Development of the Unified Z-Grid Icosahedral Model (UZIM) at CSU

Tuesday, May 13, 2014 - 07:00
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The UZIM is a nonhydrostatic global model, which is applicable to both cloud- and cyclone-resolving scales. The model is based on the unified system of equations (Arakawa and Konor, 2009; and Konor, 2014), which unifies the quasi-hydrostatic and anelastic systems to create a nonhydrostatic system of equations that filters the vertically propagating acoustic waves of all scales while maintaining enough compressibility for accurate simulations of the ultra-long atmospheric waves. No basic state is needed. The requirement of energy conservation provides a control on the mean nonhydrostatic temperature change of the atmosphere, which may be useful for the long-term climate change simulations. Not to leave any room for a computational mode in the horizontal velocity field, we have constructed the model based on the Z-grid horizontal dynamics (Randall, 1994). The Z-grid dynamics directly predicts the (vertical component of) vorticity and divergence at the cell centers. Then, the horizontal velocities are obtained from the predicted vorticity and divergence through solving two elliptic equations. These solvers increase the computations approximately 15% compared to a traditional velocity predicting dynamics with five predicted atmospheric species. We think this is a small price to pay to eliminate the computational mode completely at its origin. We generate our grid by bisecting an icosahedron. Then, we optimize the grid to yield highest order of accuracy in the computation of the Laplacian, Jacobian and divergence operators, which are used in the predictions of vorticity and divergence. The optimization method, which we call the tweaking, performs better than the methods used for different models with geodesic grids. Heikes et al. (2013) discuss the grid generation and optimization, and operator performance with a comparison to the other optimization methods. We have optimized the grids up to 1 km resolution, data of which is made available for the community. There are two versions of the UZIM: 1) Nonhydrostatic UZIM-height, which uses a height vertical coordinate and an L-grid in the vertical discretization, and 2) UZIM-sigma, which uses a hybrid sigma-pressure coordinate and a CP-grid. We have finished the construction of the quasi-hydrostatic UZIM-sigma, and are making progress for the nonhydrostatic version of the same model. The UZIM-height with a super parameterized physics has been developed. We are currently working on a version with the CAM parameterized physics. We are planning to make comparison aqua-planet runs with the both versions of the model. We are also working on the super-parameterized version of the UZIM-sigma.

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