Electrical evolution during the decay stage of New Mexico thunderstorms

The electric field (E) at the ground beneath a thunderstorm often exhibits an end‐of‐storm oscillation (EOSO) during the storm's decay phase in which E typically undergoes three polarity changes over a period of 30–75 min. To determine why the surface E oscillates, the timing of the EOSO polarity changes of a multicellular storm in New Mexico has been compared to a series of balloon, radar, and lightning data. Three in situ balloon soundings provide vertical profiles of E and inferred charge regions just before and during the EOSO. These data have been compared to the evolving precipitation structure determined from orthogonal range‐height scans of two radars and to several other parameters including lightning mapping array data. Before the EOSO began, the storm had the typical electrical structure of the late mature stage. Estimates of the total charge magnitudes in each of the principal regions before and during the EOSO ranged from 7 to 33 C. The comparisons indicate that surface E polarity changes during the EOSO were (1) primarily owing to the successive fallout of three of the four principal charge regions in the storm's mature stage charge structure, modified by (2) the growth or decay of the charge regions as they descended, and (3) changes in the screening charges at the upper and lower cloud boundaries. More limited data from two other decaying storms indicate that similar in‐cloud evolution was probably occurring during their EOSOs.