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Aerosol Climate Effects: Local Radiative Forcing and Column Closure ExperimentsIn an effort to reduce uncertainties in climate change predictions, experiments are being planned and conducted to measure anthropogenic aerosol properties and effects, including effects on radiative fields. The global average, direct anthropogenic aerosol effect on upwelling shortwave fluxes is estimated to be about +1/2 W/sq m, whereas errors in flux changes measured with airborne and spaceborne radiometers are 2 to 8 W/sq m or larger. This poses the question of whether flux changes expected in field experiments will be large enough to measure accurately. This paper obtains a new expression for the aerosol-induced change in upwelling flux, compares it to two-stream and adding-doubling (AD) results, and uses all three methods to estimate expected flux changes. The new expression accounts for the solar zenith angle dependences of aerosol transmission and reflection, as well as of surface albedo, all of which can have a strong effect in determining flux changes measured in field experiments. Despite its relative simplicity, the new expression gives results similar to previous two-stream results. Relative to AD results, it agrees within a few watts per square meter for the intermediate solar elevation angles where the flux changes peak (roughly 10 to 30 degrees), but it has negative errors for higher Sun and positive errors for lower Sun. All three techniques yield aerosol-induced changes in upwelling flux of +8 to +50 W/sq m for aerosol midvisible optical depths of 0.1 to 0.5. Because such aerosol optical depths occur frequently off the U.S. and European Atlantic coasts in summer, the flux changes they induce should be measurable by airborne, and possibly by spaceborne, radiometers, provided sufficient care is taken in experiment design (including measurements to separate aerosol radiative effects from those of absorbing gases). The expected flux changes are about 15 to 100 times larger than the global average flux change expected for the global average anthropogenic sulfate optical depth of 0.04. Reasons for the larger flux changes include the larger optical depths considered here (factor 2.5 to 12), plus restricting the measurements to cloud-free, daytime conditions over the ocean (factor 5 to 9).
Document ID
20000052534
Acquisition Source
Ames Research Center
Document Type
Reprint (Version printed in journal)
Authors
Russell, P. B.
(NASA Ames Research Center Moffett Field, CA United States)
Bergstrom, Robert W.
(Bay Area Environmental Research Inst. San Francisco, CA United States)
Kinne, S. A.
(Bay Area Environmental Research Inst. San Francisco, CA United States)
Date Acquired
August 19, 2013
Publication Date
February 28, 2000
Publication Information
Publication: Analysis of Atmospheric Aerosol Data Sets and Application of Radiative Transfer Models to Compute Aerosol Effects
Subject Category
Environment Pollution
Report/Patent Number
Paper 97JD00112
Funding Number(s)
PROJECT: RTOP 460-43-46-10
PROJECT: RTOP 537-10-01-01
Distribution Limits
Public
Copyright
Other

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