The purpose of this study is to briefly describe the global atmospheric University of Wisconsin (UW) hybrid isentropic–eta coordinate (UW θ–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 UW θ–η model eliminates the discrete interface in the UW θ–σ model 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 UW θ–σ model 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 UW θ–η model 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 UW θ–η model in simulating reversibility and potential vorticity transport over a 10-day period that corresponds with the global residence time of water vapor.
Corresponding author address: Todd K. Schaack, Space Science and Engineering Center, University of Wisconsin—Madison, 1225 W. Dayton St., Madison, WI 53706. Email: email@example.com