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| Titel |
A review of measurement-based assessments of the aerosol direct radiative effect and forcing |
| VerfasserIn |
H. Yu, 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 |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 6, no. 3 ; Nr. 6, no. 3 (2006-02-27), S.613-666 |
| Datensatznummer |
250003514
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| Publikation (Nr.) |
 copernicus.org/acp-6-613-2006.pdf |
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| Zusammenfassung |
| 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. Our goal is to
assess current observational capabilities and identify uncertainties in the
aerosol direct forcing through comparisons of different methods with
independent sources of uncertainties. Here we assess the aerosol optical
depth (τ), 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 depth (τ) on a daily
scale, with a high accuracy of ±0.03±0.05τ over ocean. The
annual average τ is about 0.14 over global ocean, of which about
21%±7% is contributed by human activities, as estimated by
MODIS fine-mode fraction. The multi-angle MISR derives an annual average AOD
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 the 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 from various methods) over the 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
oceans the surface DRE is estimated to be -8.8±0.7 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.
A number of issues remain. 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 in cloudy conditions 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. Enhanced
measurement capabilities in the next few years and high-level scientific
cooperation will further advance our knowledge. |
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