Impact of secondary ice production on thunderstorm electrification under different aerosol conditions

Aerosol and secondary ice production (SIP) processes in convective clouds are both vital to charge separation in thunderstorms, but the relative importance of different SIP processes to cloud electrification under different aerosol conditions is not well understood. In this study, using the Weather Research and Forecasting (WRF) model with a spectral bin microphysics scheme, we investigate the role of four different SIP processes in charge separation in a squall line with different cloud condensation nuclei (CCN) concentrations, including the rime-splintering process, the ice-ice collisional breakup, shattering of freezing drops, and sublimational breakup. It is found that the simulation well reproduces the macro-morphology, the occurrence location, and the eastward tendency of the storm. As the CCN concentration increases, more but smaller cloud droplets can be lifted up to mixed-phase regions. The warm rain process is suppressed, and the declined raindrop concentration leads to fewer graupel particles. In a clean environment, the shattering of freezing drops is the most important SIP process to ice production at relatively warm temperatures, and the graupel concentration can be significantly enhanced. In a polluted environment, the rime-splintering process contributes the most to the graupel and ice production at relatively warm temperatures. The ice-ice collisional break-up process contributes the most to ice production at relatively cold temperatures. The noninductive charging rates exhibit a dipole structure with upper negative and lower positive regions. The implementation of four SIP processes as well as the increase in aerosol concentration both cause an enhancement of the noninductive charging rate. However, aerosol and SIP processes have opposite impacts on the charging reversal: higher aerosol concentration results in a colder reversal temperature, while SIP processes lower the reversal level. In a clean environment, the shattering of freezing drops process has the greatest effect on the noninductive charging rate, while in a polluted environment, both rime splintering and the shattering of freezing drops processes can have a significant effect. Without any SIP process, the increase in aerosols is not capable of modifying the charge structure. It is the rime-splintering process in a polluted environment responsible for the generation of a normal charge structure. Both the addition of the SIP processes and the increase in aerosol concentration favor the enhancement of the electric field.