Coordinated video and RF measurements of positive CGs inducing both sprites and upward tower discharges

Upward lightning discharges from tall objects and towers have been investigated for decades, and have yielded much of the still limited set of quantitative measurements on positive CG charge transfers, continuing current, etc. More recently, analyses of lightning detection network and video data sets have suggested that upward propagating discharges from tall towers often (though not always) appear to be initiated by changes in the overhead in-cloud charge distributions resulting from nearby discharges; most notably +CGs. It has been suggested that towers taller than 400 m had enhanced likelihoods of being involved in such complex events which may also be concurrent with the triggering of sprites by the parent +CGs (Stanley and Heavner 2003). During summer 2010, a watch was maintained to obtain the first ever coordinated low-light TLE video, high-speed lightning video (1000 to 10,000 fps), and ULF/ELF/VLF measurements of events occurring over an array of broadcast towers located on a ridge that runs through Rapid City, SD. On 20 June 2010, seven positive ground flashes were recorded using high-speed video. Five of these +CG flashes were also associated with optically detected sprites detected by low-light cameras at Yucca Ridge (Colorado), about 425 km distant. Furthermore, three of these flashes also involved upward propagating leaders that initiated from one or more towers after the +CG return stroke. The time line of events is being constructed, and key parameters such as the impulse and extended charge moment changes and continuing current estimates are being retrieved to obtain an overview of the complicated sequence of events. These discharges will be placed in their meteorological context using WSR-88D reflectivity measurements, GOES IR imagery and mesosynoptic analyses. Another event, recorded on 17 Jun 2010, revealed an even more unusual sequence in which an upward leader from a tall tower initiated prior to a +CG return stroke and subsequent sprite, though this case will require extensive evaluation. Evidence is growing that positive ground flashes, especially in the stratiform region of large MCSs, are associated with some frequency not only with sprites but also with upward leader development from tall structures.