Magnetic Field Risetimes of Negative Lightning First Return Strokes Over Land and Ocean

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 μs 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.