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

Abstract

[1] 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 À1.0 kV m À1 to 16. kV m À1, with a mean value of 0.9 kV m À1. The median peak field was 0.29 kV m À1 . Flash rates ranged from 0 to over 27 flashes min À1 with the mean flash rate of 1.2 flashes min À1 . The median flash rate for an overpass was 0.25 flashes min À1 . The positive plus negative conductivity ranged from 0.6 pS m À1 to 3.6 pS m À1 at the nominal flight altitudes of 15 to 20 km. The mean and median total conductivity was 2.2 pS m À1 . Peak current densities during the overpasses ranged from À2.0 nA m À2 to 33. nA m À2 . The mean peak current density was 1.9 nA m À2 , and the median value was 0.6 nA m À2 . 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 À1.3 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.

Measurements of Atmospheric Electricity Aloft

“…They found that in general, the conductivity of the air above thunderstorms was not appreciably different from its value at the same altitude at distances far from the cloud. These results are supported by the Gerdien condenser measurements of Stergis et al (1957b) and Blakeslee et al (1989) both from balloon platforms, and Bailey et al (1999) and Mach et al (2009), both from aircraft platforms. Measurements of conductivity above storms using the relaxation technique from a constant level balloon platform, however, have found contradictory results (e.g.…”

“…The exponential decay to background levels after this increase was observed to occur during a timescale of ≈50 seconds, consistent with the relaxation time in ambient air at those altitudes. Transients in electric field above thunderstorms due to lightning discharges have also been observed on a large number of flights by Mach et al (2009). Their study contains the most extensive set of measurements of electrical properties above thunderstorms, taken from 850 flights over electrified clouds during a ten year period.…”

Lightning and middle atmospheric discharges in the atmosphere

“…The diurnal variation with local time for all the continents individually showed peak around 1600 h local time (Bailey et al, 2007). The land-based storms have a much larger mean flash rate (40 s À 1 ) and also larger variation between peak and trough than ocean lightning storms (mean 5 s À 1 ), which almost shows a constant value (Mach et al, 2009(Mach et al, , 2010.…”

Technique for Reducing the Effects of Nonlinear Terms on Electric Field Measurements of Electric Field Sensor Arrays on Aircraft Platforms

“…Measurements of electric fields within clouds have been made using aircraft for many years (e.g., Winn 1993;Merceret et al 2008;Mach et al 2009). Retrieving electric field components from the raw aircraft field mill data while in cloud is perhaps the most difficult aspect of the measurement process because of the usually high charging of the aircraft (e.g., Jones 1990;Koshak et al 1994;Koshak 2006;Koshak et al 2006;Mach and Koshak 2007).…”

“…Retrieving electric field components from the raw aircraft field mill data while in cloud is perhaps the most difficult aspect of the measurement process because of the usually high charging of the aircraft (e.g., Jones 1990;Koshak et al 1994;Koshak 2006;Koshak et al 2006;Mach and Koshak 2007). The electric field as measured by an instrument on the aircraft includes linear components from the external electric field e X , e Y , e Z , charge on the aircraft (e Q ), mill offset, and various other phenomenon, including measurement errors and nonlinear terms (l).…”