Multi-year simulations have been successfully conducted using a hybrid isentropic-sigma general circulation model, in which the vertical coordinate changes smoothly from sigma at the bottom to nearly isentropic in the stratosphere. The results show that the tropical precipitation and water vapor transport as well as the large-scale circulation are significantly improved, compared to a control run that uses the sigma-pressure coordinate. In the boreal winter, the South Pacific Convergence Zone rainfall band is much better simulated in position and intensity. The precipitation in South America and southern Africa is no longer overestimate as in the control run. In the boreal summer, the monsoonal rainfall increases substantially. The cahnges in the moisture convergence field are consistent with the precipitation changes, indicating that the improvement of precipitation is due to better simulation of moisture transport. The tropical large-scale circulation is also improved in many respect.
Sensitivity experiments using different coordinate coefficients were carried out to verify the mechanism for the improvements. It is found that the improvements in water vapor processes are due to the more realistic simulation of the large-scale circulation, which plays a dominant role in determining the moisture transport. The improvement of the tropical circulation is probably due to better representation of the dynamical processes by using the isentropic-sigma coordinate, rather than due to more accurate numerical representation of the moisture advection process. There exists a feedback between the large-scale circulation and the precipitation; as a result; both are better simulated.
last update: 12 March 1996
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