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The best tool for assessing the internal variability is to use large ensembles of climate model projections. By this approach, the probability of having some, or several, projections close to the actual evolution increases.
Similarly, as in weather prediction, large initial-value single model ensembles have been put forward from climate modelling groups during the last decades. Recently, a number of such large ensembles have been assessed by Deser et al. (2020). As pointed out in that study it can be important to realize that the actual evolution of the climate system is just one realization. Consequently, it may be difficult to interpret climate change aspects as result from changing forcing conditions based only on observations as what is observed may be a feature of such variability.
Most CORDEX RCMs have been used to downscale single member GCM simulations. However, for EURO-CORDEX some models have been downscaling three different ensemble members (LINK TOD1.3.2: Documentation for the role of internal variability over Europe). There are also a few examples of regional climate models that have been used for downscaling large ensembles (Addor and Fischer 2015; Aalbers et al. 2016).
References
Aalbers EE, Lenderink G, van Meijgaard E, and van den Hurk B (2016) To what extent is climate change detection at the local scale 'clouded' by internal variability? In EGU General Assembly Conference Abstracts. 18, s. 10121.
Addor N and Fischer EM (2015) The influence of natural variability and interpolation errors on bias characterization in RCM simulations. J. Geophys. Res. Atmos. 120. doi:10.1002/2014JD022824.
Deser C, Lehner F, Rodgers KB et al. (2020) Insights from Earth system model initial-condition large ensembles and future prospects. Nat. Clim. Chang. 10, 277–286, https://doi.org/10.1038/s41558-020-0731-2
Hawkins E and Sutton R (2009) The potential to narrow uncertainty in regional climate predictions Bulletin of the American Meteorological Society, 90, pp. 1095-1107, 10.1175/2009BAMS2607.1
Kjellström E, Thejll P, Rummukainen M, Christensen JH, Boberg F, Christensen OB, Fox Maule C (2013) Emerging regional climate change signals for Europe under varying large-scale circulation conditions, Clim. Res., 56, 103–119, DOI: 10.3354/cr01146.