Positive Cloud-to-Ground Lightning in Mesoscale Convective Systems

Abstract

We have examined the characteristics of positive cloud-to-ground lightning flashes in Mesoscale Convective Systems observed during the Oklahoma-Kansas PRE-STORM project in 1985. Lightning frequencies and patterns of ground strike locations are related to observed storm precipitation structures, with emphasis placed on relating observed lightning patterns to the stratiform precipitation regions. Positive ground flashes are categorized in relation to their position relative to storm radar echo patterns, and frequency of occurrence relative to storm lifecycles. Observations presented indicate that many of the positive flashes are likely produced by the advection of positive charge from the upper levels of the convective portions of the storm. We suggest that an additional class of positive ground flashes are associated with the generation of charge by stratiform precipitation processes, specifically related to ice-ice interactions in regions of low supercooled liquid water contents. A simple one-dimensional model is developed and used to show that stratiform microphysical processes are capable of producing charge densities of 2?4 C km?3. The charge structure inferred from the model is an inverted dipole with positive charge being situated below negative charge. The model results are contrasted to charge structures inferred from observational studies, one of which suggests the presence of an inverted polarity dipole.