A COMPUTATIONAL STUDY OF THE RELATIONSHIPS LINKING LIGHTNING FREQUENCY AND OTHER THUNDERCLOUD PARAMETERS

In an effort to optimize the value of global-scale measurements obtained with the NASA/MSFC satellite-borne 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 N-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 Ni 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, 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.