Visible channel development during the initial breakdown of a natural negative cloud-to-ground flash

GPS time synchronized high-speed video records (at 4000 frames per second) and electric field waveforms are discussed for one stepped leader of a negative cloud-to-ground flash whose initial development becomes visible above cloud base. The leader emerges from the opaque region of the thundercloud around 4800 m above ground, propagating with speeds of the order of 10(6) m s(-1). It is possible to see that this early, bright, and fast propagation period occurred simultaneously with the emission of initial breakdown bipolar pulse trains. The leader gradually reduces its speed and luminous intensity: When its tip is less than 3000 m above the ground, the propagation speeds oscillate between 2 and 4x10(5) m s(-1) until it makes contact approximately 16 ms after it first became visible. With time, the initial breakdown waveform is gradually replaced by unipolar pulses and the leader propagation speed drops to the 10(5) m s(-1) range. (AU)