Climate models generally underrepresent the warming by Central Africa biomass-burning aerosols over the Southeast Atlantic

Mallet, Marc; Nabat, Pierre; Johnson, Ben; Michou, Martine; Haywood, Jim M.; Chen, Cheng; Dubovik, Оleg

doi:10.1126/sciadv.abg9998

Cited by 36 publications

(31 citation statements)

References 79 publications

(138 reference statements)

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“…Southern Africa produces approximately one-third of the world's carbon emissions through biomass-burning (van der Werf et al, 2010), with the global majority of the absorbing aerosols above cloud occurring above the southeast Atlantic (Waquet et al, 2013), indicating the importance of realistic representations for this radiative climate (Mallet et al, 2021).…”

Section: Discussionmentioning

confidence: 99%

“…While climate models discern an ensemble-mean direct radiative warming, individual models disagree strongly on magnitude and even sign (Myhre et al, 2013;Zuidema et al, 2016b;Haywood et al, 2021;Mallet et al, 2021). In addition, the direct aerosol radiative effect estimated from satellites typically exceeds model estimates (de Graaf et al, 2020).…”

Section: Introductionmentioning

confidence: 99%

“…In addition, the direct aerosol radiative effect estimated from satellites typically exceeds model estimates (de Graaf et al, 2020). That the measured single-scattering albedos are lower than what is currently implemented in many models (Mallet et al, 2021), suggests one cause for an underestimated modeled direct radiative warming. This study's primary explanation for why the BBA is so highly absorbing of sunlight is that the aerosols are already highly absorbing due to a high black carbon content at emission.…”

Section: Introductionmentioning

confidence: 99%

“…The global maximum of absorbing aerosol above cloud occurs above the southeast Atlantic (Waquet et al, 2013), so that the aerosols provide a distinct radiative warming over the ocean (Keil and Haywood, 2003;Graaf et al, 2014;Zuidema et al, 2016b;Mallet et al, 2021;Doherty et al, 2021). BBA from this region is unusual for being highly absorbing of sunlight, with recent studies reporting SSA values of 0.85 or less at the green wavelength (Zuidema et al, 2018;Chylek et al, 2019;Pistone et al, 2019;Holanda et al, 2020;Taylor et al, 2020;Denjean et al, 2020;Mallet et al, 2020;Shinozuka et al, 2020;Carter et al, 2021), lower than what is currently representative of many global and regional models (Shinozuka et al, 2020;Mallet et al, 2021). This has significant implications for regional climate and its circulation (e.g., Mallet et al, 2020;Solmon et al, 2021).…”

Section: Introductionmentioning

confidence: 99%

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Non-reversible aging can increase solar absorption in African biomass burning aerosol plumes of intermediate age

Dobracki

Zuidema

Howell

et al. 2022

Preprint

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Abstract. Recent studies highlight that biomass-burning aerosol over the remote southeast Atlantic is some of the most sunlight-absorbing aerosol on the planet. In-situ measurements of single-scattering albedo at the 530 nm wavelength (SSA530nm) range from 0.83 to 0.89 within six flights (five in September, 2016 and one in late August, 2017) of the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) aircraft campaign, increasing with the organic aerosol to black carbon (OA : BC) mass ratio. OA : BC mass ratios of 10 to 14 are lower than some model values and consistent with BC-enriched source emissions, based on indirect inferences of fuel type (savannah grasslands) and dry, flame-efficient combustion conditions. These primarily explain the low single-scattering albedos. We investigate whether continued chemical aging of aerosol plumes of intermediate age (4–7 days after emission, as determined from model tracers) within the free troposphere can further lower the SSA530nm. A mean OA to organic carbon mass ratio of 2.2 indicates highly oxygenated aerosol with the chemical marker f44 indicating the free-tropospheric aerosol continues to oxidize after advecting offshore of continental Africa. Two flights, for which BC to carbon monoxide (CO) ratios remain constant with increasing chemical age, are analyzed further. In both flights, the OA : BC mass ratio decreases over the same time span, indicating continuing net aerosol loss. One flight sampled younger (∼ 4 days) aerosol within the strong zonal outflow of the 4–6 km altitude African Easterly Jet-South. This possessed the highest OA : BC mass ratio of the 2016 campaign and overlaid slightly older aerosol with proportionately less OA, although the age difference of one day is not enough to attribute to a large-scale recirculation and subsidence pattern. The other flight sampled aerosol constrained closer to the coast by a mid-latitude disturbance and found older aerosol aloft overlying younger aerosol. Its vertical increase in OA : BC and nitrate to BC was less pronounced than when younger aerosol overlaid older aerosol, consistent with compensation between a net aerosol loss through aging and a thermodynamical partitioning. Organic nitrate provided 68 % on average of the total nitrate for the 6 flights, in contrast to measurements made at Ascension Island that only found inorganic nitrate. Some evidence for the thermodynamical partitioning to the particle phase at higher altitudes with higher relative humidities for nitrate is still found. The 470–660 nm absorption Angstrom exponent is slightly higher near the African coast than further offshore (approximately 1.2 versus 1.0–1.1), indicating some brown carbon may be present near the coast. The data support the following parameterization: SSA530nm = 0.80+0056*(OA : BC). This indicates a 20 % decrease in SSA can be attributed to chemical aging, or the net 25 % reduction in OA : BC documented for constant BC : CO ratios.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Non-reversible aging can increase solar absorption in African biomass burning aerosol plumes of intermediate age

