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2 Acta Geophysica

2 2011

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Multi-scale waves in sound-proof global simulations with EULAG

Prusa J.M., Gutowski W.J.

Acta Geophysica

2011

Vol. 59, no. 6

1135-1157

EULAG is a computational model for simulating flows across a wide range of scales and physical scenarios. A standard option employs an anelastic approximation to capture nonhydrostatic effects and simultaneously filter sound waves from the solution. In this study, we examine a localized gravity wave packet generated by instabilities in Held-Suarez climates. Although still simplified versus the Earth's atmosphere, a rich set of planetary wave instabilities and ensuing radiated gravity waves can arise. Wave packets are observed that have lifetimes ≤ 2 days, are negligibly impacted by Coriolis force, and do not show the rotational effects of differential jet advection typical of inertia-gravity waves. Linear modal analysis shows that wavelength, period, and phase speed fit the dispersion equation to within a mean difference of ∼ 4 per cent, suggesting an excellent fit. However, the group velocities match poorly even though a propagation of uncertainty analysis indicates that they should be predicted as well as the phase velocities. Theoretical arguments suggest the discrepancy is due to nonlinearity - a strong southerly flow leads to a critical surface forming to the southwest of the wave packet that prevents the expected propagation.

CAM-EULAG: A non-hydrostatic atmospheric climate model with grid stretching

Abiodun B. J., Gutowski W. J., Abatan A. A., Prusa J. M.

Acta Geophysica

2011

Vol. 59, no. 6

1158-1167

This study evaluates the capability of a non-hydrostatic global climate model with grid stretching (CEU) that uses NCAR Community Atmospheric Model (CAM) physics and EULAG dynamics. We compare CEU rainfall with that produced by CAM using finite volume dynamics (CFV). Both models simulated climate from 1996 to 2000, using the same parameterization schemes. CEU and CFV both simulate well the observed global rainfall pattern. However, with same grid, CEU performs better than CFV in simulating the annual cycles of precipitation over our target region of West Africa. The reason is that it simulates African easterly jet and monsoon circulations better than CFV. CEU simulations with horizontal grid stretching to 0.5° are markedly better than those using CAM's standard 2.0°×2.5° grid.