The scavenging of particles by electrified drops: radar echo intensification following lightning

Abstract

A series of laboratory experiments were performed in an effort to improve our understanding of the physics of the echo-intensification phenomenon, in which the radar reflectivity of a region of a thundercloud increases rapidly immediately following the occurrence of lightning within it. Measurements were made of the charges, q, deposited on falling water drops when they engage in direct interactions with positive corona streamers in a uniform electric field, E, just above the limit for propagation, E0. As the drop radius increased from 12 to 950/μm, q increased from about 10−13 to about 10−10C; and q increased as E was raised above E0. These measurements yielded an estimate of 30μm and 108, respectively, for the radius of, and the number of elementary charges contained in, the tip of a propagating streamer at a pressure of one atmosphere. The collection efficiencies of drops in the radius range 40 to 120 μm carrying charges appropriate to the direct interaction process, for uncharged droplets of radius around 12 μm, were found to be about four times the non-electrical values. It was concluded that this enhancement in collection resulted largely from dipole rather than coulomb forces. A simple analysis indicates that reported echo-intensification observations cannot be explained in terms of these enhanced collection efficiencies, but could possibly be a consequence of the greatly enhanced velocities (and hence growth rates) of drops of radius ∼ 100 μm highly charged by direct interaction with corona streamers in the intense electric fields of a thundercloud.