Global Climate Simulation with the University of Wisconsin Global Hybrid Isentropic Coordinate Model

• Published in: Journal of climate 2004-08, Vol.17 (15), p.2998-3016
• Main Authors: Schaack, Todd K., Zapotocny, Tom H., Lenzen, Allen J., Johnson, Donald R.
• Format: Article
• Language: English
• Subjects: Atmospheric models; Atmospherics; Climate; Climate models; Climatology. Bioclimatology. Climate change; Coordinate systems; Earth, ocean, space; Entropy; Exact sciences and technology; External geophysics; Global climate; Global climate models; Human influences; Meteorology; Modeling; Simulation; Simulations; Troposphere; Water vapor
• ISSN: 0894-8755; 1520-0442
• DOI: 10.1175/1520-0442(2004)017<2998:GCSWTU>2.0.CO;2

The purpose of this study is to briefly describe the global atmospheric University of Wisconsin (UW) hybrid isentropic–eta coordinate (UWθ–η) model and document results from a 14-yr climate simulation. The model, developed through modification of the UW hybrid isentropic–sigma (θ–σ) coordinate model, employs a vertical coordinate that smoothly varies from terrain following at the earth’s surface to isentropic coordinates in the middle to upper troposphere. The UWθ–ηmodel eliminates the discrete interface in the UWθ–σmodel between the PBL expressed in sigma coordinates and the free atmosphere expressed in isentropic coordinates. The smooth transition of the modified model retains the excellent transport characteristics of the UWθ–σmodel while providing for straightforward application of data assimilation techniques, use of higher-order finite-difference schemes, and implementation on massively parallel computing platforms. This study sets forth the governing equations and describes the vertical structure employed by the UWθ–ηmodel after which the results from a 14-yr climate simulation detail the model’s simulation capabilities. Relative to reanalysis data and other fields, the dominant features of the global circulation, including seasonal variability, are well represented in the simulations, thus demonstrating the viability of the hybrid model for extended-length integrations. Overall the study documents that no insurmountable barriers exist to simulation of climate utilizing hybrid isentropic coordinate models. Additional results from two numerical experiments examining conservation demonstrate a high degree of numerical accuracy for the UWθ–ηmodel in simulating reversibility and potential vorticity transport over a 10-day period that corresponds with the global residence time of water vapor.