Yu, Hongbin, Y.J. Kaufman, M. Chin, G. Feingold, L.A. Remer, T.L. Anderson, Y. Balkanski, N. Bellouin, O. Boucher, S. Christopher, P. DeCola, R. Kahn, D. Koch, N. Loeb, M.S.Reddy, M. Schulz, T. Takemura, M. Zhou, 2005:
A Review of Measurement-based Assessment of Aerosol Direct Radiative Effect and Forcing.
Aerosols affect the Earth's energy budget directly by scattering and absorbing
radiation and indirectly by acting as cloud condensation nuclei and, thereby, affecting cloud
properties. However, large uncertainties exist in current estimates of aerosol forcing because of
incomplete knowledge concerning the distribution and the physical and chemical properties of
aerosols as well as aerosol-cloud interactions. In recent years, a great deal of effort has gone into
improving measurements and datasets. It is thus feasible to shift the estimates of aerosol forcing
from largely model-based to increasingly measurement-based. Here we assess the aerosol optical
depth, direct radiative effect (DRE) by natural and anthropogenic aerosols, and direct climate
forcing (DCF) by anthropogenic aerosols, focusing on satellite and ground-based measurements
supplemented by global chemical transport model (CTM) simulations. The multi-spectral
MODIS measures global distributions of aerosol optical thickness (t) on a daily scale, with a
high accuracy of ±0.03±0.05t over ocean. The annual average t is about 0.14 over global ocean,
of which about 21% is contributed by human activities, as determined by MODIS fine-mode
fraction. The multi-angle MISR derives an annual average AOT of 0.23 over global land with an
uncertainty of ~20% or ±0.05. These high-accuracy aerosol products and broadband flux
measurements from CERES make it feasible to obtain observational constraints for the aerosol
direct effect, especially over global ocean. A number of measurement-based approaches estimate
the clear-sky DRE (on solar radiation) at the top-of-atmosphere (TOA) to be about -5.5±0.2 Wm-2
(median ± standard error) over global ocean. Accounting for thin cirrus contamination of the
satellite derived aerosol field will reduce the TOA DRE to -5.0 Wm-2. Because of a lack of
measurements of aerosol absorption and difficulty in characterizing land surface reflection,
estimates of DRE over land and at the ocean surface are currently realized through a combination
of satellite retrievals, surface measurements, and model simulations, and are less constrained.
Over the ocean surface, the DRE is estimated to be -8.8±0.4 Wm-2. Over land, an integration of
satellite retrievals and model simulations derives a DRE of -4.9±0.7 Wm-2 and -11.8±1.9 Wm-2 at
the TOA and surface, respectively. CTM simulations derive a wide range of DRE estimates that
on average are smaller than the measurement-based DRE by about 30-40%, even after
accounting for thin cirrus and cloud contamination.
Despite these achievements, a number of issues remain open and more efforts are required to
address them. Current estimates of the aerosol direct effect over land are poorly constrained.
Uncertainties of DRE estimates are also larger on regional scales than on a global scale and large
discrepancies exist between different approaches. The characterization of aerosol absorption and
vertical distribution remains challenging. The aerosol direct effect in the thermal infrared range
and under cloudy condition remains relatively unexplored and quite uncertain, because of a lack
of global systematic aerosol vertical profile measurements. A coordinated research strategy
needs to be developed for integration and assimilation of satellite measurements into models to
constrain model simulations. Hopefully, enhanced measurement capabilities in the next few
years and high-level scientific cooperation, will further advance our knowledge.
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