11.10.1.1 High-Resolution Atmosphere-Only GCMs

Atmosphere-only GCMs (AGCMs) include interactive land surface schemes as in an AOGCM but require information on SST and sea ice as a lower boundary condition. Given the short time scales associated with the atmosphere and land surface components compared to those in the ocean, relatively short time slices (a few decades) can be run at high resolution. The SST and sea ice information required can be derived from observations or AOGCMs. The use of observations can improve simulations of current climate but combining these with AOGCM-derived changes for the future climate (e.g., Rowell, 2005) increases the risk of inconsistency in the projected climate. The absence of two-way feedback between the atmosphere and ocean in AGCMs can cause a significant distortion of the climatic variability (Bretherton and Battisti, 2000), as documented over regions such as the Indian Ocean and the South Asian monsoon (Douville, 2005; Inatsu and Kimoto, 2005). The large-scale climate responses of AGCMs and AOGCMs appear to be similar in many regions; when and where they differ, the consistency of the oceanic surface boundary condition may be questioned (May and Roeckner, 2001; Govindasamy et al., 2003). Further research is required to determine if the similarity is sufficient for the time-slice approach with AGCMs to be considered a robust downscaling technique.

Model grids of 100 km and finer have become feasible and 50 km will likely be the norm in the near future (Bengtsson, 1996; May and Roeckner, 2001; Déqué and Gibelin, 2002; Govindaswamy, 2003). High-performance computer systems now allow global computations at 20 km (e.g., May, 2004a; Mizuta et al., 2006), although for short time slices only. Evaluated on the scale typical of current AOGCMs, nearly all quantities simulated by high-resolution AGCMs agree better with observations, but the improvements vary significantly for different regions (Duffy et al., 2003) and specific variables, and extensive recalibration of parametrizations is often required. Notable improvements occur in orographic precipitation and dynamics of mid-latitude weather systems (see Chapter 10). The highest resolution offers the prospect of credible simulations of the climatology of tropical cyclones (e.g., May, 2004a; Mizuta et al., 2006). Coordinated multi-model experiments are needed, however, to optimise the value of these high-resolution studies for general assessment.

An alternative to uniform high-resolution AGCMs is Variable-Resolution AGCMs (VRGCMs; e.g., Déqué and Piedelievre, 1995; Krinner et al., 1997; Fox-Rabinovitz et al., 2001; McGregor et al., 2002; Gibelin and Déqué, 2003). The VRGCM approach is attractive as it permits, within a unified modelling framework, a regional increase in resolution while retaining the interaction of all regions of the atmosphere. Numerical artefacts due to stretching have been shown to be small when using modest stretching factors (e.g., Lorant and Royer, 2001). The results from VRGCMs capture, over the high-resolution region, finer-scale details than uniform-resolution models while retaining global skill similar to uniform-resolution simulations with the same number of grid points.