Changes in measured lightning return stroke peak current after the 1994 National Lightning Detection Network upgrade

Abstract

Since a comprehensive upgrade of the US National Lightning Detection Network (NLDN) in 1994, the mean peak current of detected cloud-to-ground (CG) lightning flashes has decreased, the number of detected flashes has increased, and the percentage of flashes with positive polarity has increased. NLDN archived data for the years 1989-95 was analyzed to characterize this change. Models of lightning return stroke radiation and its detection by NLDN sensors were developed to simulate the effects of the changes made to the sensors and explain the cause of the post-upgrade changes in NLDN data. The US negative mean peak current decreased from a pre-upgrade (1989-93) mean of 37.5 kA to a 1995 value of 30.2 kA, a decrease of 3.39 standard deviations. The positive mean peak current decreased from 54.4 kA to 31.6 kA, a 4.97 standard deviation decrease. The US negative flash count increased 1.16 standard deviations, from a pre-upgrade mean of 16.7 million flashes yr-1 to 20.6 million flashes in 1995. The positive flash count increased 6.22 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 positive flash count increases were predominant at low peak currents. A model of return stroke detection by NLDN sensors was used to simulate one of the adjustments made to NLDN sensors during the upgrade. Decreasing the pulsewidth detection criterion used by the sensors increases their effective detection range, which increases their sensitivity to weak flashes (due to NLDN sensor geometry, increasing sensitivity has little effect on detection of strong flashes). The increased detection of weak flashes accounts for the decrease in mean peak currents and the increase in flash counts.