Disentangling Biomass Burning Aerosol Radiative Effects in the Southeast Atlantic

Each year during African fire season, prevailing winds result in episodic smoke plumes traveling over the Southeast Atlantic Ocean where biomass burning aerosols (BBAs) overlay and mix into one of Earth’s largest stratocumulus cloud decks. African fires are the largest source of BBAs, contributing 30% of the global budget by mass. Due to the large area covered by the clouds in this region, significant effects of BBA on cloud albedo and cloud amount likely affect global climate. Changes to the amount of solar radiation that is scattered and absorbed, both directly by the aerosols, and indirectly though the influence of BBAs on cloud properties, result in quantifiable perturbations to Earth’s energy balance, or radiative effects. The goal of this project is to more accurately quantify direct, semi-direct, and indirect BBA radiative effects in the unique Southeast Atlantic environment. Unambiguously separating direct, semi-direct, and indirect radiative effects is challenging. As a result, there are large uncertainties surrounding their magnitudes today. Accurate quantification of BBA radiative effects is an important step forward in reducing climate model uncertainty and deepening our understanding of the mechanisms by which carbonaceous aerosols influence climate.

This study uses the Met Office’s Unified Model to simulate smoke episodes in the Southeast Atlantic to better understand how radiative effects are sensitive to simulation and calculation methods. We compare simulated BBA radiative effects from nudged, free-running, and simulations run as weather forecasts, where meteorology is reset at regular intervals to archived analyses produced with data assimilation. We find that radiative effects depend strongly on how the meteorology is forced, and their comparison offers insight into the strengths and shortcomings of each method. We explore the sensitivity to our calculation method by using various radiation scheme outputs to separate and quantify each individual radiative effect multiple ways. Simulation accuracy is validated through comparison to various satellite retrievals and the ORACLES, CLARIFY, and LASIC field campaigns. The major properties that impact radiative effects are smoke transport, smoke optical properties, and cloud optical properties. Model biases found in these properties are addressed to the extent possible, to ensure our derived RE values are accurate. Note this presentation will update and supersede a talk on the same topic at AMS 2023 in the aerosol-climate session, with substantially revised and more comprehensive results.