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Independent pixel and Monte Carlo estimates of stratocumulus albedoMonte Carlo radiative transfer methods are employed here to estimate the plane-parallel albedo bias for marine stratocumulus clouds. This is the bias in estimates of the mesoscale-average albedo, which arises from the assumption that cloud liquid water is uniformly distributed. The authors compare such estimates with those based on a more realistic distribution generated from a fractal model of marine stratocumulus clouds belonging to the class of 'bounded cascade' models. In this model the cloud top and base are fixed, so that all variations in cloud shape are ignored. The model generates random variations in liquid water along a single horizontal direction, forming fractal cloud streets while conserving the total liquid water in the cloud field. The model reproduces the mean, variance, and skewness of the vertically integrated cloud liquid water, as well as its observed wavenumber spectrum, which is approximately a power law. The Monte Carlo method keeps track of the three-dimensional paths solar photons take through the cloud field, using a vectorized implementation of a direct technique. The simplifications in the cloud field studied here allow the computations to be accelerated. The Monte Carlo results are compared to those of the independent pixel approximation, which neglects net horizontal photon transport. Differences between the Monte Carlo and independent pixel estimates of the mesoscale-average albedo are on the order of 1% for conservative scattering, while the plane-parallel bias itself is an order of magnitude larger. As cloud absorption increases, the independent pixel approximation agrees even more closely with the Monte Carlo estimates. This result holds for a wide range of sun angles and aspect ratios. Thus, horizontal photon transport can be safely neglected in estimates of the area-average flux for such cloud models. This result relies on the rapid falloff of the wavenumber spectrum of stratocumulus, which ensures that the smaller-scale variability, where the radiative transfer is more three-dimensional, contributes less to the plane-parallel albedo bias than the larger scales, which are more variable. The lack of significant three-dimensional effects also relies on the assumption of a relatively simple geometry. Even with these assumptions, the independent pixel approximation is accurate only for fluxes averaged over large horizontal areas, many photon mean free paths in diameter, and not for local radiance values, which depend strongly on the interaction between neighboring cloud elements.
Document ID
19950036311
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)
Gollmer, Steven
(Purdue University Lafayette, IN, United States)
HARSHVARDHAN
(Purdue University Lafayette, IN, United States)
Date Acquired
August 16, 2013
Publication Date
December 15, 1994
Publication Information
Publication: Journal of the Atmospheric Sciences
Volume: 51
ISSN: 0022-4928
Subject Category
Meteorology And Climatology
Accession Number
95A67910
Funding Number(s)
CONTRACT_GRANT: DE-A105-90ER-61069
CONTRACT_GRANT: NAS5-30430
CONTRACT_GRANT: NAS5-30440
CONTRACT_GRANT: NGT-50963
CONTRACT_GRANT: NSF ATM-89-09870
Distribution Limits
Public
Copyright
Other

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