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Title: Broken cloud field longwave-scattering effects

Abstract

Throughout most of the shortwave spectrum, atmospheric gases do not absorb the abundant amount of incoming solar radiation. The shortwave-scattering albedo of clouds is very large. The combination of large amounts of incoming solar radiation, low gaseous absorptivity, and large cloud-scattering albedo enables clouds at one level of the atmosphere to affect the shortwave radiative transfer at all other atmospheric levels. Absorption by atmospheric gases is much stronger in the longwave. This localizes the effects of clouds in the longwave. Since longwave absorption is weakest in the window region (8--12 {micro}m), cloud effects there will have the greatest chance of propagating to other levels of the atmosphere. In partially overcast conditions, individual cloud geometry and optical properties are important factors. Longwave calculations of most GCMs ignore individual cloud geometry. For liquid water clouds, the optical properties of clouds are also ignored. Previous work in the window region by Takara and Ellingson considered opaque clouds with no absorption or emission by atmospheric gases. Under those conditions, the effect of cloud scattering was comparable to cloud geometry. In this work, the comparison of longwave scattering and geometric effects in the window region is improved by including partially transparent clouds and adding absorptionmore » and emission by atmospheric gases. The results show that for optically thick water clouds, it is sufficient to model the geometry; scattering can be neglected. The window region errors are less than 5 W m{sup {minus}2} for fluxes and 0.05 K day{sup {minus}1} for heating rates. The flat-plate approximation worked for ice clouds; the window region flux errors are less than 3 W m{sup {minus}2} with heating rate errors less than 0.05 K day{sup {minus}1}.« less

Authors:
;
Publication Date:
Research Org.:
Univ. of Maryland, College Park, MD (US)
Sponsoring Org.:
USDOE
OSTI Identifier:
20075776
DOE Contract Number:  
FG02-94ER61746
Resource Type:
Journal Article
Journal Name:
Journal of the Atmospheric Sciences
Additional Journal Information:
Journal Volume: 57; Journal Issue: 9; Other Information: PBD: 1 May 2000; Journal ID: ISSN 0022-4928
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; CLOUDS; CLIMATES; HEAT FLUX; SOLAR RADIATION; ALBEDO; DATA COVARIANCES; GENERAL CIRCULATION MODELS

Citation Formats

Takara, E.E., and Ellingson, R.G. Broken cloud field longwave-scattering effects. United States: N. p., 2000. Web. doi:10.1175/1520-0469(2000)057<1298:BCFLSE>2.0.CO;2.
Takara, E.E., & Ellingson, R.G. Broken cloud field longwave-scattering effects. United States. doi:10.1175/1520-0469(2000)057<1298:BCFLSE>2.0.CO;2.
Takara, E.E., and Ellingson, R.G. Mon . "Broken cloud field longwave-scattering effects". United States. doi:10.1175/1520-0469(2000)057<1298:BCFLSE>2.0.CO;2.
@article{osti_20075776,
title = {Broken cloud field longwave-scattering effects},
author = {Takara, E.E. and Ellingson, R.G.},
abstractNote = {Throughout most of the shortwave spectrum, atmospheric gases do not absorb the abundant amount of incoming solar radiation. The shortwave-scattering albedo of clouds is very large. The combination of large amounts of incoming solar radiation, low gaseous absorptivity, and large cloud-scattering albedo enables clouds at one level of the atmosphere to affect the shortwave radiative transfer at all other atmospheric levels. Absorption by atmospheric gases is much stronger in the longwave. This localizes the effects of clouds in the longwave. Since longwave absorption is weakest in the window region (8--12 {micro}m), cloud effects there will have the greatest chance of propagating to other levels of the atmosphere. In partially overcast conditions, individual cloud geometry and optical properties are important factors. Longwave calculations of most GCMs ignore individual cloud geometry. For liquid water clouds, the optical properties of clouds are also ignored. Previous work in the window region by Takara and Ellingson considered opaque clouds with no absorption or emission by atmospheric gases. Under those conditions, the effect of cloud scattering was comparable to cloud geometry. In this work, the comparison of longwave scattering and geometric effects in the window region is improved by including partially transparent clouds and adding absorption and emission by atmospheric gases. The results show that for optically thick water clouds, it is sufficient to model the geometry; scattering can be neglected. The window region errors are less than 5 W m{sup {minus}2} for fluxes and 0.05 K day{sup {minus}1} for heating rates. The flat-plate approximation worked for ice clouds; the window region flux errors are less than 3 W m{sup {minus}2} with heating rate errors less than 0.05 K day{sup {minus}1}.},
doi = {10.1175/1520-0469(2000)057<1298:BCFLSE>2.0.CO;2},
journal = {Journal of the Atmospheric Sciences},
issn = {0022-4928},
number = 9,
volume = 57,
place = {United States},
year = {2000},
month = {5}
}