ecTrans is a high-performance numerical library for transforming meteorological fields between global grid-point space representation and a spectral representation based on spherical harmonics. It is a fundamental part of the European Centre for Medium-Range Weather Forecasts' Integrated Forecasting System (IFS), a global numerical weather prediction suite. Indeed, ecTrans was previously part of the IFS source code itself. It therefore benefits from over 30 years of development and optimisation. In 2022, ecTrans was split out from the IFS source code and released as the first open-source component of the IFS as its own project.
ecTrans is engineered to work efficiently running on many hundreds, or even thousands, of compute nodes. This is achieved through a significant optimisation of the constituent compute kernels, making use of the FFTW library for the Fourier transform in the longitudinal direction and BLAS GEMMs for the Legendre transforms in the latitudinal direction. However, given that transformed fields are distributed across compute tasks, great care has been taken to ensure that parallelism can be fully exploited at all stages in the algorithm. This is achieved through data exchange steps interleaved between the Fourier and Legendre transforms, which are implemented using the Message Passing Interface (MPI).
The result is an algorithm which stresses a high-performance computing system both on the node level and the network level, serving as an excellent overall benchmark and a target for optimisation of IFS execution speed.
This user guide contains the following sections: