Identification of asymmetries in bidirectional leader development by means of the Lightning mapping Array

Basic models of lightning flashes represent them as bidirectional tree, with a negative and a positive leader ends propagating simultaneously into cloud charges of the opposite polarity. In these models, bidirectional channels remain a zero-net-charge starting at the electric potential of its initial location. In this paper we highlight some characteristics of bidirectional leader development of various lightning flashes registered by the Ebro 3-D Lightning Mapping Array at the east coast of Spain. A time-distance-altitude graph was designed, using the flash origin or a cloud-to-ground stroke as reference. This graph allows to follow the horizontal development of positive and negative leaders over time. The observations confirm that negative and positive leaders propagate at characteristic horizontal speeds (10⁵ and 2·104 m s^-1). The low apparent speed of positive leaders corresponds to the stages when recoil processes are active. Very fast negative leaders (up to 8 · 105m s^-1) are detected in positive cloud-to-ground strokes. Negative leaders redevelop repeatedly from the origin or as a retrograde negative leader at the positive leader branch. Commonly, positive leaders remain propagating throughout the flash or until reaching ground. Assuming the bidirectional leader model, the velocity difference would shift the potential of the leader, increasing the potential gradient with cloud charge at the positive end while decreasing it at the negative end. The effect of retrograde negative leaders is to lower the leader potential. The reduction of the leader potential would eventually emit a new negative leader into the upper positive `potential well´.