Environmental Conditions Producing Thunderstorms with Anomalous Vertical Polarity of Charge Structure

Electric field soundings and Lightning Mapping Arrays have confirmed the existence of thunderstorms with vertical charge structure that is inverted from the usual polarity. This inverted charge structure can be described grossly as a large upper-level negative charge at roughly the ‑40°C level, which lies immediately above a large midlevel positive charge, at roughly the ‑20 °C level. This charge structure is often accompanied by a third charge, a smaller negative charge, at low levels. Cloud-to-ground (CG) flashes lowering positive charge to ground (+CG flashes) instead of the usual negative charge (-CG flashes) make up an unusually large fraction of CG activity in these anomalous storms.  For this study, we analyzed more than a decade of CG data from the National Lightning Detection Network throughout the contiguous United States to identify storm cells characterized by very high percentages of +CG flashes as a proxy for the existence of thunderstorms with anomalous vertical charge structure. Similarly, we identified storm cells characterized by very low percentages of +CG lightning as a proxy for the existence of normal-polarity thunderstorms. We used a moving average of 15 km x 15 km x 15 min grid cells, and stepped forward every 5 km and 5 min to approximate storm cells. We then used North American Regional Reanalysis output to analyze the environment (surface to equilibrium level CAPE and NCAPE, surface to ‑20°C CAPE and NCAPE, 0°C to ‑20°C CAPE and NCAPE, CIN, cloud base height, warm cloud depth, upper-level storm-relative flow, dew point depression, 0-3 km storm-relative helicity, equivalent potential temperature, precipitable water, and 0-3 km and 0-6 km vertical wind shear) during periods in which storms with large +CG fractions were observed nearby storms with large -CG fractions to analyze differences between the environments conducive to typical and anomalous vertical charge structure. Many hypotheses for the mechanism behind the formation of anomalous-polarity thunderstorms are based on conditions favoring positive charging of graupel during rebounding collisions with pristine ice crystals in the mixed-phase region. In mixed-phase regions with high liquid water contents, laboratory experiments indicate that graupel gains positive charge during collisions, regardless of temperature. Therefore, most of the environmental parameters we study are those thought to influence liquid water content in the mixed-phase region of the storm.