Modeling stepped leaders using a time-dependent multidipolemodel and high-speed video data

Abstract

A full negative stepped leader and portions of four negative stepped leaders preceding negative cloud-to-ground lightning return strokes were modeled; each model was constrained to match electric field change measurements recorded at three or four sites located within 30 km of the leader. The time evolution and 2-D locations of stepped leaders were obtained from data collected with a high-speed video camera operated at 50,000 frames/s. The Lu et al. (Charge transfer during intracloud lightning from a time-dependent multidipole model, Journal of Geophysical Research, 2011) time-dependent multidipole model was used with some modifications. The model used a time step equal to one video frame, 20 µs. At each time step, negative charges were deposited at stepped leader tips based on measured light intensity, and an equivalent positive charge was deposited at one of the locations of the initial breakdown pulses that preceded the stepped leaders. The method has the unique advantage of obtaining locations of CG stepped leaders including its branches all the way to the ground. Three main quantities were obtained from the model:total charge transfer of -1.50 to -7.51 C, average line charge density of -0.113 to -0.413 mC/m (mean = -0.196 mC/m), and average current of -0.084 to -0.456 kA (mean = -0.31 kA). From the video data, the estimated 2-D speeds were 2.43-4.95×10 5 m/s (mean 3.34 × 10 5 m/s), and the cumulative lengths of the all branches were 3.5-9.2 times the vertical distance traveled by the visible stepped leader.