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Lightning Phenomenology INTRODUCTION RICHARD E. ORVILLE State University of New York atAlbany Severe weather phenomena that disrupt our lives in- clude tornadoes, hail, high winds, hurricanes, snow- storms, and lightning. It is not well known that in most years, lightning ranks as the number one killer, followed closely by tornadoes. Much less dramatic than a tornado passing through an area or a severe snowstorm that par- alyzes a city, a lightning ground strike can quickly kill one or two people in less than a second with little or no warning. Annually in the United States about 100 peo- ple are killed by lightning strikes, and reliable estimates for the world would be in the thousands. Lightning on a global and regional scale is an area of science that brings together the interests of the atmospheric physicist, chemist, and meteorologist in an effort to learn its char- acteristics. The phenomenology of lightning involves the fre- quency of lightning observed over large spatial and time scales. It involves the maximum and average flashing rate per unit area and the variation of flash characteris- tics with location and storm type. Studies of lightning phenomenology can now be discussed in terms of both satellite and ground-based observations. With the use of satellites, we obtain data on the global lightning flash rates and the distribution of lightning with respect to the continents and oceans. With the extensive use of 23 ground-based observations, we can determine the flash- ing rates and flash characteristics of individual storms. In addition, we can monitor the variations of the ground flashes as a function of location and storm type. SATELLITE OBSERVATIONS OF LIGHTNING Optical Detectors Significant advances in obtaining a better estimate of global flash rates and distribution have occurred as the result of satellite lightning observations in the last dec- ade. Turman (1978, 1979), Turman et al. (1978), and Turman and Edgar (1982), using optical detectors on the Defense Meteorological Satellite Program (DMSP) satellites, showed the distribution of lightning at dawn and dusk for a period of 1 year. One example of this recent result is shown in Figure 1.1, where the dusk lightning distribution for November-December 1977 demonstrates the spatial distribution and the rate. Note that the lightning is found mostly in the southern hemi- sphere, but significant activity still occurs in the north- ern hemisphere. The latitudinal and seasonal variation of the light- ning activity is best shown by examining Figure 1.2 (Ko- walezyk, 1981; Turman and Edgar, 1982~. In this histo- gram, the lightning rate has been summed over

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LIGHTNING PHENOMENOLOGY 00: ~ SEP-OCT _ _ 0~ ~0 _ -A 20 _ NOV- DEC _ - 15 _ _ 10 ~ 1~ - Z 20F JAN-MAR 10 _ ~ O _ _ o 25 - APR 20 _ 15 _ 10 _ DAWN I ED 25 - MAY-JUN ~ tool -_,,Wf~ ~ ~ ~ ~ ~. . . . 60~ 4]S US ~ 20~ 40~ ~ ~ US PUS 0 20 LATITUDE 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). 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 sec- ondary peak at 30-40 that persists through all the dusk distributions except August-September. The dawn dis- tributions 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-lune northern hemi- sphere. At dawn there appears to be an overall mini- mum 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. Other studies using the DMSP data from a different sensor provide a glimpse at the global midnight light- ning 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, 19794. Orville plotted a series of monthly maps reproduced in Figure 1.3 for the months of September, October, and 25 November. The progression of the lightning activity to- ward 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 produc- tion 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 fre- quency since it is assumed that all the emissions are pro- duced by lightning. But the satellite optical measure- ments 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 satel- lite-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 thun- derstorms. 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?

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26 (b) (c) L -hem 0NSP SHTELL I TE HIDNI5HT L16HTNIN6 SEPT 1977 (1813 FLASHES) ~''~'~ ~ - -Gus ~ ~ -A 1 as' C:, DEEP SHTELLITE MIDN16HT L16HTNINE DCT 1977 (2121 FLASHES) RICHARD E. ORVILLE By' I; .'~ id . .. N~'-"' ..,T, ;._ ~ , ~... Or , ~ 'vm.2.-... ,, 'I 4. ~ W.. DH5P SRTELL I TE HI0N16HT L16HTNIN6 NDV 1977 (2178 ~85HE5) ~ .'e''- I- ~.~}'... 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.

