The 11 member RCM ensemble was designed primarily to allow the probabilistic projections in UKCP09 to be presented at high spatial resolution, accounting for the regional effects of atmospheric dynamics and surface characteristics such as mountains and coastlines in a more detailed way than could be achieved by relying exclusively on global climate models.
The RCM ensemble provides eleven plausible time series of 21st century climate containing full spatial and temporal consistency across the UK as a whole. Subject to appropriate evaluation, it is potentially useful for applications requiring this type of information. However, users should be aware that the RCM ensemble was not designed to provide a comprehensive basis for impacts assessments or decision making on a stand-alone basis. In this respect, limitations of the 11-member RCM ensemble are:
1. Sources of modelling uncertainty are not represented as comprehensively as they are in the probabilistic projections
Unlike the UKCP09 probabilistic projections, the 11-member RCM ensemble only represents projections from one climate model (HadRM3). The 11 members do differ in the values of multiple parameters controlling a number of important physical processes in the climate system. However, the probabilistic projections take these parametric uncertainties into account more fully than the 11-member RCM ensemble. Secondly, the RCM ensemble does not account for the effects of uncertainties in the basic structure of the climate model, whereas the probabilistic projections achieve this by including results from other IPCC model projections. Thirdly, the RCM ensemble contains only a partial representation of uncertainties in future conditions in the North Atlantic ocean, and does not account for the effects of uncertainties due to carbon cycle processes, whereas the probabilistic projections do.
It is therefore recommended that users check the 11-member RCM scenarios of future changes against the UKCP09 probabilistic distribution to see how the changes fit within the plausible range (between the 10% and 90% values). Such an assessment can provide an evaluation of the proportion of the full probabilistic range that is explored by the RCM data. Further information is available in the Spatially Coherent Projections report, from the Reports & guidance section.
2. The 11 runs of the RCM are not supplied with formal estimates of credibility
The UKCP09 probabilistic projections were constructed by weighting different variants of the Met Office Hadley Centre climate model according to their performance. However, these weights were developed to be applicable to global rather than regional climate models, so they are not supplied for the RCM runs. In fact, the 11 members of the RCM ensemble were designed to be approximately equally plausible in representing UK climate as a whole. However, the levels of skill achieved by the ensemble members for specific variables, seasons or sub-regions may vary. It is therefore recommended that users evaluate the historical period of the runs on an application-specific basis, before using them for future projections.
3. Results are only available for the medium emissions scenario
As the 11-member RCM information was created for the specific purpose of downscaling the UKCP09 probabilistic projections, it only contains information for one emissions scenario (the medium emissions scenario; IPCC SRES A1B).
The use of only one emissions scenario is an additional reason why the 11-member RCM scenarios does not span the full range of possible future climates.