The North American Lightning Detection Network (NALDN)—First Results: 1998–2000

Cloud-to-ground lightning data have been analyzed for the years 1998–2000 for North America (Canada plus the contiguous United States) for all ground flashes, positive flashes, the percentage of positive lightning, peak currents for negative and positive lightning, and for negative and positive multiplicity. The authors examined a total of 88.7 million flashes divided among the three years: 31.1 million (1998), 29.5 million (1999), and 28.2 million (2000). Annual flash densities are derived from 245–424 km2 regions and are uncorrected for flash detection efficiency. The highest flash densities in Canada are along the U.S.–Canadian border (1–3 flashes km−2), and in the United States along the Gulf of Mexico coast and Florida (exceeding 9 flashes km−2). Maximum annual positive flash densities in Canada generally range primarily from 0.1 to 0.3 flashes km−2, and in the United States to over 0.7 flashes km−2 (areas in the Midwest, the Gulf Coast, and Florida). Areas of greater than 20% positive lightning occur throughout British Columbia and the midwest United States extending into Manitoba and Ontario. High percent positive also occurs in Quebec and much of eastern Canada. The median negative peak current is 16.5 kA. The median positive peak current, with the peak currents less than 10 kA removed from the calculation, is 19.8 kA. Median positive peak currents exceed 35 kA in the Midwest from west Texas to Nebraska to the Canadian border. The area of maximum mean negative multiplicity, exceeding 2.6 strokes, occurs in western Canada from just east of the British Columbia–Alberta border to and including Saskatchewan. Mean negative multiplicity also peaks in the southeastern United States. Mean positive multiplicity is observed to have maximum values in Alberta, Saskatchewan, and in a region centered on Tennessee. The authors examined the time of maximum flash rate in North America and find it is over land in the daytime hours with the exception of a region of maximum nighttime lightning in midcontinent that extends from the midwestern United States into Canada. Over the waters surrounding the North American continent, the maximum lightning is principally at night, including the coastal Pacific, the Gulf of California, the Gulf of Mexico, and the coastal waters of the North Atlantic.