Microphysics and Kinematics of a High Flash Rate Storm With a Bottom-Heavy Charge Structure

Abstract

An atypical thunderstorm with unusual lightning and charge structure characteristics propagated through Cordoba province, Argentina, during the austral warm season in late 2018. This storm exhibited echo tops near 10.5 km, with high lightning activity concentrated at low altitudes. Storm total flash rate surpassed 120 min⁻¹, with peak initiation altitude at 6 km, the inferred altitude interface between a lower positive charge region and an overlying negative region. An in-depth analysis of kinematics, eddy dissipation rate (EDR), and microphysics was performed to understand what caused intense low-altitude flash rates, bottom-heavy charge structure, and few flashes in the upper-level positive charge region. Environment and synoptic conditions led to the unusually cold troposphere and low freezing level, which influenced storm microphysics and charge structure characteristics. Positive buoyancy was limited to the −20°C isotherm level, and maximum updraft speed varied between 10 and 14 m s⁻¹ during the period of highest flash rates. Larger volumes of graupel and hail were observed at altitudes associated with the lower positive charge layer. Larger volumes of high EDR fluctuated in altitude following surges in updraft volume, but no clear relation was found for flash initiation and propagation relative to EDR. Infrequent flashes in the upper positive charge layer suggest limited charging of ice crystals at that altitude. Small warm cloud depth and high cloud water content might have played a role in decreasing the charging reversal temperature and enhancing the development of a positive charge layer in the lower mixed-phase layer.