Lightning initiation in the anvils of two supercell storms

Abstract

[1] Previous studies of lightning in anvil clouds have reported that flashes began in or near the storm core and propagated downwind into the anvil. It had been thought that flashes could not be initiated far downwind in the anvil, because anvil charge was thought to be produced mainly in the storm's deep updraft and to decrease with distance into the anvil. Here we report observations of the in-cloud development of lightning flashes in the anvils of two supercell storms, including the first observations of flashes that began in the anvil 30-100 km from the cores of the storms and propagated upwind back toward the cores. Interaction between charge regions in the two converging anvils of adjoining storms appeared to cause some of the distant flash initiations, but a local charging mechanism in the anvil likely also contributed to the flash initiations. All flashes that struck ground beneath the distant anvil transferred negative charge to ground instead of the positive charge usually transferred to ground there, an apparent consequence of the parent storm having an inverted-polarity electrical structure. Copyright 2009 by the American Geophysical Union.