Questions on how the GCM-RCM-RCP multi-model ensemble was designed in EURO-CORDEX and how the addition of simulations contributed by the C3S was chosen. Moreover questions related to how single or several models (or simulations) can be chosen from the large ensemble for various purposes are answered.
How are GCM-RCM ensembles designed?
Traditionally, GCM-RCM ensembles have been produced on an ad hoc basis where RCMs have been downscaling a number of GCMs that have supplied boundary conditions. This implies that GCM-RCM ensembles most often do not fully represent the full spread in larger GCM ensembles such as the full CMIP5 and CMIP6 ensembles.
Future climate change is uncertain as future forcing conditions are not fully known. This uncertainty can be handled by producing simulations for different scenarios. Another source of uncertainty relates to the sensitivity of the climate system, i.e. how strong it reacts to changing forcing conditions. This response can be different in different regions. In addition, internal natural variability also contributes to the uncertainty as it may be that single years, decades or even longer periods can be warmer/colder/wetter/drier than average just by chance.
Preferably, GCM-RCM ensembles could be designed in such a way as to sample as much of the spread as possible. Notably, this requires that lateral boundary conditions are provided by all GCMs. Users of RCM projections should be aware that these usually do not represent the full spread of the available GCM ensembles and that assessments of uncertainty and robustness of results may differ.
What considerations are behind the design of the C3S contribution to EURO-CORDEX?
A decision was made to build on already existing and published EURO-CORDEX simulations at 0.11° horizontal resolution with the rationale of producing a GCM-RCM-RCP matrix as large as possible. The strategy developed was such that a number of different "dimensions" can be explored with the resulting data including sensitivity to forcing scenario, choice of GCM and choice of RCM. In addition, a number of simulations were run to make it possible to address natural variability.
Due to resource limitations it was early on decided that C3S-funded regional simulations would not do a homogeneous non-complete filling of the entire GCM-RCM-RCP matrix (several hundreds), but would rather fill up selected matrix slices (e.g. one GCM-all RCMs-one RCP, many GCMs-a few RCMs-one RCP, a few GCMs-a few RCMs-all RCPs). The plan for the collective simulations produced within C3S has therefore been to distribute the allocated effort to approximately 65 simulations (the new simulations therefore roughly speaking double the available EURO-CORDEX simulations produced within other efforts) between three main goals:
- To enable studies of variability by performing several ensemble member downscaling simulations with the same GCM-RCM-RCP combination.
- To fill as large sub-matrix as possible for the RCP8.5 emission scenario.
- To add a significant amount of simulations to the weak RCP2.6 scenario, where noise is relatively large compared to the climate change signal.
How can subsets of simulations be chosen in an optimal way?
As future climate change is uncertain it is important to address this uncertainty in any study of future conditions. A strategy for model selection can be to select single models in a way that preserves as much of the initial spread in the ensemble as possible. Another strategy may be to select only a few that are representative for different ways of evolution in a story-telling sense.
Depending on which climate change impact is studied careful consideration should be given to how any subset is chosen.
