Different aerosol effects on the daytime and nocturnal cloud-to-ground lightning in the Sichuan Basin

The effect of aerosols on lightning has been involved in many studies, but its mechanisms are complex and far from 10 understood. The relationship between cloud-to-ground (CG) lightning and aerosols on an hourly time scale in the Sichuan Basin during 2010–2018 was investigated. The effects of aerosols, dynamics-thermodynamics factors (convective available potential energy: CAPE and vertical wind shear: SHEAR) and cloud-related factors (total column cloud liquid water: TCLW and total column cloud ice water: TCIW) on the CG lightning flashes on day and night were analysed. The diurnal variation of CG lightning flashes has two peaks under clean conditions, while only one peak was found in the diurnal variation of ground 15 flash under polluted conditions. In the early morning and night, more CG lightning flashes were found under polluted conditions, but in other periods, the difference in the CG lightning flashes between polluted and clean conditions is insignificant. Similar results were also found in the percentage of positive CG lightning flashes. At night, aerosols are positively correlated with the CG lightning flashes, and the response of CG lightning flashes to CAPE, SHEAR and TCLW is more evident under high aerosol loading. In the afternoon, aerosols have no significant effects on CG lightning and its response to dynamics- 20 thermodynamics and cloud-related factors. This difference seems to be caused by the different impacts of aerosol radiative and microphysical effects in these two periods. In the afternoon, aerosols may directly (indirectly) reduce the solar radiation reaching the ground and suppress convection through aerosol radiative effects (aerosol microphysical effects). Aerosols may also stimulate convection through microphysical effects. In this period, the two opposite effects of aerosols on convection offset each other. At night, without solar radiation, the aerosol microphysical effects may play a dominant role in the entire 25 AOD range to promote convection. (ECMWF) (2300–0200 BJT) an hour earlier the clean subset. 1600 to 1800 BJT, the TCG in the polluted subset larger than in the clean subset. The TCG difference between polluted and clean days is most at night, then in the morning, and the smallest in the afternoon Fig. 4c). These results indicate that the changes in aerosol loading may lead to changes in the diurnal variation of CG lightning activity. The responses of the CG lightning flashes to aerosol loading are also different at different times of the day. In the morning or night, adding aerosols 175 tends to increase CG lightning flashes. In the afternoon, adding aerosols tends to decrease CG lightning flashes slightly. Note that the difference in the averaged CG lightning flashes between polluted and clean subsets is relatively small during the late afternoon and early night (about 1400–1900 BJT) but is relatively large during the early morning (about 0600–0900 BJT) and around midnight (about 2100–0300 BJT). This indicates the potential effects of aerosols on CG lightning may be apparent in the early morning and around midnight but may be weak during the late afternoon and early night. CG lightning (PPCG) polluted in polluted ratio positive CG