation was 94 ms.

Figure 4. Visual model of an intracloud triggering flash with negative leaders as the triggering component. Positive charge in light red, negative charge in light blue, positive leaders in dark red and negative leaders in dark blue.