First Observations and Characteristics of Positive Upward Illumination Strokes

Abstract Upward Illumination (UI) strokes are a subtype of cloud‐to‐ground (CG) return strokes characterized by two concurrent branches striking the ground a few milliseconds apart. The luminous channel of an UI stroke does not connect to the main channel, which gives this phenomenon its name. Since concurrent branches in positive CG strokes are not usual, positive UI (+UI) strokes were never reported yet. This study presents the first observations of +UI strokes. They were recorded in Utah, USA, using high‐speed cameras. Although +UI strokes share similarities with negative UI strokes, their initiation mechanisms are fundamentally different. +UI strokes result from intense recoil leader activity with their positive leader speed one order of magnitude slower than the average of typical CG downward positive leaders. Additionally, multiple +UI strokes can occur within the same flash. Finally, we gather all the information learned from the case studies to propose the basic formation mechanisms of +UI strokes.