Meteorological and Electrical Conditions of Two Mid-latitude Thunderstorms Producing Blue Discharges

Blue discharges are one type of rarely observed upward discharge emerging from the top of thunderstorms. The favorable meteorological conditions and charge structures needed to produce this marvelous phenomenon are still poorly understood. In this study, we report on two midlatitude thunderstorms producing 13 blue discharges observed simultaneously by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) and ground-based lightning detection array. In these two thunderstorms, blue discharges always occurred in the vicinity of the coldest cloud top (similar to 195 K) and clustered within a bounded area near the convective surge, leading to the overshooting thundercloud top (reaching about 18 km) into the stratosphere. The parent thunderstorms were normally electrified with a main midlevel negative and an upper positive charge layer centered at about 15 km as inferred from source heights of narrow bipolar events (NBEs). The associated negative NBEs near the tropopause indicated that they are upward positive discharges initiated between the upper positive charge layer and the negative screening layer at the cloud top. In addition, the outbreak of negative NBEs in the altitude range of 16-18 km was observed in conjunction with blue discharges observed from space. This suggests that there exists a strong upper positive layer with an intense convection at this altitude of the thunderstorm. It is inferred that an overshooting thundercloud top with intense convective updraft could lead to a strong and high upper positive charge layer and therefore create favorable charge structures for initiating upward positive blue discharges. Key Points Blue discharges are produced in association with the convection surge near overshooting thundercloud tops The outbreak of negative narrow bipolar events (NBEs) is observed during the occurrence of blue discharges Strong upper positive charge layer induced by intense updrafts would create favorable charge structure conditions for blue discharges

Automated summary

Blue discharges are rarely observed upward discharges that occur near the coldest cloud tops in thunderstorms. This study reports on two midlatitude thunderstorms where blue discharges clustered in a bounded area near the convective surge, reaching into the stratosphere. Narrow bipolar events indicated a main midlevel negative charge layer and upper positive charge layer, with the outbreak of negative NBEs suggesting the presence of a strong upper positive charge layer at high altitudes in the storm.