Precipitation and Water Vapor Transport Simulated by a Hybrid σ–θ Coordinate GCM

Abstract

A smoothly varying hybrid σ?? coordinate, which changes from sigma at the bottom to nearly isentropic in the stratosphere, was implemented into a general circulation model. Multiyear simulations of the σ?? coordinate GCM were successfully conducted. The results reveal that the tropical precipitation, water vapor transport, and large-scale circulation are significantly improved, compared to a control run using the σ?p coordinate. In boreal winter, the position and intensity of the South Pacific convergence zone is much better simulated in the position and intensity. In boreal summer, the Asian monsoonal rainfall increases substantially, and the precipitation maxima in the Bay of Bengal and west of India are more realistic. The changes in the moisture transport and convergence are found to be consistent with the precipitation changes. The tropical large-scale flow field is also improved in many aspects. Possible reasons responsible for the improvements are analyzed and verified with sensitivity experiments. The improvements of water vapor transport and precipitation are more likely due to the better representation of large-scale dynamics by using the nearly isentropic coordinate in the upper levels, rather than due to more accurate representation of moisture advection process in the lower troposphere.