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Arctic Cloud Properties and Radiative Forcing From Observations
and Their Role in Sea Ice Decline Predicted by the NCAR
CCSM3 Model During the 21st Century
Irina V. Gorodetskaya 1
Laboratoire de Glaciologie et Géophysique de l'Environnement, Centre National de la Recherche Scientifique,
Université Joseph Fourier, Grenoble, France
L.-Bruno Tremblay
Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada
Arctic sea ice is sensitive to changes in surface radiative fluxes. Clouds influence
shortwave radiation primarily through their high albedo and longwave radiation by
changing atmospheric emissivity and determining the height (temperature) of the
layer of the highest emission. We review Arctic cloud properties affecting radiative
fluxes, estimate sea ice effect on the top-of-atmosphere albedo, and discuss cloud
response and contribution to the Arctic sea ice decline during the 21st century
predicted by the National Center for Atmospheric Research Community Climate
System Model, version 3 (CCSM3). Over perennial sea ice, clouds decrease
incoming shortwave flux at the surface compared to clear skies from zero in winter
to ~100 W m -2 during the summer. On average over the Arctic Ocean, sea ice
retreat decreases the shortwave radiation reflected at the top of the atmosphere
within the same range for all-sky conditions. In addition, Arctic clouds warm the
surface increasing the annual mean downwelling longwave flux by ~40 W m -2 .
During the 21st century, CCSM3 predicts a drastic sea ice decline accompanied
by larger cloud cover and liquid water content, which increase both cloud cooling
and warming effects at the surface. The surface albedo decrease caused by sea
ice retreat is partly compensated but not canceled by stronger shortwave cloud
cooling. Warming of the near-surface atmosphere is an additional factor increasing
the downwelling longwave flux at the surface. The ultimate effect of cloud changes
in this model is facilitating the sea ice decline.
1 Formerly at Lamont-Doherty Earth Observatory and Depart-
ment of Earth and Environmental Sciences, Columbia University,
Palisades, New York, USA.
1. INTRODUCTION
During the last 30 years, satellite observations have shown
a large reduction in the Arctic sea ice cover [ Overpeck et
al. , 2005; Stroeve et al. , 2005; Serreze et al. , 2007]. Sig-
nificant future reduction in the extent and thickness of the
Arctic sea ice is predicted by coupled models participating
in the Fourth Assessment Report of Intergovernmental Panel
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