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Impacts of Solar-Absorbing Aerosol Layers on the Transition of Stratocumulus to Trade Cumulus CloudsThe 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 short-wave 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.
Document ID
20170011275
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
Authors
Zhou, Xiaoli
(McGill Univ. Montreal, Quebec, Canada)
Ackerman, Andrew S.
(NASA Goddard Inst. for Space Studies New York, NY, United States)
Fridlind, Ann M.
(NASA Goddard Inst. for Space Studies New York, NY, United States)
Wood, Robert
(Washington Univ. Seattle, WA, United States)
Kollias, Pavlos
(Stony Brook Univ. Stony Brook, NY, United States)
Date Acquired
November 27, 2017
Publication Date
October 26, 2017
Publication Information
Publication: Atmospheric Chemistry and Physics
Publisher: European Geosciences Union (EGU)
Volume: 17
Issue: 20
ISSN: 1680-7316
e-ISSN: 1680-7324
Subject Category
Meteorology And Climatology
Report/Patent Number
GSFC-E-DAA-TN48809
Distribution Limits
Public
Copyright
Other

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