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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 17, issue 20 | Copyright
Atmos. Chem. Phys., 17, 12725-12742, 2017
https://doi.org/10.5194/acp-17-12725-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 26 Oct 2017

Research article | 26 Oct 2017

Impacts of solar-absorbing aerosol layers on the transition of stratocumulus to trade cumulus clouds

Xiaoli Zhou1, Andrew S. Ackerman2, Ann M. Fridlind2, Robert Wood3, and Pavlos Kollias4,5 Xiaoli Zhou et al.
  • 1Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada
  • 2NASA Goddard Institute for Space Studies, New York, New York, USA
  • 3Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
  • 4School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, New York, USA
  • 5Department of Environmental and Climate Sciences, Brookhaven National Laboratory, Upton, New York, USA

Abstract. The effects of an initially overlying layer of solar-absorbing aerosol on the transition of stratocumulus to trade cumulus clouds are examined using large-eddy simulations. For lightly drizzling cloud the transition is generally hastened, resulting mainly from increased cloud droplet number concentration (Nc) induced by entrained aerosol. The increased Nc slows sedimentation of cloud droplets and shortens their relaxation time for diffusional growth, both of which accelerate entrainment of overlying air and thereby stratocumulus breakup. However, the decrease in albedo from cloud breakup is more than offset by redistributing cloud water over a greater number of droplets, such that the diurnal-average shortwave forcing at the top of the atmosphere is negative. The negative radiative forcing is enhanced by sizable longwave contributions, which result from the greater cloud breakup and a reduced boundary layer height associated with aerosol heating. A perturbation of moisture instead of aerosol aloft leads to a greater liquid water path and a more gradual transition. Adding absorbing aerosol to that atmosphere results in substantial reductions in liquid water path (LWP) and cloud cover that lead to positive shortwave and negative longwave forcings on average canceling each other. Only for heavily drizzling clouds is the breakup delayed, as inhibition of precipitation overcomes cloud water loss from enhanced entrainment. Considering these simulations as an imperfect proxy for biomass burning plumes influencing Namibian stratocumulus, we expect regional indirect plus semi-direct forcings to be substantially negative to negligible at the top of the atmosphere, with its magnitude sensitive to background and perturbation properties.

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Shallow maritime clouds make a well-known transition from stratocumulus to trade cumulus with flow from the subtropics equatorward. Three-day large-eddy simulations that investigate the potential influence of overlying African biomass burning plumes during that transition indicate that cloud-related impacts are likely substantially cooling to negligible at the top of the atmosphere, with magnitude sensitive to background and perturbation aerosol and cloud properties.
Shallow maritime clouds make a well-known transition from stratocumulus to trade cumulus with...
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