Initial leader velocities during intracloud lightning: Possible evidence for a runaway breakdown effect

Using three-dimensional lightning mapping observations, the initial leaders of intracloud flashes have been found to start at a median speed of about 1.6 x 10(5) m s(-1) and to decelerate during the first 10-15 ms of the discharge. The results disagree with the predictions that the speed should increase with time as the developing leader shorts out an increasing potential difference in the storm. The observations can be explained if the flash initiation region is preconditioned in some manner to give a high initial speed and if the preconditioning decays with time and/or decreases with distance to give the observed speed decrease. Such preconditioning could be the result of ionization and excited molecules produced by energetic electron avalanches.