VHF Lightning Mapping Observations of Screening Charge Flashes at Thunderstorm Tops

Abstract:

Deeply convective thunderstorms occasionally produce series of continual small-scale electrical discharges near the cloud top. Over the past years a number of such cases in normal-polarity thunderstorms have been observed with the Lightning Mapping Array (LMA) deployed near Langmuir Laboratory in south-central New Mexico. The LMA observations show most of the discharges as single-point VHF sources, which can occur at rates of ten or more discharges per second in a localized region near the cloud top.

Correlated visual observations of storm tops indicate that the small discharges occur near the cloud boundary of overshooting tops during their upward development, suggesting that the discharges are screening charge flashes between upper positive charge and inferred negative screening charge at the cloud boundary. Some screening charge flashes develop into more extensive breakdown, with negative leaders propagating down several km into the upper positive charge region, while the positive leaders are undetected by the LMA. The more extensive screening charge flashes are characteristic of breakdown associated with blue starters and jets.

Screening charge flashes can temporarily become the dominant form of negative breakdown into the upper positive charge region, associated with an almost complete lack of intracloud flashes. We speculate that vigorous overshooting convection at high altitude, where the atmosphere has a higher conductivity, produces a rapid accumulation of negative screening charge at the cloud boundary. The accumulated screening charge causes localized regions of strong electric fields between it and the upper positive charge region before diffusing and mixing in.