Global lightning: Total, cloud and ground flash estimates

Abstract

A worldwide survey of the ratio of cloud flashes to ground flashes in thunderstorms using CGR3 (Cloud‐Ground Ratio #3) instruments at sites covering a latitude range from 59.9°N to 27.3°S between 1986 and 1991 inclusive has provided information on the latitudinal variation of total flash density and the ratio of cloud flashes to total flashes. Two surveys of global lightning occurrence using satellites, the Defense Meteorological Satellite Program (DMSP) survey using optical sensors to detect lightning at near local midnight and the Ionospheric Sounding Satellite‐b (ISS‐b) survey using radio‐frequency lightning detection equipment, were used in conjunction with the CGR3 observations to obtain the latitudinal variation of total flash density over each major land mass and each major ocean. These three surveys and other sources provided data on the annual and diurnal variation in lightning occurrence at various latitudes. We have combined the above information in a computational model of global lightning occurrence and have separated total flashes into cloud flash and ground flash components. Using this model, we estimate the global annual number of total flashes to be 2.05×109 yr−1, corresponding to a mean rate of 65 s−1. This estimate is probably low because of inadequate detection of weak cloud flashes. Depending on the assumption made concerning the latitudinal variation of the ratio of cloud flashes to total flashes, the mean rates of occurrence are between about 51 s−1 and 55 s−1 for cloud flashes and between about 10 s−1 and 14 s−1 for ground flashes. The uncertainty in these estimates is a factor of 2 in either direction. The calculated global diurnal variation has a peak rate of occurrence of about 1.6 times the mean rate at about 1400 universal time (UT) in the northern hemisphere spring and has its lowest rate of about 0.5 times the mean rate at about 0300 UT in the northern hemisphere summer. About 54% of all lightning occurs in the northern hemisphere. The mean global land total flash density is about 8.3 km−2 yr−1. This is about 3.4 times the mean global total flash density over oceans.