The impacts of shipping emissions on lightning: roles of aerosol-radiation-interactions and aerosol-cloud-interactions

Abstract

Long-term (2005-2021) observations from the World Wide Lightning Location Network (WWLLN) reveal significantly larger flash counts over heavily trafficked shipping lanes in the northeastern Indian Ocean compared to nearby regions. Here, we use the online coupled meteorology-chemistry model WRF-Chem to examine the impact of aerosols from shipping emissions on lightning activity over one of the world’s busiest shipping lanes (5-7°N, 82-92°E). We conduct three case simulations in January 2013, 2015, and 2018 when frequent lightning activities were recorded. The WRF-Chem model with lightning parameterized based on convective cloud-top height captures the temporal evolution of lightning activities but overestimate the magnitude of lightning counts. We find that all three cases consistently show enhanced lightning flash counts due to shipping-induced aerosols by 6.2-22.3%, accompanied with increased cloud droplet number, cloud water concentration, radar reflectivity, and cloud-top height. Sensitivity simulations show that aerosol-cloud-interactions (ACIs) from shipping aerosols consistently enhance the strength of convection and lightning frequency in all three cases, by increasing cloud condensation nuclei (CCN) and cloud droplet numbers, delaying precipitation of cloud water, allowing the lift and accumulation of water vapor, increasing release of latent heat, and finally invigorating convections. In comparison, aerosol-radiation-interactions (ARIs) show inconsistent contributions to convection and lightning, which enhance lightning frequency in the 2013 and 2015 cases but decrease lightning frequency in the 2018 case. This inconsistency may be attributed to differences in the number and vertical structure of shipping aerosols and cloud droplet in the three cases. Our study thus approves the certain but complicated anthropogenic impacts on lightning activities through shipping-induced aerosols, which has important implications for understanding future trends in lightning and its impact on atmospheric chemistry and climate.