The albedo of fractal stratocumulus clouds

Journal Article · Mon Aug 15 04:00:00 EDT 1994 · Journal of the Atmospheric Sciences

DOI:https://doi.org/10.1175/1520-0469(1994)051<2434:TAOFSC>2.0.CO;2· OSTI ID:28477

Cahalan, R F; Wiscombe, W J; Bell, T L ^[1]; Ridgway, W ^[1]

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1. NASA/Goddard Space Flight Center, Greenbelt, MD (United States)

An 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. Past work has addressed the effect of cloud shape on albedo; here the focus is on the within-cloud variability of the vertically integrated liquid water. The main result is an estimate of the {open_quotes}plane-parallel albedo bias{close_quotes} using the {open_quotes}independent pixel approximation,{close_quotes} which ignores net horizontal photon transport, from a simple fractal model of marine stratocumulus clouds that ignores the cloud shape. The use of the independent pixel approximation in this context will be justified in a separate Monte Carlo study. 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). 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. 52 refs., 11 figs., 2 tabs.

• OSTI ID: 28477
• Journal Information: Journal of the Atmospheric Sciences, Journal Name: Journal of the Atmospheric Sciences Journal Issue: 16 Vol. 51; ISSN 0022-4928; ISSN JAHSAK
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
ALBEDO
AMBIENT TEMPERATURE
CLOUDS
EARTH PLANET
FRACTALS
GLOBAL ASPECTS
WATER VAPOR