The LMDZ4 general circulation model: climate performance and
sensitivity to parametrized physics with emphasis on tropical convection
Climate Dynamics, Volume 27, Numbers 7-8 / December, 2006. DOI:
10.1007/s00382-006-0158-0
Frédéric Hourdin, Ionela Musat,
Sandrine Bony, Pascale Braconnot, Francis Codron,
Jean-Louis Dufresne, Laurent Fairhead,
Marie-Angèle Filiberti, Pierre Friedlingstein,
Jean-Yves Grandpeix, Gerhard Krinner, Phu LeVan,
Zhao-Xin Li and François Lott
Abstract
The LMDZ4 general circulation model
is the atmospheric component of the
IPSL–CM4 coupled model which has been used to perform climate change
simulations for the 4th IPCC assessment report. The main aspects of the
model climatology (forced by observed sea surface temperature) are
documented here, as well as the major improvements with respect to the
previous versions, which mainly come form the parametrization of
tropical convection. A methodology is proposed to help analyse the
sensitivity of the tropical Hadley–Walker circulation to the
parametrization of cumulus convection and clouds. The tropical
circulation is characterized using scalar potentials associated with
the horizontal wind and horizontal transport of geopotential (the
Laplacian of which is proportional to the total vertical momentum in
the atmospheric column). The effect of parametrized physics is analysed
in a regime sorted framework using the vertical velocity at
500 hPa as
a proxy for large scale vertical motion. Compared to Tiedtke’s
convection scheme, used in previous versions, the Emanuel’s scheme
improves the representation of the Hadley–Walker circulation, with a
relatively stronger and deeper large scale vertical ascent over
tropical continents, and suppresses the marked patterns of concentrated
rainfall over oceans. Thanks to the regime sorted analyses, these
differences are attributed to intrinsic differences in the vertical
distribution of convective heating, and to the lack of self-inhibition
by precipitating downdraughts in Tiedtke’s parametrization. Both the
convection and cloud schemes are shown to control the relative
importance of large scale convection over land and ocean, an important
point for the behaviour of the coupled model.
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