LMA system recorded and triangulated the peak amplitude very high frequency (VHF) emission in sequential 80 µs time windows. Figure 2. Layout of the 10 towers relative to the electric field sensing instrumentation and cameras. National Lightning Detection Network (NLDN) stroke data [Cummins and Murphy, 2009] for a 200 km radius around the towers was obtained and analyzed for this study. Analysis provided timing and location of impulsive events detected by the NLDN during triggering and upward flashes. The physics sign convention [Rakov and Uman, 2003, Section 1.4.2] will be used when referring to the electric field and its change. A positive electric field is an upward pointing field when the ground is positively charged and there is negative charge overhead. A positive field change would result if there were an increase in negative charge overhead.

was 2.9 with a maximum of 8 upward flashes during a single storm.

## IV. VISUAL MODEL

Since UPLs are dominantly observed during LTUL in Rapid City, our visual model will focus on this type of upward flash, and we will address UNL triggering separately after the UPL cases. Our analysis suggests that two flash components are responsible for the initiation of UPLs from tall objects. Firstly, negative leader development passing near or over the tall objects can create quasielectrostatic stress at the tower tip due to the rapid positioning of negative charge near the towers that results in the initiation of an UPL. The addition of negative charge over the tower causes a positive field change as the electric field strengthens according to Coulomb’s Law. If this electric field strengthening occurs rapidly enough to overcome the increased induced positive corona space charge production near ground objects, which acts to reduce the sensed electric field at the tower tip, a self-propagating upward positive leader may form. This negative leader development can take place during an intracloud flash or prior to or after a positive cloud-to- ground (+CG) return stroke. Horizontally extensive flashes in MCSs, where negative leaders travel large distances through layered positive charge frequently found near cloud base, are the most effective in triggering UPLs [Mazur and Ruhnke, 2011; Warner, 2012; 2012a; 2012b; Saba et al., 2012, 2016; Schumann et al., 2014a, 2014b, 2014c]. Following a +CG return stroke, which effectively raises ground potential to the upper extent of the bidirectional leader network that formed prior to the return stroke, extensive negative leader growth can occur due to the potential difference between the upper extent of the return stroke channel (near ground potential) and the surrounding ambient cloud potential. The negative leader development can further extend previously formed leader channels and branches or create new branches which propagate in new directions as dictated by available charge regions (potential wells). This renewed development can travel large horizontal distances in the horizontally stratified positive charge regions frequently found in stratiform precipitation areas of MCSs [Lu et al., 2009]. As a result, the initiation time for an UPL may be well after the +CG return stroke if the renewed negative leader development has to travel significant distances to reach the tower area. The delay from a +CG return stroke to UPL initiation can exceed 100 ms [Warner et al., 2012a; Saba et al., 2016]. The second flash component that can trigger UPLs is the return stroke of a +CG flash. The return stroke, which effectively raises negative charge along the return stroke channel path, can again cause a large positive electric field change. If the return stroke channel path is near or over the towers, the speed of the return stroke and its associated quasielectrostatic influence on the tower can again exceed the induced positive corona space charge production resulting in an UPL. Unlike the negative leader triggering flash component that can precede or follow a return stroke, the relatively short duration but intense field change associated with the return stroke results in a very short delay in initiating an UPL based on our observations, usually within 2 ms. Analysis of the ambient electric field at the surface prior to upward flashes showed that it was preferentially positive. For

## III. OBSERVATIONS

The UPLIGHTS formal observation timeframe fell within three summer thunderstorm seasons (Apr – Sep) from 2012 – 2014. A total of 38 upward flashes during 13 storms were observed from the 10 towers located in Rapid City. All 38 upward flashes were LTUL with the upward leaders being triggered by nearby preceding lightning activity. 37 upward flashes had upward propagating positive leaders develop from one or more towers, and we observed one upward flash in which an upward propagating negative leader developed from one tower. This was the first and only upward negative leader ever observed in Rapid City since observations began back in 2004. 18 of the 38 upward flashes (47%) had an upward positive leader develop from more than one tower during the flash. The maximum number of towers that initiated an UPL during a single upward flash was six and this occurred twice. Four storms had only one upward flash occur and nine storms had more than one upward flash occur as it passed over the towers. The average number of upward flashes per storm