On phenomenology of compact intracloud lightning discharges

[1] We examined wideband electric fields, electric and magnetic field derivatives, and narrowband VHF (36 MHz) radiation bursts produced by 157 compact intracloud discharges (CIDs). These poorly understood lightning events appear to be the strongest natural producers of HF-VHF radiation. All the events transported negative charge upward (or lowered positive charge), 150 were located by the U.S. National Lightning Detection Network (NLDN), and 149 of them were correctly identified as cloud discharges. NLDN-reported distances from the measurement station were 5–132 km. Three types of wideband electric field waveforms were observed. About 73% of CIDs occurred in isolation; 24% occurred prior to, during, or following cloud-to-ground or “normal” cloud lightning; and 4% occurred in pairs, separated by less than 200 ms (“multiple” CIDs). For a subset of 48 CIDs, the geometric mean of radiation source height was estimated to be 16 km. It appears that some CIDs actually occurred above cloud tops in clear air or in convective surges (plumes) overshooting the tropopause and penetrating deep into the stratosphere. For the same 48 CIDs, the geometric mean electric field peak normalized to 100 km (inclined distance) was as high as 20 V/m, and for 22 events within 10–30 km (horizontal distance), it was 15 V/m, both of which are higher than that for first strokes in negative cloud-to-ground lightning. The geometric means of total electric field pulse duration, width of initial half cycle, and ratio of initial peak to opposite polarity overshoot were 23 μs, 5.6 μs, and 5.7, respectively.