Improved electric field measurements with the T‐28 armored research airplane

Abstract

The South Dakota School of Mines and Technology armored T‐28 airplane has been used to collect electric field data inside thunderstorms since 1986. However, the derivation of the Ex field component in the flight direction has been problematic, and the accuracy of other field components derived has been suspect when heavy precipitation was encountered. The problems are due to (1) poor understanding of when and where corona ions are emitted from the airplane and how they affect the measurements, (2) lack of a complete and advanced calibration of the electric field meters' response to the ambient electric field components and airplane charge, and (3) poor understanding of this particular airplane's charging characteristics in the heavy precipitation and strong electric field environment of severe thunderstorms. By using data obtained during intercomparison/formation flights of the T‐28 with another well‐calibrated airplane in 1997, a complete calibration matrix for the T‐28 is presented in this paper. The calibration matrix provides improved and more robust estimates of all electric field components Ex, Ey and Ez. The data from the intercomparison flights are used to characterize the corona emission phenomena and its effects on the meter measurements. A methodology to detect emissions and their effect on the measurements is presented. Findings from this work and work from the Special Test Vehicle for Atmospheric Research (SPTVAR) at New Mexico Institute of Mining and Technology indicate that electric meters should not be located downstream of any airplane's propeller, the most likely source of corona ion emissions in large ambient electric fields. Alternative computations for field components and techniques for calculating airplane charge are also presented. In addition, we show that when flying through heavy precipitation, the effect of precipitation charging on a field meter's reading is small in comparison to the effect of typical ambient electric fields found in thunderstorms.