Emission Heights of Energetic Lightning Pulses in Thunderstorm Clouds

Abstract

Since 2004 the newly developed 3D lightning detection network LINET utilizes VLF/LF techniques for measurements of distinct pulses with waveforms and amplitudes that often resemble the ones typical for return strokes but occur inside thunder-clouds and do not connect to ground. For simplicity, these distinct signals are termed ‘IC-strokes’. The determined emission heights represent a new data source and call for further explanation. In a first attempt to understand the nature of these IC-strokes, 3D scans from polarimetric high-resolution radar of DLR (POLDIRAD) have been used for comparison, giving verification that the heights coincide with regions of graupel and ice aloft. A more specific IC-stroke analysis was carried out in 2010: a 7-sensor LINET system was set up at Kennedy Space Center (Florida) enabling a comparison with 3D lightning data from 4DLSS (also called LDAR2), the high-resolution VHF network operated by NASA. It turned out that the emission heights of LINET IC-strokes fit perfectly into channels defined by radio source points from 4DLSS, no matter whether these channels were located in lower (<7 km) or higher (>10 km) altitudes, and independent of whether the discharge was a ground- or a cloud flash. In fact, the accuracy of LINET heights was mostly better than ~10%. Furthermore, earlier observations [Betz et al., 2008] from comparisons with 2D VHF (Safir type) systems in Europe have been confirmed: in a substantial fraction of discharges the first measurable signal is an IC-stroke, whereby the VHF system does not record any prior electric activity.