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LIGHTNING PHENOMENOLOGY 25 longitude at both dawn and dusk and presented as a function of latitude. The dusk distributions show a smooth change as the seasons change. But note the secondary peak at 30-40Â° that persists through all the dusk distributions except August-September. The dawn distributions do not change as smoothly, but the seasonal shift is apparent. The enhanced polar-front activity at dawn is quite evident in the November-December southern hemisphere and the April- June northern hemisphere. At dawn there appears to be an overall minimum for the period January-March. Analysis of the lightning rates for an entire year show a variation of 10 percent from a global value that is estimated to range from 40 to 120 flashes per second. Figure 1.2 The latitudinal variation of the dawn and dusk global lightning activity as a function of season. Adapted from Kowalczyk and Bauer (1981) and Turman and Edgar (1982). Other studies using the DMSP data from a different sensor provide a glimpse at the global midnight lightning activity with a spatial resolution of approximately 100 km (Orville and Spencer, 1979; Orville 1981). A study of the global midnight lightning activity yields a lightning rate of 96 flashes per second, but this could be in error by a factor of 2 (Orville and Spencer, 1979). Orville plotted a series of monthly maps reproduced in Figure 1.3 for the months of September, October, and November. The progression of the lightning activity toward the southern hemisphere as summer approaches in that hemisphere is evident. The striking absence of lightning over the ocean is apparent in all three months and clearly shows the importance of land in the production of thunderstorms. Radio Detectors Recent measurements of high-frequency radio noise by the Ionosphere Sounding Satellite-B have been used by Kotaki et al. (1981) to estimate a global lightning flash rate of approximately 300 per second, in contrast to the optical measurements discussed previously. The radio measurements may overestimate the lightning frequency since it is assumed that all the emissions are produced by lightning. But the satellite optical measurements are uncertain in estimating the lightning rate since the fraction of lightning that is actually detected depends on a calibration factor that represents a best estimate. Despite the availability of satellites to estimate the global lightning activity, we have made little progress in obtaining a flash rate with small error bars owing to the present experimental limitations of sensor sensitivity, area coverage, and the number of satellite platforms. Nevertheless, the satellite observation provides us with the first reliable estimate of the distribution of global lightning. The resolution of the varying global flash rate estimates may depend on the close coordination of satellite-based and ground-based observations of lightning and the availability of larger-coverage-area platforms, such as geosynchronous satellites. GROUND OBSERVATIONS OF LIGHTNING Most of our information on the characteristics of lightning has come, and will continue to come, from ground- based observations of the lightning flash. Many of these studies have focused on the ratio of intracloud to cloud-to- ground flashes, the lightning ground-flash density, and the flashing rate of different types of thunderstorms. Intracloud Versus Cloud-to-Ground Lightning The ratio of intracloud to cloud-to-ground lightning is of fundamental importance. How does this ratio vary with latitude and longitude, and how does this ratio vary in the lifetime of a storm? Are there storms that have nearly all intracloud flashes and consequently are less damaging, and are there storms that have almost all ground flashes and consequently are of greater concern?
LIGHTNING PHENOMENOLOGY 26 Figure 1.3 Three maps showing the progression of monthly lightning for (a) September, (b) October, and (c) November. From Orville (1981), reproduced with permission of the American Meteorological Society.