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The albedo of fractal stratocumulus cloudsAn increase in the planetary albedo of the earth-atmosphere system by only 10% can decrease the equilibrium surface temperature to that of the last ice age. Nevertheless, albedo biases of 10% or greater would be introduced into large regions of current climate models if clouds were given their observed liquid water amounts, because of the treatment of clouds as plane parallel. The focus on marine stratocumulus clouds is due to their important role in cloud radiative forcing and also that, of the wide variety of earth's cloud types, they are most nearly plane parallel, so that they have the least albedo bias. The fractal model employed here reproduces both the probability distribution and the wavenumber spectrum of the stratocumulus liquid water path, as observed during the First ISCCP Regional Experiment (FIRE). A single new fractal parameter 0 less than or equal to f less than or equal to 1, is introduced and determined empirically by the variance of the logarithm of the vertically integrated liquid water. The reduced reflectivity of fractal stratocumulus clouds is approximately given by the plane-parallel reflectivity evaluated at a reduced 'effective optical thickness,' which when f = 0.5 is tau(sub eff) approximately equal to 10. Study of the diurnal cycle of stratocumulus liquid water during FIRE leads to a key unexpected result: the plane-parallel albedo bias is largest when the cloud fraction reaches 100%, that is, when any bias associated with the cloud fraction vanishes. This is primarily due to the variability increase with cloud fraction. Thus, the within-cloud fractal structure of stratocumulus has a more significant impact on estimates of its mesoscale-average albedo than does the cloud fraction.
Document ID
19950035312
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
Authors
Cahalan, Robert F.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Ridgway, William
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Wiscombe, Warren J.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Bell, Thomas L.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Snider, Jack B.
(NOAA/ERL/Wave Propagation Laboratory, Boulder, CO United States)
Date Acquired
August 16, 2013
Publication Date
August 15, 1994
Publication Information
Publication: Journal of the Atmospheric Sciences
Volume: 51
Issue: 16
ISSN: 0022-4928
Subject Category
Meteorology And Climatology
Accession Number
95A66911
Funding Number(s)
CONTRACT_GRANT: DE-A105-90ER61069
CONTRACT_GRANT: NAS5-30430
CONTRACT_GRANT: NAS5-30440
Distribution Limits
Public
Copyright
Other

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