Direct and semi-direct radiative forcing of biomass-burning aerosols over the southeast Atlantic (SEA) and its sensitivity to absorbing properties: a regional climate modeling study
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Date
2020-11-10
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Citation of Original Publication
Mallet, M., Solmon, F., Nabat, P., Elguindi, N., Waquet, F., Bouniol, D., Sayer, A. M., Meyer, K., Roehrig, R., Michou, M., Zuidema, P., Flamant, C., Redemann, J., and Formenti, P.: Direct and semi-direct radiative forcing of biomass-burning aerosols over the southeast Atlantic (SEA) and its sensitivity to absorbing properties: a regional climate modeling study, Atmos. Chem. Phys., 20, 13191–13216, https://doi.org/10.5194/acp-20-13191-2020, 2020.
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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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Abstract
. Simulations are performed for the period 2000–
2015 by two different regional climate models, ALADIN
and RegCM, to quantify the direct and semi-direct radiative effects of biomass-burning aerosols (BBAs) in the southeast Atlantic (SEA) region. Different simulations have been
performed using strongly absorbing BBAs in accordance
with recent in situ observations over the SEA. For the
July–August–September (JAS) season, the single scattering
albedo (SSA) and total aerosol optical depth (AOD) simulated by the ALADIN and RegCM models are consistent
with the MACv2 climatology and MERRA-2 and CAMSRA reanalyses near the biomass-burning emission sources.
However, the above-cloud AOD is slightly underestimated
compared to satellite (MODIS and POLDER) data during the
transport over the SEA. The direct radiative effect exerted at
the continental and oceanic surfaces by BBAs is significant
in both models and the radiative effects at the top of the atmosphere indicate a remarkable regional contrast over SEA (in
all-sky conditions), with a cooling (warming) north (south)
of 10 ◦S, which is in agreement with the recent MACv2 climatology. In addition, the two models indicate that BBAs
are responsible for an important shortwave radiative heating
of ∼ 0.5–1 K per day over SEA during JAS with maxima between 2 and 4 km a.m.s.l. (above mean sea level). At these altitudes, BBAs increase air temperature by ∼ 0.2–0.5 K, with
the highest values being co-located with low stratocumulus
clouds. Vertical changes in air temperature limit the subsidence of air mass over SEA, creating a cyclonic anomaly.
The opposite effect is simulated over the continent due to
the increase in lower troposphere stability. The BBA semidirect effect on the lower troposphere circulation is found to
be consistent between the two models. Changes in the cloud
fraction are moderate in response to the presence of smoke,
and the models differ over the Gulf of Guinea. Finally, the results indicate an important sensitivity of the direct and semidirect effects to the absorbing properties of BBAs. Over the
stratocumulus (Sc) region, DRE varies from +0.94 W m−2
(scattering BBAs) to +3.93 W m−2
(most absorbing BBAs).