Lightning and electrical structure of mesoscale convective systems

Abstract

Recent observations indicate that cloud-to-ground lightning discharges are common in the stratiform portions of Mesoscale Convective Systems. A bipolar lightning pattern is often found, with negative flashes preferred in the convective region and positive flashes preferred in the stratiform region. We first review a charge advection process offered to explain lightning bipoles. In this process, positive charge (contained on the surface of small ice particles) is transported into the stratiform region by storm-relative winds. Calculations for the expansion rate of a spherical volume of charge by Coulombic forces indicate slow expansion, thus maintaining high charge concentrations in the stratiform region. However, when turbulent diffusion of the charge volume is considered, expansion rates through mixing are large compared to expansion rates by Coulombic forces. We therefore argue that the charge advection process is not a plausible mechanism for explaining lightning bipoles. The charge structures observed in stratiform clouds are reviewed and an in-situ mechanism, where the stratiform clouds electrify independent from the deep convection, is offered as an explanation for lightning bipoles. New observations from a tropical field program are presented in support of this in-situ charging mechanism.