This catalogue entry provides daily Regional Climate Model (RCM) data on single levels from a number of experiments, models, members and time periods computed in the framework of the Coordinated Regional Climate Downscaling Experiment (CORDEX). The term "single levels" is used to express that the variables are computed at one vertical level which can be surface (or a level close to the surface) or a dedicated pressure level in the atmosphere. Multiple vertical levels are excluded from this catalogue entry.

High-resolution RCMs can provide climate change information on regional and local scales in relatively fine detail, which cannot be obtained from coarse scale Global Climate Models (GCMs). This is manifested in better description of small-scale regional climate characteristics and also in more accurate representation of extreme events. Consequently, outputs of such RCMs are indispensable in supporting regional and local climate impact studies and adaptation decisions. RCMs are not independent from the GCMs, since the GCMs provide lateral and lower boundary conditions to the regional models. In that sense RCMs can be viewed as magnifying glasses of the GCMs.

The CORDEX experiments consist of a large number of RCM simulations representing different future forcing scenarios, different combinations of GCMs and RCMs and different ensemble members of the same GCM-RCM combinations. This experiment design allows for studies addressing questions related to the key uncertainties in future climate change:

• what will future climate forcing be?
• what will be the response of the climate system to changes in forcing?
• what is the uncertainty related to natural variability of the climate system?

The term "experiment" in the CDS form refers to three main categories:

• Evaluation: CORDEX experiment driven by ECMWF ERA-Interim reanalysis for a past period. These experiments can be used to evaluate the quality of the RCMs using perfect boundary conditions as provided by a reanalysis system. The period covered is typically 1980-2010;
• Historical: CORDEX experiment which covers a period for which modern climate observations exist. Boundary conditions are provided by GCMs. These experiments, that follow the observed changes in climate forcing, show how the RCMs perform for the past climate when forced by GCMs and can be used as a reference period for comparison with scenario runs for the future. The period covered is typically 1950-2005.;
• Scenario (RCP2.6, RCP4.5, RCP8.5): Ensemble of CORDEX climate projection experiments using RCP (Representative Concentration Pathways) forcing scenarios. These scenarios are the RCP 2.6, 4.5 and 8.5 scenarios providing different pathways of the future climate forcing. Boundary conditions are provided by GCMs. The period covered is typically 2006-2100.

In CORDEX, the same experiments were done using different RCMs (labelled as “Regional Climate Model” in the CDS form).

In addition, for each RCM, there is a variety of GCMs, which can be used as lateral boundary conditions. The GCMs used are coming from the CMIP5 (5th phase of the Coupled Model Intercomparison Project) archive. These GCM boundary conditions are labelled as “Global Climate Model” in the form.

Additionally, the uncertainty related to internal variability is sampled by several members of a few RCM-GCM combinations. Each member of the ensemble is named after a triad of integers associated to the letters r (as realization), i (as initialization) and p (as physics representing parameter settings in the models). These three letters cover variations in initial conditions as well as in the physical parameters of the models. The associated numbers are typically assigned sequentially. For instance, the member "r1i1p1" and the member "r2i1p1" being two realizations for the same model and experiment indicate that the corresponding simulations differ only in which GCM realization that has been downscaled. Different GCM realizations, in turn, differ only in definition of the initial conditions applied as being representative of the pre-industrial time, i.e. typically 1850. More details behind these sequential ensemble numbers will be available in the detailed documentation.

More details about the product are given in the Documentation section.