Analysis of NLDN-estimated peak currents for positive cloud-to-ground lightning

We infer peak currents from radiation electric field peaks of 48 positive return strokes acquired at the Lightning Observatory in Gainesville (LOG), Florida, in 2007-2008. In doing so, we use the transmission line model, NLDN-reported distances, and assumed return-stroke speed. From a similar analysis of negative first and subsequent strokes, it appears that the implied return-stroke speed in the NLDN field-to-current conversion equation is 1.8 x 10 m/s. The NLDN uses the same field-to-current conversion procedure (and hence the same implied return-stroke speed) for positive return strokes. However, for positive return strokes and an assumed return stroke speed of 1.8 x 10 m/s, NLDN-reported peak currents are not consistent with the transmission line model predicted peak currents (differ by 27% on average). The discrepancy between regression equations for negative first and subsequent strokes on the one hand and positive return strokes on the other hand suggests that the NLDN procedure to compensate for field propagation effects and find the average range-normalized signal strength (RNSS) works differently for these two groups of strokes. We find that the difference can be explained by the bias towards NLDN sensor reports from larger distances for positive strokes combined with the higher relative sensor gain at larger distances. Keywordspositive lightning; NLDN; peak current