This paper on AMS "
UPLIGHTS: Upward Lightning Triggering Study" is a study on lightning from broadcast towers in Rapid City, SD.
It has some good shots of upward positive leaders emanating from the towers.
http://journals.ametsoc.org/toc/bams/currentIt is the third article down #631. Click on the PDF link. It says that it is "Open Access Content" so it is viewable.
Greg H