Online climate change projections report 5.1 Regional climate models
A Regional Climate Model contains the same representations of atmospheric dynamical and physical processes as in a global model. It is run at a higher horizontal resolution (in our case 25 km) but over a sub-global domain (typically 5000 km square), and is driven at the boundary of the domain by of variables (such as temperature and winds) saved from a GCM projection. and sea-ice extents are also prescribed from the GCM, since HadRM3 (like most RCMs) does not include an interactive ocean component. The purpose of RCMs is to provide a high resolution climate projection consistent with its driving GCM projection at spatial scales skilfully resolved by the latter, but adding realistic detail at finer scales. This is the downscaling process referred to above. The advantages of projections from RCMs over those from GCMs are:
- RCMs simulate spatial contrasts in time-averaged climate at a scale much smaller than that of the driving GCM, in particular where there are significant regional influences arising from surface features such as mountains and coastlines (see Figure 5.1).
- The higher resolution of RCMs also allows improved representation of climate variability, particularly aspects associated with small scale meteorological processes. As a result, they can provide skilful (though not perfect) projections of regional climate extremes, such as localised intense precipitation events, which cannot be captured in GCMs.
- The higher resolution of RCMs allows small islands to be explicitly represented in the model.
- While RCM projections are designed to be consistent with their driving GCM projections at large scales, some types of climate impact, such as changes in river flow, are likely to be so strongly dependent on the fine scale detail that the use of downscaling, either based on RCM data or a statistical method, is essential for the generation of a credible assessment of future changes.
General guidelines for applying RCM data can be seen in a report from the IPCC Task Group on Climate Impacts Assessments (Mearns et al, 2003). A key caveat is that while RCMs are now well established as skilful and sophisticated downscaling tools, they inevitably inherit all the uncertainties in large scale aspects of climate change present in their driving GCM simulations (see Annex 2), so the enhanced detail in their projections should not be taken to imply higher accuracy (see also Annex 3 and Annex 6). The same caveat applies to fine scale projections derived from the UKCP09 Weather Generator (see further discussion in the next section).