Changes in measured lightning flash count and return stroke peak current after the 1994 US National Lightning Detection Network upgrade - 1. Observations
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A total of more than 134 million cloud-to-ground lightning flashes (127 million negative, 7 million positive), occurring during 1989-1995 in the continental United States, have been studied on a monthly and yearly basis for variations in flash count, first stroke peak current, and polarity. The years 1989-1993 cover a period in which similar instrumentation was used throughout the United States. In 1994 the National Lightning Detection Network (NLDN) underwent a system-wide upgrade to improve location accuracy and detection efficiency. As a result of this upgrade, we observe in the NLDN that the negative mean peak current decreased from a preupgrade (1989-1993) mean of 37.5 kA to a 1995 value of 30.2 kA, a decrease of 3.4 standard deviations. The positive mean peak current decreased from 54.4 to 31.6 kA, a 5.0 standard deviation decrease. The NLDN negative flash count increased 1.2 standard deviations, from a preupgrade mean of 16.7 million flashes yr-1 to 20.6 million flashes in 1995. The positive flash count increased 6.2 standard deviations, from an average of 696,000 flashes yr-1 before the upgrade to 2.1 million flashes in 1995. Both the negative and the positive flash count increases were predominantly at low peak currents. Copyright 1999 by the American Geophysical Union.