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LIGHTNING PHENOMENOLOGY Studies by Prentice and Mackerras (1977) have sum- marized much of the available data on the ratio of intra- cloud to cloud-to-ground flashes (Nt.IN.,). From an anal- ysis of 29 data sets from 13 countries, they obtain the following relationship for an average thunderstorm: Nt.IN~ = (4.16 + 2.16 cos 3~) (06+ 72-098~' (1.1) where T. the number of thunder days per year, is less than or equal to 84 and X, the latitude in the northern and southern hemispheres, is less than 60. If the num- ber of thunder days is unknown, then the ratio can be estimated from the relation Nt.IN~ = (4.16 + 2.16 cos 3~. (1.2) This result is plotted in Figure 1.4. Note that the ratio has the highest value in the tropics where most of the lightning was shown to occur by the satellite data. Re- call, however, that the satellite data were composed of both intracloud and cloud-to-ground lightning flashes. There is at the moment no way to distinguish between a ground flash and an intracloud flash from a satellite. Lightning Flash Density The number of lightning strikes per unit time per unit area, or the flash density, is a fundamental quantity of interest. Most of the available information has been ob- tained with lightning flash counters. Prentice (1977) summarized the values for several ge- ographical areas and reported 5 flashes per km2 per year in Queensland, Australia; 0.2 to 3 flashes per km2 per year in Norway, Sweden, and Finland; and 0.05 to 15 flashes per km2 per year in South Africa depending on the location. Piepgrass et al. (1982) reported the results of studying 79 summer storms at the Kennedy Space Center, Flor- ida, which produced 10 or more discharges, during the years 1974-1980. Using field mill sites covering an effec- tive area of 625 km2, they observed an area flash density for all discharges during June, July, and August to range from 4 to 27 discharges per km2 per month, with a sys- tematic uncertainty of perhaps a factor of 2 in the sam- ple area. The mean and the standard deviation of the monthly area density over the above years was 12 + 8 discharges per km2. Approximately 38 percent of the dis- charges were ground flashes. Therefore, they were able to estimate the ground flash density to be 4.6 + 3.1 flashes per km2 per month. The most recent estimate of the ground flash density 27 in the United States has been made by Maier and Piotro- wicz (1983) using thunderstorm hour statistics and is re- produced as Figure 1.5. They used thunderstorm dura- tion data from approximately 450 aviation weather reporting stations, each with an uninterrupted 30 years of records. The station density available is twice that of any previous thunderstorm frequency analysis of the United States. The maximum annual ground flash den- sities of 18 per km2 are found in the western interior of Florida. High flash densities greater than 12 per km2 are found over much of Florida and westward to eastern Texas. Flash densities greater than 8 per km2 are found in most of Oklahoma, Kansas, Missouri, Arkansas, Lou- isiana, Mississippi, and Tennessee. Most western and northeastern states have flash densities that are less than 4 per km2. Lightning Flash and Related Characteristics Data from two summers at the Kennedy Space Cen- ter, Florida, have been used to estimate the flashing rates in thunderstorms (Livingston and Krider, 1978~. It was observed that large storms evolve through an ini- tial, an active, and a final phase of activity. Most of the lightning activity was observed to occur in the active phase with 71 percent of the lightning, although this phase of the storm occupied only 27 percent of the total storm duration. During the active phase, 42-52 percent of all lightning was to ground, while during the final storm period, only about 20 percent of the lightning was to ground. The discharge rate for all storms observed in 1975 was approximately 4 flashes per minute with a maximum flashing rate of 26 discharges per minute dur- ing any 5-minute period. The highest flashing rate aver- aged over an entire storm was about 9 discharges per minute for over 200 minutes. More recent data from a 4- year interval indicates that the mean rate of flashes is about 2.4 discharges per minute per storm (Piepgrass et al., 1982~. The relationship of rainfall to lightning flash rates has been investigated by Piepgrass et al. (1982~. They re- ported that when the meteorological conditions favor the production of lightning, there is almost a direct pro- portionality between the total rain volume and the total number of flashes. Maier et al. (1978) noted in an earlier paper that the lightning counts were proportional to the total storm rainfall and that the proportionality in- creased with the rain volume until the rainfall reached about 1.2 to 2.7 X 104 m3 per flash. Beyond these vol- umes, storms that produced more rainfall tended to pro- duce proportionally less lightning. Piepgrass et al. (1982) point out that, "Clearly, these problems warrant further study."

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28 8 4 2 _ 10 . O 1 1 1 1 1 1 1 0 10 20 30 40 50 60 70 X, DEGREES FIGURE 1.4 The ratio of intracloud to cloud-to-ground lightning as function of latitude. From Prentice and Mackerras (1977). Lightning Location Networks The study of lightning phenomenology has made a major advance in the last decade with the introduction of new magnetic direction-finding techniques (Krider et al., 1980) that provide the means to monitor ground strikes over areas exceeding 106 km2. Extensive networks of lightning direction finders have been established for forest-fire detection in the western United States, Can- ada, and Alaska. Figure 1.6 shows the coverage as of the summer of 1984, and it can be predicted that within the next few years the entire United States will be covered. One expanding lightning detection network covers FIGURE 1.5 Lightning flash density esti- mates on an annual basis. Adapted from Maier and Piotrowicz (1983) and MacGor- manetal. (1984). RICHARD E. ORVILLE the East Coast and is approaching the Mississippi River to provide coverage of the eastern part of the United States (Orville et al., 1983~. This network is operated by the State University of New York at Albany in a multi- drop communication network that links all the direction finders to one computer. Data are now retrieved on the time, location, number of strokes in the flash, polarity of the charge lowered to ground, and amplitude of the peak magnetic radiation field that can be related to the maximum current in the first stroke. These data, in turn, can be analyzed and related to the meteorological patterns producing the observed phenomena. To report all initial results would far exceed the space available in this brief paper; nevertheless, it is interest- ing to note a few observations. The highest ground flash rate recorded by the East Coast Network occurred on June 13, 1984, when 50,836 flashes were detected in a 24-hour period over an area of approximately 250,000 km2. The highest hourly summary was 7800 flashes with the highest S-minute rate exceeding 10,000 ground flashes per hour. These results are remarkable when it is realized that these flash rates were from storms in only three states- Pennsylvania, New Jersey, and part of New York and at the time were producing approxi- mately 3 percent of the entire global lightning activity. Other results indicate that lightning is recorded in every week of the year along the East Coast and that the polarity of the lightning ground strikes shows a change from negative to positive in the fall and a shift back to negative in the spring. A discussion of positive lightning and its characteristics is presented by Rust (Chapter 3, this volume). 45 4( 35' 30' 25 _ Mean Annuol Ground Flosh Density (flashes/km2 ) ~ I _. loo105 loon as 90 as 80.

