Electric fields, conductivity, and estimated currents from aircraft overflights of electrified clouds

Abstract

Using rotating vane electric field mills and Gerdien capacitors, we measured the electric field profile and conductivity during 850 overflights of clouds and thunderstorms. The measurements were made with NASA ER‐2 and Altus‐II aircrafts. Peak electric fields, with lightning transients removed, ranged from −1.0 kV m−1 to 16. kV m−1, with a mean value of 0.9 kV m−1. The median peak field was 0.29 kV m−1. Flash rates ranged from 0 to over 27 flashes min−1 with the mean flash rate of 1.2 flashes min−1. The median flash rate for an overpass was 0.25 flashes min−1. The positive plus negative conductivity ranged from 0.6 pS m−1 to 3.6 pS m−1 at the nominal flight altitudes of 15 to 20 km. The mean and median total conductivity was 2.2 pS m−1. Peak current densities during the overpasses ranged from −2.0 nA m−2 to 33. nA m−2. The mean peak current density was 1.9 nA m−2, and the median value was 0.6 nA m−2. Using the peak electric fields, a median field falloff with distance based on all overflights, and cylindrical storm symmetry, the total upward current flow from storms in our data set ranges from −1.3 to 9.4 A with a mean value of 0.8 A. The median total current was 0.27 A. The contributions from lightning field changes do not significantly affect the total derived currents. We found that 7% of the storms were producing current flows above the storms that were opposite in polarity from the standard role that thunderstorms play in the global electric circuit. Approximately one third of the storms had no detectable lightning during the overpasses but still had significant electric fields. Owing to a possible sampling bias, the fraction of nonlightning storms with electric fields may not reflect the global probability of these clouds.