horizontal propagation just below cloud base and then branch extensively [Warner et al., 2012c; Schumann et al., 2014a; Saba et al., 2016]. This paper reports on the observations and initial findings from the Upward Lightning Triggering Study (UPLIGHTS) that took place in Rapid City, South Dakota, USA from 2012 – 2014. This study sought to further define and characterize the nearby lightning triggering process that leads to upward leader initiation and development from tall objects. We first summarize the observations made during the campaign, and then present a visual model of the triggering processes based on findings from our analyses. Using this framework, we then discuss specific LTUL examples relating to components and processes incorporated in the model. Findings from the analysis of weather conditions and charge distributions favorable for upward lightning during UPLIGHTS will be provided via separate papers at this conference.

towers (see Figure 2). The two vehicles were spatially separated so that recordings could provide some level of 3-dimensionality when comparatively analyzed. Variable vehicle locations were chosen based on storm direction of movement, storm type and expected visibility. Each vehicle had at least two high-speed cameras with one having a wide field of view, and the other having a moderate field of view on selected towers. Each vehicle also had one camera that operated in the 1,000 – 2,000 ips range and another in the 10,000 – 35,000 ips range. The two vehicles also contained standard-speed video cameras recording in standard- and high-definition that provided 60 ips continuous video at extremely wide field of views and moderate field of views on selected towers. A single digital still camera located in each vehicle captured entire flashes employing continuous 10 second exposures at night. During the day, these cameras utilized an infrared trigger with the lens aperture set at f/8 and ISO at 100 resulting in images with “camera determined” exposure times based on ambient light conditions. Three fixed optical sites located on opposing sides of Rapid City captured continuously recorded wide field of view standard-speed video at 60 ips as well as digital still images. Distances from these sites to each of the towers ranged from 2.5 km to 11 km. The National Institute for Space Research (INPE), Brazil operated flat-plate, fast antenna systems with a sample rate of 5 million samples/second and a lower and upper frequency bandwidth of 306 Hz and 1.5 MHz respectively. Both a high- and low-gain fast antenna were placed together approximately 5 km west of the towers (West Sensors in Figure 2). At this location, the dual gain system provided increased dynamic range for sensing electric field changes primarily associated with triggering flashes. A low-gain fast antenna was placed between towers 1 and 2 (North Sensors in Figure 2). At this location, the sensor was within 1.6 km of the three towers that historically produced the most upward leaders (Towers 1, 4 and 6), optimizing the recording of both the triggering flash electric field change near the tower locations and the field change resulting from the development of the upward leaders. Co-located with the fast antenna systems were rotating vane electric field meters (EFMs) that sampled the ambient electric field at 20 samples/second. An additional EFM was also located on one of the mobile vehicles. The field meters were factory- calibrated but were not calibrated to account for local field enhancement at each individual sensor site. Therefore, they were not absolutely calibrated so electric field magnitude could not be compared relatively between the three different site locations. Data obtained from the EFMs provided insight into the initial electric field polarity as well as direction and magnitude of change during upward flashes at varying distances and directions from the towers. For the 2014 observation season, a nine-station LMA provided by New Mexico Tech was installed with the centroid approximately 50 km east-southeast of the towers [Krehbiel et al., 2000, Thomas et al., 2004]. The LMA centroid was located southeast of the towers due to additional research objectives not related to UPLIGHTS. This location did, however, provide good 3-dimensional resolution of lightning generated source points for triggering and upward flashes from the towers. The

## II. PROJECT SUMMARY

### A. Overview

UPLIGHTS utilized multiple optical assets that included high- and standard-speed video cameras along with digital still image cameras to record visual observations of triggering flashes and upward leader initiation and development. Electric field sensing equipment, which included fast antenna systems and electric field meters, provided data on the ambient electric field before as well as its change during upward flashes. The addition of a Lightning Mapping Array (LMA) for the 2014 season allowed for the 3-dimensional visualization of lightning leader development and propagation relative to the towers. The National Weather Service KUDX WSR-88D, located 35 km east of the towers, recorded weather radar data including dual- polarity data beginning with the 2013 season. The National Weather Service Forecast Office in Rapid City, located 3 km east of the closest tower, recorded meteorological surface data and upper air soundings twice daily (see Figure 1). Figure 1. Layout of the 10 towers relative to the National Weather Service forecast office and weather radar.

### B. Instrumentation

Optical assets included high- and standard-speed video cameras along with digital still image cameras. Up to five high- speed cameras operating between 1,000 and 35,000 images per second (ips) were used to record lightning flash activity. Cameras were grouped into two mobile platforms, which were positioned at various locations from 600 m to 9 km from the