Performance evaluation of the U.S. National Lightning Detection Network in eastern New York: 1. Detection efficiency

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The detection efficiency (DE) of the U.S. National Lightning Detection Network (NLDN) has been evaluated using a large data set of video observations of cloud‐to‐ground lightning activity in the vicinity of Albany, New York. These data were acquired during the summers of 1993, 1994, and 1995, the latter being the year of completion of a major upgrade of the network to the improved accuracy from combined technology (IMPACT) configuration. For 1993, we find a flash DE value of 67% based upon 517 cloud‐to‐ground flashes documented on video. The latter two years yielded both flash and stroke DEs: in 1994, 86% of 893 flashes and 67% of 2162 strokes were detected; in 1995, 72% of 433 flashes and 47% of 1242 strokes were detected. The higher DEs of 1994 relative to 1995 are likely due to additional sensors deployed locally during the initial stage of the IMPACT upgrade. Detection efficiencies were found to vary significantly from storm to storm in each season, likely due to the inherent variability of return stroke characteristics between storms. For a special subset of 92 strokes of known location and measured electric‐field change, peak current estimates were generated using the transmission‐line model and a return stroke speed of 1.2×l08 m/s. This speed was selected, as it is the effective speed used in present NLDN peak current estimates. For this 92‐stroke data subset, the stroke DE depended upon peak current: strokes with peak currents greater than 14 kA were almost always detected (39 of 40); below 14 kA, the DE dropped until by 6–10 kA, the stroke DE was only 18% (three of 17). None of 14 strokes with estimated peak currents below 6 kA was detected. If the IMPACT design constraint of an effective 5‐kA minimum peak current is applied to our 92‐stroke subset, the respective flash and stroke DEs are 84% and 69%; this is consistent with NLDN model predicted performance in this area. As a faster return stroke speed, possibly 1.8×108 m/s, would seem appropriate, the above cited current values would need to be scaled downward by a factor of 2/3, implying greater actual sensitivity of the NLDN to weaker strokes. However, a commensurate adjustment downward would be required of present NLDN‐derived peak current estimates as well.