Electric field values observed near lightning flash initiations

[1] From a dataset of about 250 soundings of electric field (E), nine were adversely affected by lightning. These soundings are interpreted as ending near lightning initiation locations. Scaled to standard pressure, the largest observed E was 626 kV m⁻¹ and the largest estimated E was 929 kV m⁻¹. E exceeded runaway breakdown threshold, RB_th, by factors of 1.1–3.3 before each flash, and overvoltages were 1.4–4.3. Seven cases had rapid E increases (rates of 11–100 kV m⁻¹ s⁻¹) in the few seconds before the flash, and in three the maximum E occurred 3 s or more before the flash. A tenth sounding with E > RB_th for 38 s had subsequent lightning initiate 2 km from the balloon; one channel came within 400 m, but the flash and large E did not adversely affect the instruments. The findings suggest that E > RB_th is a necessary condition for lightning initiation, but it is not sufficient.