Parametric Sensitivity to Regional Mesh Refinement in the Community Atmosphere Model (CAM)

Wednesday, May 14, 2014 - 07:00
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The expectation is that many regional climate problems will soon be routinely examined in the framework of low-resolution global climate simulations, with local regions of increased horizontal resolution. This represents both significant cost and data savings over equivalent global high resolution simulations. Although CAM is currently able to functionally carry out such experiments the fidelity of the simulations remains unclear. In particular, the behavior of the existing physical parameterizations, built for a uniform resolution grid, require analysis and understanding. This presentation will examine the sensitivity of idealized aqua-planet simulations to regional mesh refinement in response to refinement location, CAM physics package and perturbations to key physics parameters. As with previous work, we find much reduced sensitivity to tropical mesh refinement with version 5 (CAM5) of the model compared to CAM4, indicating a more 'scale-aware' behavior. This sensitivity resides almost entirely in the deep convection response, and is clearly identifiable from a shift in the balance of tendency terms between inside and outside the mesh region. Perturbations to the parameters controlling the strength of deep convection illustrate its role in the magnitude of the mesh sensitivity. A reduced timescale leads to lower sensitivity and an increased timescale to a higher sensitivity in the mean climate. The sensitivities seen in the mean climate due to refinement are not necessarily reflected in the transient wave activity. Interestingly the time and space scales of equatorial waves are impacted differently through tropical refinement regions. We show that the faster Kelvin-type waves are impeded more by the refinement whereas the slower MJO/Kelvin waves see a reduced impact on transit through the mesh region. Away from the tropics and the dominance of deep convection, grid sensitivities are comparatively much reduced, but we show that there are some non-local impacts of mid-latitude refinement back on the tropical mean circulation.

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