Atmos. Chem. Phys. Discuss., 5, 7647-7768, 2005
www.atmos-chem-phys-discuss.net/5/7647/2005/
doi:10.5194/acpd-5-7647-2005
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
A review of measurement-based assessment of aerosol direct radiative effect and forcing
H. Yu1,2, Y. J. Kaufman1, M. Chin1, G. Feingold3, L. A. Remer1, T. L. Anderson4, Y. Balkanski5, N. Bellouin6, O. Boucher6,12, S. Christopher7, P. DeCola8, R. Kahn9, D. Koch10, N. Loeb11, M. S. Reddy12,13, M. Schulz5, T. Takemura14, and M. Zhou15
1Laboratory for Atmospheres, NASA Goddard Space Flight Center (GSFC), Greenbelt, USA
2Goddard Earth Science and Technology Center, University of Maryland, Baltimore County, Baltimore, USA
3NOAA Environmental Technology Laboratory (ETL), Boulder, USA
4University of Washington, Seattle, USA
5Laboratoire des Sciences du Climat et de l’Environnement, CEA/CNRS – LSCE, L’Orme des Merisiers, France
6Met Office, Exeter, UK
7University of Alabama, Huntsville, USA
8NASA Headquarters, Washington, D.C., USA
9NASA Jet Propulsion Laboratory (JPL), Pasadena, USA
10NASA Goddard Institute for Space Studies (GISS), New York, USA
11NASA Langley Atmospheric Research Center (LaRC), Hampton, USA
12Laboratoire d’Optique Atmosphérique (LOA), Villeneuve d’Ascq, France
13NOAA Geophysical Fluid Dynamics Laboratory (GFDL), Princeton, USA
14Kyushu University, Fukuoka, Japan
15Georgia Institute of Technology, Atlanta, USA

Abstract. 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 (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% 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.


Citation: Yu, H., Kaufman, Y. J., Chin, M., Feingold, G., Remer, L. A., Anderson, T. L., Balkanski, Y., Bellouin, N., Boucher, O., Christopher, S., DeCola, P., Kahn, R., Koch, D., Loeb, N., Reddy, M. S., Schulz, M., Takemura, T., and Zhou, M.: A review of measurement-based assessment of aerosol direct radiative effect and forcing, Atmos. Chem. Phys. Discuss., 5, 7647-7768, doi:10.5194/acpd-5-7647-2005, 2005.
 
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