Understanding the Effects of Aerosols on Electrification and Lightning Polarity in an Idealized Supercell Thunderstorm via Model Emulation

Aerosol effects on the lightning intensity and polarity of a continental supercell storm were investigated using a three-dimensional lightning scheme within the Weather Research and Forecasting model. We find that both intra-cloud (IC) and cloud-to-ground (CG) flashes are enhanced by the increasing number of cloud condensation nuclei (CCN), especially the percentage of positive CG (+CG) strokes peaking at 42%. Electrical characteristics of the storm varied in different aerosol scenarios through microphysical processes. Added aerosols increase the number of cloud droplets and ice-phase hydrometeors. The greater ice-crystal concentration and larger graupel size ensure sufficient charge separation, leading to higher charge density and more lightning discharges. In addition, an inverted polarity charge structure with a strong positive-charge region in the mid-levels was formed mainly due to the positively charged graupel in the presence of higher supercooled cloud water content. Positive lightning channels originating from this positive-charge region propagated to the ground, producing more +CG strokes. When the aerosol concentration was low, the charge density in the upper positive-charge region was much lower due to smaller ice-particle content. Consequently, there were barely any +CG strokes. Most of the negative CG flashes deposited positive charge in the lower negative-charge region.