Negative first stroke leader characteristics in cloud-to-ground lightning over land and ocean

Abstract

We examine downward leader characteristics for negative first return strokes, along with estimated first stroke peak currents, for lightning occurring over land and ocean reported by the U. S. National Lightning Detection Network (NLDN). For the first time, to the best of our knowledge, we report independent evidence that supports the observations by lightning locating systems of higher first stroke peak currents for lightning occurring over ocean than land. We analyzed lightning occurring in five circular regions, each with 50 km diameter. In western Florida, the median stepped-leader duration was 17% shorter over ocean than over land and in eastern Florida the median durations were 21% and 39% shorter over two oceanic regions than over land. We infer that the shorter durations over ocean simply reflect the higher (25% in western Florida and 11%-16% in eastern Florida) oceanic return stroke peak currents reported by the NLDN. These findings indicate that the cloud charge structure for (at least some) oceanic storms are different than those for storms over land. The percentage of flashes that had at least one NLDN-reported negative cloud pulse prior to the first negative cloud-to-ground stroke was found to be about the same over land and ocean. Using regression analysis, we found no evidence that the relationship between leader duration and first return stroke peak current is different over land and ocean.