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In an effort to assess lightning occurrence in the Kwajalein Atoll, a series of Model 141 Advanced Lightning Direction Finder (ALDF) sensors were installed at four locations in the Kwajalein Atoll. The sensors, which are located at Kwajalein (8.6 degrees north latitude and 167.7 degrees east longitude), Illeginni (9.1 degrees north latitude and 167.4 degrees east latitude), Meck (9.0 degrees north latitude and 167.4 degrees east longitude), and Gegan (9.3 degrees north latitude and 167.6 degrees east longitude) became operational in 1996 and have been collecting valuable data concerning cloud-to-surface lightning in the vicinity of the atoll since then.

The data collected by the sensors serves as a means to discover relationships that may exist between the tropical environment of the Kwajalein Atoll and the lightning that occurs there. By examining these relationships, those who are involved in operations at the U. S. Army base on the atoll can access information that will aid them in critical decisions concerning mission planning and execution.

This study concerns the data for 1996 and 1997. Since there are only two years of data, it seems logical that the two years could be compared to each other as well as to data that constitutes a typical year in the tropics. The comparison structure of the study makes relationships between the two years easier to extract from the data. Charts were made depicting the number of lightning strikes for each month of the year for the two years. In addition, there are charts portraying the number of strikes within range bins that are concentric to specified locations within the atoll. Finally, there are charts that compare the number of lightning days with strikes within 30 miles of specific locations for both 1996 and 1997.

The Advanced Lightning Direction Finder (ALDF) sensors are classified as low gain sensors that detect cloud-to-surface lightning strikes. The sensors operate with an accuracy of approximately 0.5 km. The lightning detectors, aided by the Model 280 Advanced Position Analyzer (APA) measure the azimuth between the detecting sensor and the actual lightning strike. By determining the intersection of the azimuths for multiple sensors, it is possible to more accurately determine strike location. For each strike, the system records the date, time, and latitude/longitude position.

First, charts were made depicting the number of lightning strikes that occurred within each month for both 1996 and 1997. Figures 3a and 3b show these numbers.

Figure 3a is more indicative of a typical year in the tropics. The rainy season, which begins May and normally peaks in September and October, roughly corresponds with the peak in August and high strike counts that continue through October.

Figure 3b displays a non-typical year in the tropical environment of the Marshall Islands. The peak in lightning activity that occurred in April of 1997 may be explained as a phenomena associated with the onset of El Nio. Despite the fact that the effects of El Nio are not entirely understood at this point, it is accepted that El Nio brings slightly warmer water to various locations in the Western Pacific, including Kwajalein. At the time, the water in the area was warmer than normal by about one degree Celsius. It is possible that an increase in convective activity was brought about by the presence of the warmer water coupled with cooler than normal air aloft, which was observed in the region during the month of April. Next, range bins were set up to count the number of lightning strikes that occurred near the center of the Kwajalein Atoll. Recorded location data was converted to miles from a point designated as the center of the atoll. Then, a histogram analysis was performed for concentric range bins of ten miles each, going out to 100 miles. Figure 3c displays the values for each of the range bins.

Figure 3c shows that the sensors detected forty-three lightning strikes within ten miles of the center of the Kwajalein Atoll in 1997. This area is mainly within the lagoon, and therefore this information is most useful to those boating in the area. Most atoll land surfaces, however, are found within thirty miles of the atoll's center. By adding each of the range bins out to thirty miles it is observed that there were 390 cloud-to-surface strikes detected by the sensors that occurred roughly within the atoll in 1997. A chart of this type was also made for 1996. In addition, charts were made for both 1996 and 1997 for particular locations that are important to operations in the Kwajalein Atoll. The specific locations include Kwajalein, Meck, and Roi-Namur, which are the sites of crucial equipment and mission personnel. The results from these particular charts will be presented at the conference. Finally, charts were made that show the number of lightning days where strikes occurred within 30 miles of the center of the Kwajalein Atoll. The charts for this data are set up as a comparison between 1996 and 1997. In other words, the information for 1996 and 1997 was put onto one chart so conclusions could be drawn more easily. These charts are particularly important because they help to determine whether lightning occurs in small amounts over many days or in greater amounts over fewer days. Using this analysis, one can determine whether lightning is a constant worry in the atoll or if it is a concern only on the few days that it actually occurs. Figure 3d shows the relationship between 1996 and 1997 in terms of the number of lightning days.

Figure 3d depicts a somewhat typical situation for 1996 with the number of lightning days being highest in the rainy season. The chart also shows that the spring of 1997 was especially unusual. This period, which is typically the dry season in the Kwajalein Atoll, shows greater than normal convective activity in the month of April. Once again, the onset of El Nio is the probable cause of the unusual lightning events. The chart also shows that in 1997, what is typically considered the rainy season offered very little in terms of lightning. At this time, the atoll was entering a period of severe drought. The drought was temporarily interrupted in December by Typhoon Paka, but the lightning count continued to stay low in that month due to the fact that lightning is not typically associated with tropical storms (Riehl, 1979). As with the range bin charts, other charts were created and analyzed for Kwajalein, Meck, and Roi-Namur for both 1996 and 1997. Further results about the center of the atoll, as well as the other locations specified will be presented at the conference.

• Global Atmospherics, Inc., 1996: Introduction to Lightning Detection.
• Riehl, Herbert, 1979: Climate and Weather in the Tropics. Academic Press, London.
• Southeast Regional Climate Center (SERCC). Sea Surface Temperatures - Mean and Anomalous Sea Surface Temperatures for 1997[Homepage of South Carolina Department of Natural Resources], Formerly available at: http://water.dnr.state.sc.us/climate/sercc/sst_97.html .
• United States Army Kwajalein Atoll/Kwajalein Missile Range (USAKA/RTS), 1997: Instrumentation and Support Facilities Manual.
• United States Army Kwajalein Atoll/Kwajalein Missile Range (USAKA/RTS), 1992: Range Users Manual.

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