Changes in measured lightning flash count and return stroke peak current after the 1994 US National Lightning Detection Network upgrade - 1. Observations

A total of more than 134 million cloud-to-ground lightning flashes (127 mil lion negative, 7 million positive), occurring during 1989-1995 in the conti nental United States, have been studied on a monthly and yearly basis for v ariations in flash count, first stroke peak current, and polarity. The year s 1989-1993 cover a period in which similar instrumentation was used throug hout the United States. In 1994 the National Lightning Detection Network (N LDN) underwent a system-wide upgrade to improve location accuracy and detec tion efficiency. As a result of this upgrade, we observe in the NLDN that t he 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 deviation s. The positive mean peak current decreased from 54.4 to 31.6 kA, a 5.0 sta ndard deviation decrease. The NLDN negative flash count increased 1.2 stand ard 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 t o 2.1 million flashes in 1995. Both the negative and the positive flash cou nt increases were predominantly at low peak currents.