Upward lightning observations from towers in Rapid City, South Dakota and comparison with National Lightning Detection Network data, 2004–2010

We report on upward lightning observations from ten tall towers (91–191 m) in Rapid City, South Dakota, USA and compare with National Lightning Detection Network (NLDN) data. A total of 81 upward flashes were observed from 2004–2010 using GPS time‐stamped optical sensors, and in all but one case, visible flash activity preceded the development of the upward leaders. Time‐correlated analysis showed that the NLDN recorded an event within 50 km of towers and within 500 ms prior to upward leader development from the tower(s) for 83% (67/81) of the upward flashes. A preceding positive cloud‐to‐ground stroke (+CG) was detected in 57% (46/81) of the cases, and a preceding positive intracloud flash (+IC) in 23% (19/81) of the cases. However, 8 of the 19 NLDN‐indicated +IC events were actually +CG strokes based on optical observations. Preceding negative intracloud flashes (−IC) were recorded for 2% (2/81) of the cases. Analysis also showed that for 44% (36/81) of the upward flashes, the NLDN reported subsequent negative cloud‐to‐ground (−CG) strokes and/or −IC events at one or more tower locations. Of the 151 subsequent events, 70% (105/151) were −CG reports and 30% (46/151) were listed as −IC events. The geometric mean/median location accuracy and peak current for subsequent events were 194 m/206 m and −12.9 kA/−12.4 kA respectively. These correlated observations suggest that a majority of the upward lightning flashes were triggered by a preceding flash with the dominant triggering type being the +CG flash.