COLA Report 13
Improvement in Stratosphere and Upper-Troposphere Simulation with a
Hybrid Isentropic-Sigma Coordinate GCM
Zhengxin Zhu and Edwin K. Schneider
June 1995
Abstract
A hybrid isentropic-sigma vertical scheme was implemented into a general circulation model
(GCM). The vertical coordinate changes smoothly from sigma at the bottom to nearly isentropic in
the stratosphere. Multi-year integrations of the GCM were successfully conducted with this new
vertical coordinate. Improvements in the simulated stratospheric and upper tropospheric circulation
are found compared to a control run using the original GCM, which employed a hybrid pressure-sigma vertical coordinate. The greatest improvements are a significant reduction of the cold bias in
the winter polar stratosphere and the wind speed of the polar night jet. These improvements are
shown to be related to significant changes in the eddy fluxes of heat and momentum. Some
improvement is also noted in the simulation of stationary waves in the upper troposphere and the
tropical precipitation distribution.
The Eliassen-Palm flux and other diagnostics show that the upward transfer of wave activity
transfer is larger in the isentropic-sigma simulations than in the control run. Accordingly, the
planetary waves are stronger in the stratosphere, and they transfer more heat to the polar region. The
wave forcing of the zonal flow in the polar night jet region is excessively strong at the top level in
the control run, but largely reduced with the isentropic-sigma coordinate. The isentropic-sigma
coordinate is apparently more accurate in representing the wave/mean flow interactions than the
pressure-sigma coordinate.
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last update: 25 July 1995
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