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. The possible effects of aerosols on lightning intensity and polarity were investigated using an explicit lightning scheme within a weather forecast model. Added aerosols invigorated intra-cloud flashes and cloud-to-ground (CG) flashes in continental convection. A noticeably high fraction of positive CG lightning was also identified under polluted conditions. Elevated cloud condensation nuclei concentration increased the ice particle content and graupel size, leading to enhanced charge separation and lightning activity. An inverted-polarity charge structure appeared owing to the positively charged graupel in the presence of greater supercooled cloud water amounts. This charge distribution was manifested in the lightning activity, showing that positive channels propagated to the ground, generating more positive CG strokes under polluted conditions. Aerosols enhance charge separation and lightning discharges due to a greater ice-crystal concentration and larger graupel sizeAerosols lead to inverted charge structure with strong positive-charge region, producing high percentage of positive cloud-to-ground strokesGraupel particles gain positive charge at upper levels in the presence of higher supercooled cloud water under polluted conditions