Dobracki

Zuidema

Howell

et al. 2022

Preprint

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“…One of the frequently used variables to define model representation for BBA is aerosol optical depth (AOD) which depends on both aerosol abundance and optical properties in the atmosphere. Previous studies have reported that global models produced substantial underestimations of AOD over BB regions with highly varying extent despite using different emission inventories (Kaiser et al, 2012;Veira et al, 2015;Johnson et al, 2016;Reddington et al, 2016;Mallet et al, 2021). For example, Kaiser et al (2012) showed the global Monitoring Atmospheric Composition and Change (MACC) aerosol model driven by emissions from the Global Fire Assimilation System (GFAS) underestimated AOD by a factor of 2-4 for BBA.…”

Section: Introductionmentioning

confidence: 99%

Satellite-based evaluation of AeroCom model bias in biomass burning regions

Zhong

Schutgens

Werf

et al. 2022

Preprint

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Abstract. Global models are widely used to simulate biomass burning aerosols (BBA). Exhaustive evaluations on model representation of aerosol distributions and properties are fundamental to assess health and climate impacts of BBA. Here we conducted a comprehensive comparison of Aerosol Comparisons between Observation project (AeroCom) model simulations with satellite observations. A total of 59 runs by 18 models from three AeroCom Phase III experiments (i.e., Biomass Burning Emissions, CTRL16, and CTRL19) and 14 satellite products of aerosols were used in the study. Aerosol optical depth (AOD) at 550 nm was investigated during the fire season over three key fire regions reflecting different fire dynamics (i.e., deforestation-dominated Amazon, Southern Hemisphere Africa where savannas are the key source of emissions, and boreal forest burning on boreal North America). The 14 satellite products were first evaluated against AErosol RObotic NETwork (AERONET) observations, with large uncertainties found. But these uncertainties had small impacts on the model evaluation that was dominated by modeling bias. Through a comparison with Polarization and Directionality of the Earth’s Reflectances (POLDER-GRASP) observations, we found that the modeled AOD values were biased by -93–152 %, with most models showing significant underestimations even for the state-of-art aerosol modeling techniques (i.e., CTRL19). By scaling up BBA emissions, the negative biases in modeled AOD were significantly mitigated, although it yielded only negligible improvements in the correlation between models and observations, and the spatial and temporal variations of AOD biases did not change much. For models in CTRL16 and CTRL19, the large diversity in modeled AOD was in almost equal measures caused by diversity in emissions, lifetime, and mass extinction coefficient (MEC). We found that in the AEROCOM ensemble, BBA lifetime correlated significantly with particle deposition (as expected) and in turn correlated strongly with precipitation. Additional analysis based on Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) aerosol profiles suggested that the altitude of the aerosol layer in the current models was generally too low, which also contributed to the bias in modeled lifetime. Modeled MECs exhibited significant correlations with the Ångström Exponent (AE, an indicator of particle size). Comparisons with the POLDER-GRASP observed AE suggested that the models tended to overestimate AE (underestimated particle size), indicating a possible underestimation of MECs in models. The hygroscopic growth in most models generally agreed with observations and might not explain the overall underestimation of modeled AOD. Our results imply that current global models comprise biases in important aerosol processes for BBA (e.g., emissions, removal, and optical properties) that remain to be addressed in future research.

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The representation of dry-season low-level clouds over Western Equatorial Africa in reanalyses and historical CMIP6 simulations

et al. 2023

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Conflicts of interest/Competing interests: Not applicable Availability of data and material: all the data used in this work are publicly available. ISD cloud data are accessible from www.ncdc.noaa.gov/isd/data-access, and EECRA cloud data from NCAR at climateguide.ucar.edu/climate-data. The CALIOP satellite data come from the GCM-Oriented CALIPSO Cloud Product (GOCCP) at climserv.ipsl.polytechnique.fr/cfmip-obs/Calipso_goccp.html. ERA5 reanalyses are available from https://cds.climate.copernicus.eu and MERRA-2 from gmao.gsfc.nasa.gov/reanalysis/MERRA-2/data_access. CMIP6 data can be downloaded from esgfnode.llnl.gov/search/cmip6.

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Climate models generally underrepresent the warming by Central Africa biomass-burning aerosols over the Southeast Atlantic

Cited by 36 publications

References 79 publications

Non-reversible aging can increase solar absorption in African biomass burning aerosol plumes of intermediate age

Non-reversible aging can increase solar absorption in African biomass burning aerosol plumes of intermediate age

Satellite-based evaluation of AeroCom model bias in biomass burning regions

The representation of dry-season low-level clouds over Western Equatorial Africa in reanalyses and historical CMIP6 simulations

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