The algorithms underlying numerical weather prediction (NWP) and climate models that have been developed in the past few decades face an increasing challenge to adapt to paradigm shifts imposed by new hardware developments. The emerging diverse and complex hardware solutions have a large impact on the programming models traditionally used in NWP software, triggering a rethink of design choices for future software frameworks. On the other hand, there is a drive to increase the model complexity to include ever more processes of the whole Earth system. Some of these processes may require computations on grids of different type or resolution than the atmospheric grid. Multiple grid structures may be required as part of the numerical filtering strategy for atmospheric wave motions or to simply save computational cost of selected physical processes. These different grids may have different domain decompositions for parallel computations, and different parallelisation strategies. Moreover the internal memory layout for a field that is optimal for one numerical algorithm may not be optimal for another. These complexities will inevitably break NWP modelling infrastructures that did not take these aspects into consideration 30+ years ago.

To address the above mentioned challenges, the European Centre for Medium Range Weather Forecasts (ECMWF) is developing Atlas, a new modern software framework that is designed to take into account these new developments. Atlas helps to accommodate flexibility in hardware and software choices as well as increasing model complexity. Atlas is not a new model but rather forms a foundation layer that new models can be built with, or for existing models to be complemented or redesigned with.

In this talk, we demonstrate how Atlas is used to complement ECMWF's Integrated Forecasting System (IFS) model to enable a number of physical processes to be implemented on multiple grids.