Analysis of ERA40-driven CLM simulations for Europe

The Climate Local Model (CLM) is a community Regional Climate Model (RCM) based on the COSMO weather forecast model. We present a validation of long-term ERA40-driven CLM simulations performed with different model versions. In particular we analyse three simulations with differences in boundary nudging and horizontal resolution performed for the EU-project ENSEMBLES with the model version 2.4.6, and one with the latest version 4.0. Moreover, we include for comparison a long-term simulation with the RCM CHRM previously used at ETH Zurich. We provide a thorough validation of temperature, precipitation, net radiation, cloud cover, circulation, evaporation and terrestrial water storage for winter and summer. For temperature and precipitation the interannual variability is additionally assessed. While simulations with CLM version 2.4.6 are generally too warm and dry in summer but still within the typical error of PRUDENCE simulations, version 4.0 has an anomalous cold and wet bias. This is partly due to a strong underestimation of the net radiation associated with cloud cover overestimation. Two similar CLM 2.4.6 simulations with different spatial resolutions (0.44 degrees and 0.22 degrees) reveal for the analysed fields no clear benefit of the higher resolution except for better resolved fine-scale structures. While the large-scale circulation is represented more realistically with spectral nudging, temperature and precipitation are not. Overall, CLM performs comparatively to other state-of-the-art RCMs over Europe.