European Solar “Dimming” and “Brightening” as a Diagnostic for Aerosol Simulation in GCMs
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Transcript of European Solar “Dimming” and “Brightening” as a Diagnostic for Aerosol Simulation in GCMs
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European Solar “Dimming” and “Brightening” as a Diagnostic for
Aerosol Simulation in GCMs
Joel Norris
Scripps Institution of Oceanography
20 March 2006
Special thanks to Martin Wild
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Motivation
• Historical trends in direct and indirect radiative forcing by anthropogenic aerosols are highly uncertain.
• GCMs have previously estimated the magnitude of aerosol radiative forcing by matching the observed 20th century temperature record.
• Unfortunately GCMs cannot obtain a unique solution due to a trade-off between aerosol radiative forcing and climate sensitivity.
• Quantification of observed changes in regional aerosol radiative forcing would provide additional constraints on GCM aerosol/cloud parameterizations.
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Datasets
• Monthly downward all-sky SW radiation flux from Global Energy Budget Archive (GEBA) stations during ~1965-2003 (clear-sky not available)
• Monthly gridded daytime total cloud amount from ISCCP D2 during 1983-2004
• Monthly gridded downward all-sky and clear-sky SW radiation flux at the surface from ISCCP Flux Dataset (FD) during 1983-2004
• Monthly daytime total cloud cover from synoptic reports at WMO stations during 1971-1996 (Hahn and Warren NDP026D)
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× = GEBA station● = cloud station− = ISCCP grid box
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SW Flux Estimated from Synoptic Reports
• Use synoptic cloud reports to bridge between satellite observations and pre-1983 GEBA measurements
estimated SW CRF anomaly =(mean SW CRF / mean cloud cover) total cloud cover anomaly ) thickness factor
The “thickness factor” accounts for the positive correlation between cloud cover and cloud thickness anomalies
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SW Flux Estimated from Synoptic Reports
correlation = 0.87
no adjustment for thickness
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SW Flux Estimated from Synoptic Reports
correlation = 0.87
×1.8 thickness factor
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“dimming” “brightening”
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“dimming” “brightening”
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Observational Summary
• Cloud cover and related thickness variations dominate surface solar radiation flux on monthly to subdecadal time scales.
• Once cloud cover and related effects are removed, the residual time series shows a deceasing trend before 1984 and an increasing trend after 1992.
• The residual trend represents changes in clear-sky flux and changes in cloud optical thickness that are uncorrelated to changes in cloud cover.
• The most likely cause of the trends is direct and indirect forcing (first effect) by anthropogenic aerosol.
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CAM3 IPCC AMIP Ensemble
• Five simulations for 1950-2000 with prescribed historical SST and same 20th century radiative forcing used in coupled IPCC runs
• Time-varying aerosol, but direct radiative forcing only (no influence on cloud microphysical properties)
• Monthly output was processed the same way as in the observational study
CLDTOT (total cloud)
FSDS (all-sky downward solar flux at surface)
FSDSC (clear-sky downward solar flux at surface)
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+ = CAM3 grid box center− = ISCCP grid box
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“dimming” “brightening”
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“dimming” “brightening”
cyan = individual runs
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Conclusions
• CAM3 overproduces interannual variations in surface downward solar flux over Europe by a factor of two.
• CAM3 does not reproduce the observed solar “dimming” and “brightening” trends over Europe when best guess 20th century anthropogenic aerosol radiative forcing is applied.
• Incorrect aerosol input?• Poor representation of transport, sources, and sinks?• Poor representation of clear-sky radiative forcing?• Lack of aerosol influence on cloud albedo?