Optical observations of recoil leader-return stroke sequences in bipolar flashes during the first RAMMER campaign in Southeastern Brazil

In this work we present a discussion on the process that may lead to the formation of subsequent return strokes in multiple cloud-to-ground flashes, based on optical lightning observations of the RAMMER network during the summer season of 2011/2012 in southeastern Brazil. The RAMMER network is an array of high-speed cameras that automatically record lightning flashes and was installed in São José dos Campos, Brazil, since 2011. During the event of 03/13/2012 one of the high-speed cameras registered a peculiar thunderstorm in which six consecutive flashes presented recoil leaders that were visible below the cloud base. In two of these cases, it was possible to observe that the recoil leaders were responsible for the formation of negative subsequent strokes after a first positive stroke, i. e., they were responsive for the formation of bipolar flashes. These events are the base for a proposed mechanism for the initiation of subsequent strokes of bipolar flashes. Additionally, they are complementary to previous studies that report the occurrence of recoil activity prior to subsequent strokes in negative flashes, as observed from VHF records. Our data suggest that the optically visible recoil leaders can be responsible for the initiation of a dart leader-return stroke sequence in both negative and bipolar cloud-to-ground flashes.