The goals of this project are to (1) assess and quantify the sensitivity and scale-dependency of unresolved subgrid-scale mixing processes in the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM), and (2) to improve the accuracy and skill of forthcoming CESM configurations on modern cubed-sphere and variable-resolution computational grids. The research thereby contributes to the description and quantification of uncertainties in CESM's dynamical cores and their physics-dynamics interactions. This will inform NCAR's and DOE's contributions to climate change assessments.

All dynamical cores of atmospheric General Circulation Models (GCMs) need some form of subgrid-scale dissipation, either explicitly specified or inherent in the chosen numerical schemes. The representation of the subgrid scale in GCMs is complex. Besides physical processes to be represented, numerical errors manifest themselves as subgrid-scale diffusion and ad-hoc mixing is used to assure numerical stability and to compensate for numerical dispersion errors. The impact of such mixing processes triggered by the dynamical core and its tracer advection scheme on climate simulations is poorly understood. The research will isolate and quantify such climate signals, and shed light on the nonlinear interactions between the dynamical core, tracer transport processes and the physical parameterizations. This is done via a hierarchy of CESM simulations that range from dry dynamical core assessments, simulations with simplified moisture processes to full-physics experiments in aqua-planet mode. We will utilize specific process studies. They are the study of idealized tropical cyclones at moderate to extreme (12 km) resolutions, the study of CESM's tracer transport algorithms in both tropospheric and stratospheric flow regimes, and a novel evaluation of the physics-dynamics coupling and "believable" scales.

The research will focus on two traditional and two forthcoming CESM dynamical cores. The former are the Eulerian spectral transform model and the Finite-Volume dynamical core on the latitude-longitude grid. The latter are the High-Order Method Modeling Environment (HOMME) and the non-hydrostatic FVcubed model, developed at the National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory and NASA. HOMME and FVcubed are built upon the highly scalable cubed-sphere grid geometry. HOMME will become the default for future CESM developments in late 2011. In addition, we will include an assessment of the prototype NCAR/DOE Model for Prediction Across Scales. The research will provide an in-depth analysis of cubed-sphere model characteristics such as potential grid imprinting and HOMME's forthcoming variable-resolution mesh configuration. The research will thereby assess multi-scale interactions, evaluate dissipative and wave reflection properties in refined domains, and shed light on scale-aware physics interactions in order to prepare pathways for future adaptive mesh refinement capabilities in GCMs. The research is built upon a strong partnership between the University of Michigan, Sandia National Laboratories, and the National Center for Atmospheric Research.