The role of electrical forces in charge separation by falling precipitation in thunderclouds

Abstract

Theoretical expressions for the maximum electric field, the total and leakage currents, and the recovery time of the electric field have been developed for the failing precipitation mechanism of charge separation in thunderclouds. The derivations take into consideration the electrical forces acting on the precipitation and smaller particles. Quantitative dependence of the derived electrical parameters on the precipitation intensity and on the charge density of the precipitation particles has been studied in the case of the ice crystal-hailstone collision mechanism and the drop-splintering mechanism. Certain conditions for optimum efficiency of this charge separation process have been established and the minimum rates of precipitation required to generate different values of electric field have been calculated. It is inferred that precipitation particles of larger sizes will contribute more efficiently than the smaller ones in generating high electric fields. The results suggest lower and upper limits for the average charge density on precipitation particles so that the theoretical results might match the experimental data. Furthermore, it is shown that when the precipitation intensity and the charge density on the precipitation particles are high, the currents due to electrical forces acting on precipitation particles, which hitherto have been ignored, become comparable or even larger than the sum of currents due to conductivity in the thundercloud and due to the point discharge from the ground. It is concluded from a comparison of the results to the available data that failing precipitation may not he the dominant cause in separating charges in thunderclouds with intense electrification.