The Climate Impact of Aerosols on Lightning: Is it Detectable from Long-term Aerosol and Meteorological Data?

Abstract. Aerosol effect on lightning is still under debate. In this study, the relative roles of meteorology and aerosols on lightning activities in Africa are investigated from a climatological perspective, based on the 11-year worth of lightning flashes from Lightning Imaging Sensor onboard Tropical Rainfall Measuring Mission, aerosol optical depth (AOD) from Moderate Resolution Imaging Spectroradiometer onboard Aqua, meteorological variables from the Medium-Range Weather Forecasting ERA-Interim reanalysis and aerosol composition data from the Modern-Era Retrospective analysis for Research and Application. Six meteorological variables are selected: sea level pressure (SLP), potential temperature ( θ ) at 2 m above ground level, mid-level relative humidity (RH), convective availabe potential energy (CAPE), vertical wind shear (SHEAR), and 200 hPa divergence (Div). To differentiate two dominant aerosol types in Africa and to account for their distinct climate regimes that all affect lightning, we separate two regions of interest (ROIs): the northern Africa (ROI_1) and the southern Africa (ROI_2) dominated by dust and smoke aerosols, respectively. As compared with dust in ROI_1, smoke aerosols in ROI_2 exhibit huge contrast between dry and wet season. Irrespective of regions, the lightning exhibits large diurnal variation with an afternoon peak and strong seasonality with a summertime peak, while the pronounced differences in lightning under relatively clean and polluted conditions signify the potential influences of aerosols. Lightning is dictated mostly by RH and CAPE in the dust dominant region (ROI_1). In the smoke dominant region (ROI_2), the effect of SLP is also significant. Systematic changes of boomerang shape was found in lightning frequency with AOD, with a turning point at around AOD = 0.3, below which lightning flashes increase monotonously with increasing AOD in both ROIs. As AOD approaches the optimal value, lightning activity seems to be saturated under smoky condition, likely due to the tradeoff between the aerosol invigoration effect and the radiative effect that tends to enhance and suppress lightning, respectively. In contrast, lightning activity in ROI_1 is suppressed by dust aerosol presumably due to the more dominant radiative heating effect of dust aerosol under dry environment.