Latitudinal variation of lightning occurrence characteristics

Abstract

Lightning occurrence has been recorded at 14 sites in 11 countries covering latitudes from 60°N to 27°S between 1987 and 1991 inclusive using lightning detection instruments denoted CGR3. These instruments have an effective range between about 12 and 16 km depending upon site latitude and type of lightning and have separate registers for negative ground flashes, positive ground flashes, and cloud flashes. From the registrations we have obtained approximate values of the annual lightning total flash density, Nt, and the cloud flash‐to‐ground flash ratio, z, at each site. We have observed a well‐marked variation of Nt with latitude from 61 km−2 yr−1 at 1° to 0.66 km−2 yr−1 at 60° latitude. Nt falls by a factor of about 10 for a 30° increase in latitude, in reasonable agreement with estimates based on satellite observations. Earlier studies concerning the latitudinal variation of z showed that z was high (about 6 to 9) in the tropics, was about 3 to 4 in the subtropics and temperate regions, and was about 1 to 1.5 at high latitudes. However, we have found tropical values of z in the range 0.5 to 3.4 (weighted mean 2.3 for latitude range 0° to 20°) and similar values in subtropical and temperate regions (range from 1.1 to 3.8 and weighted mean 2.2 for the latitude range 20° to 40°). At high latitudes (40° to 60°) our measured values (range 1.0 to 1.5 and weighted mean 1.3) agree with the earlier studies. The weighting was by years of observation at the site. The mean of the values of z measured at 14 sites was 1.9. Values of z at 13 of the sites were within a factor of 2 of this mean. We estimate the global mean value of z to be 2.3, based on weighting site z values by site total flash density. We have compared lightning occurrence characteristics at eight sites between 60°N and 27°S.