From AMS Glossary
A lightning pattern revealed in cloud-to-ground lightning locations in which areas of predominantly negative flash locations are separated horizontally from positive flash locations.
This pattern has been identified in midlatitude mesoscale convective systems (MCSs). The convective regions of the MCSs (those with radar echoes greater than 35 dBZ) typically have negative lightning flashes, whereas the positive flashes are typically located in the stratiform region (<35 dBZ) adjacent to the convective region. The bipolar pattern is hypothesized to arise from 1) the advection of positively charged particles from the upper portions of the convective region to the stratiform region due to vertical wind shear; 2) in situ charging of ice particles that acquire positive charge in an environment of supercooled liquid water and mesoscale ascent in the stratiform cloud layer; or 3) some combination of the two processes.