ON THE VELOCITY OF LIGHTNING UPWARD CONNECTING POSITIVE LEADERS

Abstract- The point of interception of the downward moving lightning leader by a newly created upward connecting leader depends upon the velocity of both leaders. Even though measurements and estimates of the velocity of downward moving lightning leaders are more common in the literature, only few values of the velocity of upward connecting leaders have been reported. In this paper, a self-consistent leader propagation model is used to estimate the velocity of upward connecting positive leaders initiated from a tall tower under the influence of downward negative lightning leaders. It is predicted that an upward connecting leader propagates with low velocity (lower than about 5 x 103 m s-1) immediately after the creation of the first leader segment. It is not until the electric field produced by the downward leader is high enough for the continuous acceleration of the leader channel, when the upward leader velocity reaches values close to the ones observed in laboratory (about 1-2 x 104 m s-1). Then, the connecting leader continues its movement with increasing velocity until it intercepts the downward moving leader. The propagation of upward connecting leaders has been found to be influenced by the prospective return stroke current, the average velocity and the lateral position of the downward leader channel as well as by the ambient field. This result clearly shows that the velocity and propagation time of upward connecting positive leaders change from flash to flash and they cannot be generalized by assuming a given velocity ratio of both leaders as it is assumed by the existing leader progression models