Lightning response to smoke-laden air from Australian wildfires

Abstract:

Australia experienced severely dry conditions under the combined effects of a positive Indian Ocean Dipole (IOD) and the El Niño-Southern Oscillation (ENSO, El Niño), with a negative Southern Annual Mode (SAM) for the latter half of 2019. The notorious 2019-2020 Australian bushfire season broke the historical wildfire record by burning roughly 46 million acres and heavily contaminating the atmosphere offshore to the east of New South Wales. This offers a favorable natural experiment for understanding the aerosol effects on lightning in conditions of moist convection over the polluted ocean (Aerosol Optical Depth>0.3), where the massive heat from wildfires on land is beyond reach and the aerosol effects can be disentangled from the convolution of thermodynamic factors in consideration of the lightning response. This work focuses on this exceptional aerosol-lightning experiment to address the lightning changes to the added aerosol during the 2019-2020 Australian bushfire season, through a comprehensive examination of both the thermodynamic parameters and cloud microphysics. Taking advantage of the high-resolution lightning data from the Earth Networks Total Lightning Network (ENTLN), alterations of lightning characteristics induced by the aerosol effects are discussed over the moist conditions for both land and ocean and further compared with the dry conditions. Over the ocean east of Australia, total strokes increase by 270% and the enhancement ratio of negative to positive stokes is 2.6 during the bushfire season. In contrast, on land the total increase in stroke count for the strong fire season is only 73% and the enhancement ratio of negative to positive stokes is 0.47, showing a stronger positive stoke enhancement over land than ocean. Possible mechanisms are elaborated to study the complex interactions of aerosol contamination of the cloud convection system and the charge separation process in the mixed-phase region.