Significant influence of aerosol on cloud-to-ground lightning in the Sichuan Basin

Highlights

• The lightning activities in Sichuan Basin is sensitive to aerosol optical depth.

• The convection in Sichuan Basin generally occurs at night.

• Aerosol-radiation interaction has little influence on the convection.

• The aerosol-cloud interaction can explain the decrease of lightning production.

Abstract

The effects of aerosols under special terrain on lightning activities in the Sichuan Basin (SB) are studied using the cloud-to-ground (CG) lightning data and aerosol optical depth (AOD) data from 2010 to 2018. Interesting results show that the topographic drop plays an important role in the process of aerosol affecting lightning. The great topographic drop in the western SB is conducive to the production of CG lightning. Both the CG lightning production and AOD in the northwest and southwest of the basin decrease in fluctuation during the study period. It can be inferred that the reduction of aerosol results in the decrease of lightning activity. Due to the topographic drop in the northwest of the basin being greater than that in the southwest, the correlation of 0.64 between AOD and CG lightning in the northwest of the basin is larger than that of 0.31 in the southwest. Since the lightning activity in SB generally occurs at night, the aerosol-radiation interaction (ARI) has an insignificant influence on the occurrence of convection. Under this condition, it can be inferred that the aerosol-cloud interactions (ACI) are dominant over SB region. These could better support the reduction of lightning generation.

Introduction

To date, a host of studies have reported the effect of aerosols on the electrification in thunderstorms (Guo et al., 2016; Yang and Li, 2014; Zhao et al., 2020). However, as the different results reported in different areas, the influence of aerosol particles on lightning activity remains a debatable topic. It has been suggested that both the aerosol-cloud interactions (ACI) and aerosol-radiation interactions (ARI) can be explained the relationship between aerosols and lightning activities. It has been hypothesized that the enhancement of lightning production is probably a response to micro-physical effect of aerosols, while some studies revealed that the occurrence of lightning is inhibited as a result of radiation effect (Fan et al., 2016; Li et al., 2016, Li et al., 2017; Tan et al., 2016; Tao et al., 2012; Yang et al., 2013). However, the internal physical mechanisms are hard to be investigated because of the effects of aerosol properties and meteorological conditions.

Aerosol particles can serve as condensation nuclei (CCN) and cloud ice nuclei (IN), and thus the aerosol particles have a significant influence on the physical processes of liquid clouds and ice clouds. Since lightning activities are closely connected with the microphysical development in thunderstorms, aerosols perhaps have a great impact on the lightning production in thunderstorms (Fan et al., 2016; Li et al., 2016, Li et al., 2017; Tao et al., 2012). For example, some studies have shown that aerosols emitted by ships can increase lightning activities in shipping lanes (Thornton et al., 2017; Liu et al., 2020). Pérez-Invernón et al. (2021) reported that the apparent reduction in lightning activities during the COVID-19 lockdown was partly due to lower anthropogenic aerosol emissions. However, Yang et al. (2013) and Tan et al. (2016) have found a negative correlation between aerosol and lightning activity in Xi'an and Nanjing, China. Therefore, one can conclude that aerosols can inhibit the occurrence of lightning activities. Since aerosols can reduce the arrival of solar radiation to the ground by absorbing and scattering it, the surface temperature and atmospheric instability may be reduced. Thereby, the formation of convections and lightning discharge may be indirectly inhibited (Rosenfeld, 1999; Koren et al., 2004). A recent study by Shi et al. (2020) has revealed that the lightning flash rate is positively correlated with AOD under relatively clean conditions (AOD < 1.0), while in the situation of high aerosol concentration (AOD > 1.0), the correlation between AOD and lightning flash rate is not obvious. As the development of thunderstorms also depends on meteorological factors such as topography and environmental airflow (Wu et al., 2016; Zhao et al., 2021a; b), convective available potential energy (CAPE) (Fuchs et al., 2015; Stolz et al., 2017; Williams, 2005) and temperature (Markson, 2007; Price, 1993; Williams, 1994; Yang et al., 2018) and humidity (Fan et al., 2007; Wall et al., 2014), a causal relationship between aerosol particles and lightning activities is hard to establish. Therefore, an important objective of this study is to investigate whether the micro-physical effect can be well explained in the relationship between aerosol and lightning activities.

The topography of the Sichuan Basin (SB) is complex and embosomed in mountains (Zhao et al., 2021a). The pollutants are difficult to diffuse under the condition of the special terrain, so SB has become an area with high aerosol particle concentration (Zhang et al., 2019a; Zhao et al., 2021a). The Qinghai Tibet Plateau (TP) resides in the west of the basin, and a great topographic drop can be found between TP and SB. The terrain is conducive to the forced lifting of airflow, inducing the occurrence and development of the convective cell. Therefore, lightning can be dramatically produced under these conditions. The low-level warm and humid airflow is hindered by the inclined terrain, and then covered by the warm and dry air from the TP. Therefore, the premature release of unstable performance is prevented and unstable energy is accumulated (Sawyer, 1947; Houze et al., 2007; Wu et al., 2016). Thereby, the cold winds brought by radiative cooling at night may be the key factor in triggering and forming convection. It is different from the thunderstorms that mainly occur in the afternoon along the southern foothills of the Himalayas (Houze et al., 2007; Wu et al., 2016; Zhao et al., 2021a). At present, studies have shown that aerosol particles can enter the topographic cloud with the updraft and affect the convective activity by changing the cloud micro-physical structure (Mansell and Ziegler, 2013; Tan et al., 2017). Therefore, under the special topographic conditions of SB, how aerosols affect the thunderstorm process is a question that needs to be deeply discussed.

Therefore, this paper is conducted to investigate the influence of aerosol on lightning activity in SB. The role of aerosol-radiation interaction (ARI) and aerosol-cloud interactions (ACI) on the relationship between aerosols and lightning activities are surveyed. The rest of the paper is organized as follows. The various data sets and data processing methods used in this study are described in detail in section 2. Section 3 compares spatial distribution and annual variation of CG lightning and meteorological factors, together with a detailed analysis of the mechanism of lightning occurrence in SB. A summary of all results is presented in section 4.