Dependency of the Horizontal Resolution in the Charge Structure of a Convective Cloud in a Bulk Lightning Model

This study investigates the dependency of the charge structure in a summer convective cloud on the horizontal resolution of a bulk lightning model coupled with a meteorological model. For simulations involving a bulk lightning model with fine grid resolution, a multigrid solver is applied to the model, and the simulation results show that the charge separation numerically converges when the resolution reaches 200 m. The reason for this convergence is that with finer resolution, the strong vertical updraft actively transports liquid water and water vapor to higher altitudes, generating an environment conducive to riming, with sufficient supercooled water, transported by vertical updraft and produced from water vapor by condensation, present at altitudes of 9-11 km. Consequently, with higher resolution, the graupel mixing ratio increases at these altitudes, and this graupel formation leads to increased charge separation, which results in a higher lightning frequency. These results suggest that simulations with a resolution of 200 m or finer are necessary to simulate the lightning frequency accurately in numerical models coupled with a bulk lightning model.