Characteristics of Cloud-to-Ground Lightning Return Strokes Occurring over Land and Ocean

Abstract:

Various studies have reported differences between the characteristics of cloud-to-ground lightning return strokes occurring over land and ocean. Orville et al. [2011] used peak currents estimated by the NALDN for negative cloud-to-ground return strokes to report that their magnitudes are, on average, larger over the ocean than over land. This observation was confirmed by Hutchins et al. [2013] and Said et al. [2013]. However, this discrepancy in peak current magnitudes has not been found for positive return strokes over land and ocean [Cummins et al., 2005; Orville et al., 2011].

Nag and Rakov [2014] reported that, for positive return strokes and propagation over relatively long distances (ranging from about 10 to 160 km) over land, the peak electric field derivative (dE/dt) is considerably smaller and the dE/dt half-peak width is much longer than the values reported in the literature [Cooray et al., 2004] for the case of propagation over salt water. Interestingly, Nag and Rakov’s values are comparable to those reported by Heidler and Hopf [1998] for propagation over a few kilometers over land, for which propagation effects due to finitely conducting soil were expected to be relatively small. This implies that there may be a discrepancy between the values of peak electric field derivative and dE/dt half-peak width for positive return strokes occurring over land versus salt water related to the source, as opposed to propagation effects. On the other hand, Cummins et al. [2005] examined U.S. NLDN reported risetimes of both positive and negative first strokes and found them to be similar for lightning over land and ocean.

In this study, we examine magnetic field risetimes and estimated peak currents of return strokes measured by NLDN sensors located close to the coastline for lightning occurring over land and ocean. For “oceanic” return stroke magnetic fields propagating over ocean and several tens of kilometers over relatively high conductivity (>10 mS/m) soil, risetimes were found to be similar to those for land-based return stroke magnetic fields propagating several tens of kilometers over relatively high conductivity soil. We will discuss the significance of these results and examine (via modeling) possible reasons for apparent differences between the characteristics of cloud-to-ground lightning over land and ocean.