Upward Electrical Discharges From Thunderstorm Tops

Abstract

A variety of storm top electrical discharges have been observed using several types of low-light imagers, film, and the human eye. Recently, a video recorded an unprecedented, bright blue upward discharge from a tropical thunderstorm top near Puerto Rico. The event reached the base of the ionosphere. The horizontal dimensions of cloud top discharges can range from 100 m to several kilometers. Upward extents vary from 100 m to 70 km. Shapes include “points” of light, upwardly flaring trumpets, and narrow, vertical, lightning-like channels, some topped with expanding blue, flame-like features. Visual appearances range from brilliant white lightning-like channels to a grainy, almost particulate appearing jets of dim blue light, and sometimes as a blue flame within which a brilliant white channel appears. The classical blue jet is at the lower limit of human night vision whereas some upward discharges have been clearly seen during daylight. Cloud top “pixies” last no longer than 16.7 ms, whereas upward lightning-like channels are often characterized as long lasting (2.0 s or more). To date, optical measurements have not associated cloud-top events with specific lightning flashes. There is a strong tendency for all such events to occur above the convective dome of rapidly intensifying thunderstorms. It is possible that the great diversity of forms illustrates the complexity inherent in the upward streamer mechanism for blue jets. It is also possible that the basic blue jet is only one of several distinct classes of discharges from highly electrified storm cloud tops. Future research should focus on rapidly growing convective storm tops, including supercells and intense oceanic storms, as opposed to the stratiform regions of large mesoscale convective systems that have characterized sprite observations to date.