Development of Atlas, a flexible data structure framework.

This document is one of the deliverable reports created for the ESCAPE project. ESCAPE stands for Energy-efficient Scalable Algorithms for Weather Prediction at Exascale. The project develops world-class, extreme-scale computing capabilities for European operational numerical weather prediction and future climate models. This is done by identifying Weather & Climate dwarfs which are key patterns in terms of computation and communication (in the spirit of the Berkeley dwarfs). These dwarfs are then optimised for different hardware architectures (single and multi-node) and alternative algorithms are explored. Performance portability is addressed through the use of domain specific languages. In this deliverable report, we present Atlas, a new software library that is currently being developed at the European Centre for Medium-Range Weather Forecasts (ECMWF), with the scope of handling data structures required for NWP applications in a flexible and massively parallel way. Atlas provides a versatile framework for the future development of efficient NWP and climate applications on emerging HPC architectures. The applications range from full Earth system models, to specific tools required for post-processing weather forecast products. Atlas provides data structures for building various numerical strategies to solve equations on the sphere or limited area's on the sphere. These data structures may contain a distribution of points (grid) and, possibly, a composition of elements (mesh), required to implement the numerical operations required. Atlas can also represent a given field within a specific spatial projection. Atlas is capable of mapping fields between different grids as part of pre- and post-processing stages or as part of coupling processes whose respective fields are discretised on different grids or meshes.