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

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.

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)

× = GEBA station● = cloud station− = ISCCP grid box

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

SW Flux Estimated from Synoptic Reports

correlation = 0.87

no adjustment for thickness

SW Flux Estimated from Synoptic Reports

correlation = 0.87

×1.8 thickness factor

“dimming” “brightening”

“dimming” “brightening”

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.

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)

+ = CAM3 grid box center− = ISCCP grid box

“dimming” “brightening”

“dimming” “brightening”

cyan = individual runs

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?