  We construct a two-dimensional electric circuit model for creeping discharge.
Two types of discharge, surface corona and surface leader, are modeled by a
two-step function of conductance. Branched patterns of surface leaders
surrounded by the surface corona appear in numerical simulation. The fractal
dimension of branched discharge patterns is calculated by changing voltage and
capacitance. We find that surface leaders often grow stepwise in time, as is
observed in lightning leaders of thunder.
