Storm Characteristics Determining Dominant Cloud-to-Ground Lightning Polarity

Detailed analyses of storms from the Severe Thunderstorm Electrification and Precipitation Study (STEPS) in 2000 have established relationships between radar observables and lightning flash rate and polarity. We combine visual and in situ aircraft microphysical observations with these earlier radar analyses to establish additional relationships. In particular, we show that storms forming in an environment with the right balance between vertical shear of the horizontal wind, and convective available potential energy, develop in such a way that initial convective development occurs in relatively isolated flanking cells. When these cells develop vigorously without entraining precipitation from more mature neighboring cells, precipitation formation in the new cells is delayed. This means that precipitation finally develops at higher altitudes, higher cloud liquid water concentrations, and lower temperatures, compared to precipitation formation in storms in which new cells are less vigorous and/or entrain precipitation debris from earlier cells. The storms in which initial convective development occurs in isolated flanking cells go through an extended stage of almost purely intracloud lightning production, and then into a phase where there is a mixture of intracloud and cloud-to-ground lightning lowering positive charge. Storms in which precipitation develops earlier and lower in new cells, due to entrainment of precipitation debris from older neighboring portions of the storm, tend to produce predominantly negative cloud- to-ground lightning. The relationship between microphysical conditions in these different types of storms, and results of laboratory microphysical experiments concerning charge separation during particle collisions, are not straightforward.