Lightning, electric field, and radar observations of the STEPS 25 June 2000 storm

The Severe Thunderstorm Electrification and Precipitation Study (STEPS) took place in the late spring and early summer of 2000 in NE Colorado, NW Kansas, and SW Nebraska. One focus of the study was to learn more about electrification in thunderstorms where the majority of ground flashes lowered positive charge to the ground. A storm occurring on 25 June was chosen for investigation because it produced a large number of positive ground flashes. An overview of techniques used to analyze the charge structure found in the storm is given. Using data from a three-dimensional lightning mapping array (LMA) and balloon-borne electric field meters, this storm’s electrical structure near the balloon was analyzed. Using a one-dimensional approximation of Gauss’s law, ten regions of charge were inferred from the two electric field soundings in this multicellular storm. The LMA data, however, indicates only four regions of charge in the same part of the storm. Because the different techniques have different limitations, a combination of these two techniques plus three-dimensional electric field vectors (3-D vectors) plotted along the balloon paths was used to gain a better understanding of the storm’s charge structure near the balloon paths. Because the charge structure was complex, modeling studies of charge and electric field profiles are suggested for future work.

This study also found that the charge structure was different in various sections of the storm. The main body of the storm contained a quadrapole structure, with the lowest charge layer having positive polarity and alternating charge polarities stacked vertically above it. The electrical structures varied temporally as well. A transition of charge structure within the main body of the storm from normal polarity to inverted polarity was inferred from lightning observations. This transition was coincident with both the beginning of positive ground flashes and the appearance of an outflow-produced fineline on radar.