Edited by James Haywood, University of Exeter, Exeter, United Kingdom, and accepted by Editorial Board Member A. R. Ravishankara February 6, 2018 (received for review August 6, 2017)
Each year biomass burning (BB) aerosols emitted in southern Africa during the fire season (July–October) transport westward over the southeast Atlantic Ocean and interact with underlying stratocumulus decks. Previous studies showed that BB aerosols greatly perturb the top of atmosphere radiation balance by scattering and absorbing solar radiation and by altering cloud properties via changing the lower tropospheric stability (direct and semidirect effects). Using a state-of-the-art model in combination with satellite observations, we found that BB aerosols that are entrained into the clouds function as cloud condensation nuclei, and increase the brightness of stratocumulus clouds (indirect effect). In these models, this indirect effect dominates the direct and semidirect effects of BB aerosols over this region.
Marine stratocumulus clouds cover nearly one-quarter of the ocean surface and thus play an extremely important role in determining the global radiative balance. The semipermanent marine stratocumulus deck over the southeastern Atlantic Ocean is of particular interest, because of its interactions with seasonal biomass burning aerosols that are emitted in southern Africa. Understanding the impacts of biomass burning aerosols on stratocumulus clouds and the implications for regional and global radiative balance is still very limited. Previous studies have focused on assessing the magnitude of the warming caused by solar scattering and absorption by biomass burning aerosols over stratocumulus (the direct radiative effect) or cloud adjustments to the direct radiative effect (the semidirect effect). Here, using a nested modeling approach in conjunction with observations from multiple satellites, we demonstrate that cloud condensation nuclei activated from biomass burning aerosols entrained into the stratocumulus (the microphysical effect) can play a dominant role in determining the total radiative forcing at the top of the atmosphere, compared with their direct and semidirect radiative effects. Biomass burning aerosols over the region and period with heavy loadings can cause a substantial cooling (daily mean −8.05 W m−2), primarily as a result of clouds brightening by reducing the cloud droplet size (the Twomey effect) and secondarily through modulating the diurnal cycle of cloud liquid water path and coverage (the cloud lifetime effect). Our results highlight the importance of realistically representing the interactions of stratocumulus with biomass burning aerosols in global climate models in this region.
Author contributions: Z.L., X.L., and Z.Z. designed research; Z.L., X.L., and Z.Z. performed research; Z.L., Z.Z., C.Z., K.M., C.R., C.W., and Z.Y. analyzed data; and Z.L., X.L., and J.E.P. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. J.H. is a guest editor invited by the Editorial Board.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1713703115/-/DCSupplemental.
Published under the PNAS license.
