A Critical Examination of the Observed First Aerosol Indirect Effect

  • Shao H
  • Liu G
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The relative change in cloud droplet number concentration with respect
to the relative change in aerosol number concentration, α, is
an indicator of the strength of the aerosol indirect effect and is
commonly used in models to parameterize this effect. Based on Twomey’s
analytical expression, the values of α derived from measurements
of an individual cloud (i.e., αT) can be as large as 0.60–0.90.
In contrast, the values of α derived from direct measurements
of polluted and clean clouds (i.e., αΔ) typically range
from 0.25 to 0.85, corresponding to a weaker but more uncertain cooling
effect. Clearly, reconciling αΔ with αT is necessary
to properly calculate the indirect aerosol forcing. In this study,
the terms that are involved in determining αT and αΔ
are first analytically examined. Then, by analyzing satellite data
over subtropical oceans, the satellite-observed αΔ can
be successfully related to Twomey’s analytical solution. It
is found that except for the dust-influenced region of the northeastern
Atlantic Ocean, injecting continental aerosols into a marine background
may significantly reduce the average aerosols’ ability to act
as cloud condensation nuclei. Taking this competing effect into account
may reduce the cooling effect proposed by Twomey from 0.76 to 0.28.
It is also found that the variability of the adiabaticity (i.e.,
the cloud dilution state with respect to adiabatic cloud) among different
clouds accounts for ∼50% uncertainty in αΔ. Based
on these results, the authors explain the claimed discrepancies in
the first aerosol indirect effect (AIE) from different methods and
on different scales and present an improved parameterization of the
first AIE that can be used in global climate models.

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  • Hongfei Shao

  • Guosheng Liu

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