Observations of Blue Discharges Associated With Negative Narrow Bipolar Events in Active Deep Convection

On 19 August 2012, the Imager of Sprites and Upper Atmospheric Lightning on board the FORMOSAT-2 satellite captured a sequence of seven blue discharges within 1min that emanated from a parent thunderstorm over Lake Taihu in East China. The analysis of lightning activity produced in the thunderstorm indicates that at least six of these events occurred in association with negative narrow bipolar events (NBEs) that were concurrent with the blue discharge by less than 1ms, and negative cloud-to-ground occurred within 6s before each blue discharge, which is in agreement with the modeling presented by Krehbiel et al. (2008). Therefore, the frequent occurrence of negative cloud-to-ground could provide the favorable condition for the production of blue discharges, and negative NBEs are probably the initial event of blue discharges. The detection of negative NBEs might provide a convenient approach to detect the occurrence of blue discharges as lightning bolt shooting upward from the top of energetic thunderstorms. Plain Language Summary Energetic thunderstorms in the troposphere may shoot blue lightning bolts upward into the stratosphere, while their origin inside the parent thunderstorm remains a mystery. The analyses on a sequence of seven blue discharges emanating from an active thunderstorm over Lake Taihu in East China, as observed by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL), provide the first compelling evidence that they are physically related to the narrow bipolar events (NBEs) of negative polarity. These intracloud lightning discharges are located by a ground-based lightning location network with accuracy better than 2km. The close temporal (within 1ms) and spatial relationship (typically within 5km) between blue discharges and negative NBEs strongly suggests that the negative NBEs are probably the inception event of blue discharges, showing that the detection of negative NBEs might provide a convenient approach to monitor the occurrence of blue discharges as the electrical coupling between thunderstorms and middle atmosphere. Moreover, the same thunderstorm produced frequent negative cloud-to-ground (-CGs), confirming the hypothesis of previous modeling results that -CGs might favor the production of blue discharges. Our work enriched the insight of the relationship between blue discharges and ordinary lightning discharges.