Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> Lightning can cause natural disasters that engender human and animal injuries or fatalities, infrastructure destruction, and wildfire ignition. Lightning-produced NO<sub>x</sub> (LNO<sub>x</sub>), a major NO<sub>x</sub> (NO<sub>x </sub>= NO + NO<sub>2</sub>) source, plays a vital role in atmospheric chemistry and global climate. The Earth has experienced marked global warming and changes in aerosol and aerosol precursor emissions (AeroPEs) since the 1960s. Investigating long-term historical (1960–2014) lightning and LNO<sub>x</sub> trends can provide important indicators for all lightning-related phenomena and for LNO<sub>x</sub> effects on atmospheric chemistry and global climate. Understanding how global warming and changes in AeroPEs influence historical lightning–LNO<sub>x</sub> trends is also helpful because it can provide a scientific basis for assessing future lightning–LNO<sub>x</sub> trends. Moreover, global lightning activities’ responses to large volcanic eruptions (such as the 1991 Pinatubo eruption) are not well elucidated, and are worth exploring. This study used the widely used cloud top height lightning scheme (CTH scheme) and the newly developed ice-based ECMWF-McCAUL lightning scheme to investigate historical (1960–2014) lightning–LNO<sub>x</sub> trends and variations and their controlling factors (global warming, increases in AeroPEs, and Pinatubo eruption) in the framework of the CHASER (MIROC) chemistry–climate model. Results of sensitive experiments indicate that both lightning schemes simulated almost flat global mean lightning flash rate trends during 1960–2014 in CHASER. Moreover, both lightning schemes suggest that past global warming enhances historical trends of global mean lightning density and global LNO<sub>x</sub> emissions in a positive direction (around 0.03 % yr<sup>−1</sup> or 3 % K<sup>−1</sup>). However, past increases in AeroPEs exert an opposite effect to the lightning–LNO<sub>x</sub> trends (−0.07 % yr<sup>−1 </sup>– −0.04 % yr<sup>−1</sup> for lightning and −0.08 % yr<sup>–1 </sup>– −0.03 % yr<sup>–1</sup> for LNO<sub>x</sub>). Additionally, effects of past global warming and increases in AeroPEs on lightning trends were found to be heterogeneous across different regions when analyzing lightning trends on the global map. Lastly, this study is the first to suggest that global lightning activities were suppressed markedly during the first year after the Pinatubo eruption shown in both lightning schemes (global lightning activities decreased by as much as 17.02 % simulated by the ECMWF-McCAUL scheme). Based on the simulated suppressed lightning activities after the Pinatubo eruption, our study also indicates that global LNO<sub>x</sub> emissions decreased after the Pinatubo eruption (2.41 % – 8.72 % for the annual percentage reduction), which lasted 2–3 years. Model intercomparisons of lightning flash rate trends and variations between our study (CHASER) and other Coupled Model Intercomparison Project Phase 6 (CMIP6) models indicate significant uncertainties in historical (1960–2014) global lightning trend simulations. Such uncertainties must be investigated further.
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