Impact of Convective Intensity on Mixed‐Phase Evolution and Upper Positive Charge Region Formation in an Isolated Convective Cloud

Abstract

Lightning initiation depends on the charge separation within the mixed‐phase areas of convective clouds, where graupel, ice, and supercooled water interact. However, the specific mixed‐phase structures that enhance charge separation remain unclear. In this study, charge separation refers to non‐inductive charging, whereas the resulting upper positive charge region represents a cloud‐scale net charge structure. We conducted idealized numerical simulations of isolated convective clouds using a meteorological model coupled with a bulk lightning model by varying the initial warm bubble perturbation to control the convective intensity. We quantified the heterogeneous spatiotemporal distributions of mixed‐phase evolution; liquid water, graupel, and ice presence, thereby identifying the mixed‐phase structures associated with charge separation using the RGB hexagram framework. The results show that the formation of the upper positively charged region is closely linked to mixed‐phase areas where graupel presence dominates alongside minimal liquid water, particularly in areas with numerous lightweight graupel particles. Faster convective development transported these particles to the upper levels, enhancing graupel–snow collisions and non‐inductive charge separation producing negative charging of graupel (positive charging of snow), thereby promoting upper positive charge formation. In contrast, slower‐developing convection produces heavier graupel with less active charge separation. Importantly, areas with actively developed graupel masses do not always correspond to areas favorable for charge separation. These findings demonstrate that convective intensity influences charge separation indirectly by regulating the branching of mixed‐phase structure developments and associated hydrometeor properties, providing a quantitative framework for identifying cloud structures conducive to lightning initiation.