The Center of Excellence in Simulation of Weather and Climate in Europe (ESiWACE) will substantially improve efficiency and productivity of numerical weather and climate simulation on HPC platforms by supporting the end-to-end workflows in global Earth system modelling for top-of-the-edge HPC environments. This will be obtained by improving and supporting: scalability of models, tools and data management on state-of-the-art supercomputer systems; usability of models and tools throughout the European HPC ecosystem; exploitability of the huge amount of resulting data. By developing solutions for Europe and at European scale, ESiWACE will directly impact on the competitiveness of the European HPC industry by engendering new products, providing opportunities for exploitation beyond the project itself, and by enhancing the skills base of staff in both industry and academia. ESiWACE will be at once thematic, as it focuses on the HPC application domain of climate and weather modeling, transversal, as it covers several aspects of computational science, and challenge-driven, as climate and weather predictability represents a major societal issue.