Journal article

Impact of an improved shortwave radiation scheme in the MAECHAM5 General Circulation Model

Cagnazzo C, Manzini E, Giorgetta M, Forster P, Morcrette J ...see all

Atmospheric Chemistry and Physics, vol. 7, issue 10 (2007) pp. 2503-2515

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In order to improve the representation of ozone absorption in the
stratosphere of the MAECHAM5 general circulation model, the spectral
resolution of the shortwave radiation parameterization used in the
model has been increased from 4 to 6 bands. Two 20-years simulations
with the general circulation model have been performed, one with
the standard and the other with the newly introduced parameterization
respectively, to evaluate the temperature and dynamical changes arising
from the two different representations of the shortwave radiative
transfer. In the simulation with the increased spectral resolution
in the radiation parameterization, a significant warming of almost
the entire model domain is reported. At the summer stratopause the
temperature increase is about 6 K and alleviates the cold bias present
in the model when the standard radiation scheme is used. These general
circulation model results are consistent both with previous validation
of the radiation scheme and with the offline clear-sky comparison
performed in the current work with a discrete ordinate 4 stream scattering
line by line radiative transfer model. The offline validation shows
a substantial reduction of the daily averaged shortwave heating rate
bias (1?2 K/day cooling) that occurs for the standard radiation parameterization
in the upper stratosphere, present under a range of atmospheric conditions.
Therefore, the 6 band shortwave radiation parameterization is considered
to be better suited for the representation of the ozone absorption
in the stratosphere than the 4 band parameterization. Concerning
the dynamical response in the general circulation model, it is found
that the reported warming at the summer stratopause induces stronger
zonal mean zonal winds in the middle atmosphere. These stronger zonal
mean zonal winds thereafter appear to produce a dynamical feedback
that results in a dynamical warming (cooling) of the polar winter
(summer) mesosphere, caused by an increased downward (upward) circulation
in the winter (summer) hemisphere. In addition, the comparison of
the two simulations performed with the general circulation model
shows that the increase in the spectral resolution of the shortwave
radiation and the associated changes in the cloud optical properties
result in a warming (0.5?1 K) and moistening (3%?12%) of the upper
tropical troposphere. By comparing these modeled differences with
previous works, it appears that the reported changes in the solar
radiation scheme contribute to improve the model mean temperature
also in the troposphere.

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  • C. Cagnazzo

  • E. Manzini

  • M. a. Giorgetta

  • P. M. De F. Forster

  • J. J. Morcrette

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