Abstract
Abstract. The effect of aerosols on lightning has been noted in many case studies, but much less is known about the long-term impact, relative importance of dynamics–thermodynamics versus aerosol, and any difference by different types of aerosols. Attempts are made to tackle all these factors, whose distinct roles are discovered by analyzing 11-year datasets of lightning, aerosol loading and composition, and dynamic–thermodynamic data from satellite and model reanalysis. Variations in the lightning rate are analyzed with respect to changes in dynamic–thermodynamic variables and indices such as the convective available potential energy (CAPE) and vertical wind shear. In general, lightning has strong diurnal and seasonal variations, peaking in the afternoon and during the summer. The lightning flash rate is much higher in moist central Africa than in dry northern Africa presumably because of the combined influences of surface heating, CAPE, relative humidity (RH), and aerosol type. In both regions, the lightning flash rate changes with aerosol optical depth (AOD) in a boomerang shape: first increasing with AOD, tailing off around AOD = 0.3, and then behaving differently, i.e., decreasing for dust and flattening for smoke aerosols. The deviation is arguably caused by the tangled influences of different thermodynamics (in particular humidity and CAPE) and aerosol type between the two regions. In northern Africa, the two branches of the opposite trends seem to echo the different dominant influences of the aerosol microphysical effect and the aerosol radiative effect that are more pronounced under low and high aerosol loading conditions, respectively. Under low-AOD conditions, the aerosol microphysical effect more likely invigorates deep convection. This may gradually yield to the suppression effect as AOD increases, leading to more and smaller cloud droplets that are highly susceptible to evaporation under the dry conditions of northern Africa. For smoke aerosols in moist central Africa, the aerosol invigoration effect can be sustained across the entire range of AOD by the high humidity and CAPE. This, plus a potential heating effect of the smoke layer, jointly offsets the suppression of convection due to the radiative cooling at the surface by smoke aerosols. Various analyses were done that tend to support this hypothesis.
Highlights
Lightning can be considered a key indicator of strong atmospheric convection (Betz et al, 2009)
Variations in dust loading change little throughout the year (Fig. 2c), while smoke shows a pronounced seasonal variation with a large contrast between dry and wet seasons (Fig. 2d). Lightning activity in both regions is most active in summer and rarely occurs in winter, which is consistent with the seasonal feature of convective available potential energy (CAPE), implying that the seasonal variation in lightning activity is mainly controlled by thermodynamic conditions
To provide a visual comparison of the dust- and smokedominant regions, we show the spatial distributions of the correlation coefficients of the regressions between the lightning flash rate and dynamic–thermodynamic variables
Summary
Lightning can be considered a key indicator of strong atmospheric convection (Betz et al, 2009). By acting as cloud condensation nuclei (CCN) with fixed liquid water content, increasing the aerosol loading tends to reduce the mean size of cloud droplets, suppress coalescence, and delay the onset of warmrain processes (Rosenfeld and Lensky, 1998). This permits more liquid water to ascend higher into the mixed-phase region of the atmosphere where it fuels lightning. Biomassburning activities, anthropogenic emissions, and desert dust are the three major atmospheric aerosol sources (Rosenfeld et al, 2001; Fan et al, 2018) that have different climate effects. The effect of dust on cloud properties tends to decrease precipitation through a feedback loop (Rosenfeld et al, 2001; Huang et al, 2014a, b) especially for drizzle and light rain
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