Abstract. The U.S. National Lightning Detection Network TM (NLDN) has provided lightning data coveting the continental United States since 1989. Using information gathered from more than 100 sensors, the NLDN provides both real-time and historical lightning data to the electric utility industry, the National Weather Service, and other government and commercial users. It is also the primary source of lightning data for use in research and climatological studies in the United States. In this paper we discuss the design, implementation, and data from the time-of-arrival/magnetic direction finder (TOA/MDF) network following a recent system-wide upgrade. The location accuracy (the maximum dimension of a confidence region around the stroke location) has been improved by a factor of 4 to 8 since 1991, resulting in a median accuracy of 500 m. The expected flash detection efficiency ranges from 80% to 90% for those events with peak currents above 5 kA, varying slightly by region. Subsequent strokes and strokes with peak currents less than 5 kA can now be detected and located; however, the detection efficiency for these events is not quantified in this study because their peak current distribution is not well known.

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Extensive networks of magnetic direction-finding (DF) stations have been installed throughout the western United States and Alaska to facilitate early detection of lightningcaused fires. Each station contains a new wideband directionfinder that responds primarily to cloud-to-ground lightning and discriminates against cloud discharges and background noise. Good angle accuracy is obtained by measuring the lightning direction at just the time the return-stroke electromagnetic field reaches its initial peak. Lightning locations are calculated from the intersections of direction vectors and/or from the ratio of signal strengths recorded simultaneously at two, three, or four DF sites. The development of these systems has proved to be a significant aid in the detection of lightning-caused fires and in fire weather forecasting.

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