A computational study of the relationships linking lightning frequency and other thundercloud parameters

Abstract

Abstract In an effort to optimize the value of global‐scale measurements obtained with the NASA/MSFC satelliteborne Lightning Imaging System (LIS), a simple computational model of thundercloud electrification has been developed, from which it is possible to derive crude relationships between lightning frequency f (which LIS will measure) and cloud parameters such as radar reflectivity Z, precipitation rate P , updraught speed w , cloud radius R , ice‐crystal concentration i and graupel‐pellet concentration N g. Electric field‐growth is assumed to occur via the non‐inductive charging mechanism, for both Fletcher and Hallett‐Mossop types of glaciation mechanisms. A simple criterion is used to distinguish between cloud‐to‐ground and intracloud lightning discharges. f is found to be especially sensitive to w in situations where, as updraught speed increases, the temperature at balance level, T bal , of the upper boundary of the charging zone falls. In these circumstances N 1 and the sizes of the ice hydrometeors are significantly increased, with a corresponding enhancement of the effectiveness of charge transfer. Over a wide range of conditions, f is found to be roughly proportional to the first power of the parameters R 1 N i N g and Z and (in some circumstances) to at least the sixth power of w. the relationship between f and P depends critically on whether or not w and T bal are strongly linked. Hallett‐Mossop glaciation is capable of producing inverted‐polarity lightning from thunderclouds; Fletcher glaciation is not.