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LIGHTNING PHENOMENOLOGY ~i~ ~REFERENCES FIGURE 1.6 Coverage of North America by time of arrival (dashed lines) and wide-band magnetic direction finders (solid lines) as of the summer of 1985. CONCLUSIONS The past decade has been a period of significant ad- vances in lightning knowledge. Satellite studies have provided the first confirmation of the early estimates of the global lightning flash rates and added new informa- tion on the distribution of lightning over the land and over the ocean. The development of widely distributed ground-based lightning networks provides for the first time the ability to monitor and calculate lightning char- acteristics in near real time. Relating these parameters to the meteorological observations of visible and infra- red images from space and to radar observations from the ground poses a major challenge in the near future. 29 Kotaki, M., I. Kuriki, C. Katoh, and H. Sugiuchij (1981). Global dis- tribution of thunderstorm activity, J. Radio Res. Labs. Japan 66. Kowalczyk, M., and E. Bauer (1981). Lightning as a source of NOX in the troposphere, final report FAA-EE-82-4. Krider, E. P., A. E. Pifer, and D. L. Vance (1980). Lightning direc- tion-finding systems for forest fire detection, Bull. Am. Meteorol. Soc. 61, 980-986. Livingston, J. M., and E. P. Krider (1978~. Electric fields produced by Florida thunderstorm, J. Geophys. Res. 83, 385-401. MacGorman, D. R., M. W. Maier, and W. D. Rust (1984). Lightning strike density for the contiguous United States from thunderstorm duration records, prepared for Division of Health, Siting and Waste Management, Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission, Washington, D.C., 44 pp. Maier, M. W., and J. M. Piotrowicz (1983~. Improved estimates of the area density of cloud-to-ground lightning over the United States, presented at 8th International Aerospace and Ground Conference on Lightning and Static Electricity, June 21-23, 1983, Forth Worth, F ~ exas. Maier, M. W., A. G. Boulanger, and J. Sarlet (1978). Cloud-to- ground lightning frequency over south Florida, preprint, Confer- ence on Cloud Physics and Atmospheric Electricity (Issaquah, Wash.), American Meteorological Society, Boston, Mass., pp. 605- 610. Orville, R. E. (1981). Global distribution of midnight lightning Sep- tember to November 1977, Mon. Weather Rev. 109, 391-395. Orville, R. E., and D. W. Spencer (1979). Global lightning flash fre- quency Mon. Weather Rev. 107, 934-943. Orville, R. E., R. W. Henderson, and L. F. Bosart (1983~. An East Coast lightning detection network, Bull. Am. Meteorol. Soc. 64, 1029-1037. Piepgrass, M. V., E. P. Krider, and C. B. Moore (1982). Lightning and surface rainfall during Florida thunderstorms, J. Geophys. Res. 87, 11193-11201. Prentice, S. A. (1977). Frequencies of lightning discharges, in Physics of Lightning, R. H. Golde, ea., Academic Press, New York, pp. 465-496. Prentice, S. A., and D. Mackerras (1977~. The ratio of cloud to cloud- ground lightning flashes in thunderstorms, J. Appl. Meteorol. 16, 545-549. Turman, B. N. (1978). Analysis of lightning data from the DMSP satel- lite, J. Geophys. Res. 83, 5019-5024. Turman, B. N. (1979~. Lightning detection from space, Am. Scientist 67, 321-329. Turman, B. N., and B. C. Edgar (1982~. Global lightning distribu- tions at dawn and dusk, J. Geophys. Res. 87, 1191-1206. Turman, B. N., B. C. Edgar, and L. N. Friesen (1978~. Global light- ning distribution at dawn and dusk for August-September 1977, EOS 59, 285.