Interactive aerosol effects during an extreme biomass burning episode over South America simulated with WRF-Chem

Large-scale biomass burning (BB) events in South America constitute a major source of atmospheric aerosols, with profound implications for regional radiative budgets, cloud microphysics, and precipitation processes. However, most operational and regional weather prediction models still neglect interactive atmospheric chemistry, limiting their ability to realistically represent aerosol radiative effects and associated feedbacks on clouds and precipitation during extreme BB episodes. Despite extensive observational evidence, the representation of aerosol–meteorology interactions linked to intense biomass burning remains a major source of uncertainty in regional climate and weather simulations.

In this study, we investigate the atmospheric impacts of an intense and persistent BB episode that affected South America during September 2022, using the fully coupled Weather Research and Forecasting model with online chemistry (WRF-Chem). The event was identified based on active fire detections from the FIRMS web-portal, consistently observed through enhanced Aerosol Optical Depth (AOD) from MODIS and elevated carbon monoxide (CO) columns retrieved from IASI. Model simulations were conducted for the period from 25 August to 12 September 2022, employing the MOZCART chemical/aerosol mechanism (MOZART and GOCART), Morrison double-moment microphysics, and the RRTMG radiation scheme. To isolate aerosol-driven perturbations from the large-scale meteorological forcing, a control experiment without interactive chemistry was performed and used as a baseline. The analysis focuses on aerosol-induced modifications to cloud microphysical properties and precipitation, evaluated over four distinct geographical subregions representative of the most affected areas.

Model performance was assessed through a comprehensive comparison with observations. The chemical component was evaluated by analyzing the spatial and temporal evolution of simulated AOD and CO against MODIS and IASI satellite products. The meteorological consistency of the simulations was independently verified using surface observations, with statistical metrics computed for near-surface temperature, wind speed, and relative humidity across the domain. The results highlight the critical role of interactive aerosol–radiation and aerosol–cloud processes in shaping the atmospheric response to extreme biomass burning events. This study demonstrates the added value of fully coupled chemistry–meteorology modeling and spatially resolved diagnostics for improving the representation of biomass burning impacts in regional weather and climate simulations over South America.