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We examined risetimes for negative first strokes occurring over land (3,427 strokes) and ocean (4,302 strokes) in Florida reported by the U.S. National Lightning Detection Network. The waveform characteristics were measured by a single National Lightning Detection Network sensor located less than 2 km from the coastline in Florida. All risetime measurements (10-50, 50-90, 10-90%, and threshold to peak) over both land and ocean exhibited statistically significant positive correlation with estimated peak current, but with small correlation coefficients. For a subset of 1,247 strokes selected to mitigate bias in our data, the median 10-50% magnetic field risetime was found to be 11% longer and the proportion of strokes having 10-50% risetimes in the range of 2.6 to 4.8 mu s was 42% larger over ocean. We speculate that the slow-front portion of radiation field waveforms for negative first strokes is affected by differences in surface properties over land versus over ocean that impact upward leader initiation.

Plain Language Summary Lightning, a high current electrical spark in the Earth's atmosphere, has significant public safety and economic impacts. It kills or injures hundreds of people per year in the United States and tens of thousands per year globally. In the United States alone, the annual infrastructure-related economic impact of lightning is estimated to be in the billions of dollars. There has been sustained interest in the differences between characteristics of cloud-to-ground lightning occurring over land versus over ocean. For example, it is known that lightning over ocean is significantly more powerful. While some theories exist, the exact reason for this observation remains unknown. Differences between land-based and oceanic lightning may signify underlying dissimilarities in the nature and development of deep convection over ocean versus over land. In this paper, we examine the rising portion of cloud-to-ground lightning current by studying remotely measured magnetic field waveforms. We find that the 10-50% risetimes for negative first return strokes over ocean were somewhat longer than those for strokes over land. We speculate that this difference may be due to lightning attaching differently to the ocean surface versus to the land surface due to the relative smoothness of the ocean surface.