François Lott,
Laurent Fairhead, Frederic Hourdin and Phu Levan
Climate Dynamics,Volume 25, Numbers 7-8 / December, 2005,
DOI:10.1007/s00382-005-0064-x
(pdf here)
Abstract
A climatology of the stratosphere is
determined from a 20-year
integration with the stratospheric version of the Atmospheric General
Circulation Model LMDz. The model has an upper boundary at near
65 km,
uses a Doppler spread non-orographic gravity waves drag
parameterization and a subgrid-scale orography parameterization. It
also has a Rayleigh damping layer for resolved waves only (not the
zonal mean flow) over the top 5 km. This paper describes the basic
features of the model and some aspects of its radiative-dynamical
climatology. Standard first order diagnostics are presented but some
emphasis is given to the model’s ability to reproduce the low frequency
variability of the stratosphere in the winter northern hemisphere. In
this model, the stratospheric variability is dominated at each
altitudes by patterns which have some similarities with the arctic
oscillation (AO). For those patterns, the signal sometimes descends
from the stratosphere to the troposphere. In an experiment where the
parameterized orographic gravity waves that reach the stratosphere are
exaggerated, the model stratosphere in the NH presents much less
variability. Although the stratospheric variability is still dominated
by patterns that resemble to the AO, the downward influence of the
stratosphere along these patterns is near entirely lost. In the same
time, the persistence of the surface AO decreases, which is consistent
with the picture that this persistence is linked to the descent of the
AO signal from the stratosphere to the troposphere. A comparison
between the stratospheric version of the model, and its routinely used
tropospheric version is also done. It shows that the introduction of
the stratosphere in a model that already has a realistic AO persistence
can lead to overestimate the actual influence of the stratospheric
dynamics onto the surface AO. Although this result is certainly model
dependent, it suggests that the introduction of the stratosphere in a
GCM also call for a new adjustment of the model parameters that affect
the tropospheric variability.
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