NUKLEUS – A First Kilometre Scale Multi-model Climate Ensemble for Germany: Evaluation

Abstract. This study presents the evaluation of NUKLEUS, the first kilometre-scale, multi-model convection-permitting regional climate ensemble (CPM) for Germany. Three state-of-the-art regional climate models (ICON-CLM, COSMO-CLM, and REMO) were run at ~3 km horizontal resolution using a two-step downscaling chain driven by ERA5 reanalysis. The ensemble provides high-resolution climate information for Central Europe. In particular, we evaluate the CPM simulation results for Germany with a focus on six representative pilot regions selected within the German RegIKlim programme. Temperature, precipitation, global radiation, and near-surface wind, including their spatial patterns, annual and diurnal cycles, distributional characteristics, are compared to observational datasets. Moreover, selected climate indices relevant for heat and precipitation extremes are analysed. Overall, the ensemble demonstrates substantial added value compared to coarser-scale regional climate modelling, particularly in capturing regional climatic features and fine-scale variability. Temperature is reproduced with small biases (mostly within ± 0.5 K), with ICON-CLM performing best, while COSMO-CLM shows a weak cold bias and REMO a warm bias in parts of southern Germany. The CPM models realistically capture daily temperature distributions, though REMO underestimates minimum-temperature extremes. The annual cycle of precipitation is generally well represented, but all CPM models tend to overestimate totals in several regions, e.g., REMO exhibits a distinct spatial bias pattern with stronger deviations along topographic gradients. Extreme precipitation frequencies are generally overestimated, while regional contrasts such as stronger extremes in mountainous regions are preserved. Diurnal cycles show deficiencies specific to the models, including timing errors of afternoon precipitation peaks and misrepresentation of nocturnal precipitation revivals. For global radiation, ICON-CLM achieves the smallest biases, benefiting from its modern radiation scheme (ecRad), whereas COSMO-CLM and REMO show region specific over- and underestimations linked to cloud representation. 10 m wind speed diurnal cycles are best simulated by ICON-CLM, which captures the nocturnal wind minimum, while COSMO-CLM and REMO generally overestimate nighttime wind. Climate indices reveal underestimation of heat-related metrics (summer days and hot days) and systematic overestimation of heavy precipitation indices, although spatial patterns and regional differences are reproduced. In summary, we conclude that NUKLEUS provides valuable climate information for Germany, supporting climate-impact assessments and adaptation planning at municipal to regional scales.