Using XIOS to manage ensemble simulations with the IPSL-ESM model

Ensemble simulations are used to better estimate the uncertainty of climate model results. The production of ensemble simulations with current IPSL model requires to run independently as many simulations as there are members in the ensemble, producing a very large number of inodes as a consequence of writing files for each member. In 2022, we have developed a functionality that allows several simulations to be carried out simultaneously, all simulation writing its results in a common single file. This new functionality has required developments in IPSL's XIOS IO server (in its version 2) in order to handle an additional "ensemble" dimension in the result file, as well as in OASIS coupler to provide MPI inter-communicators for all of the coupled members. The IPSL libIGCM runtime environment library was also adapted to manage efficiently the use of different types of files (forcing, parameters, restart, results) by several members within the same computing job. This job allows now "by itself" to simultaneously perform simulations of several members of an ensemble. The number of files resulting from these ensemble simulations matches (without any post-processing step) with criteria required by computing centers filesystems. Finally, the results of all the members of an ensemble are available in a single file, thus facilitating statistical processing and scientific analysis. This development is used in several IPSL scientific applications : the reconstruction of paleoclimate and historical periods by data assimilation methods and the simulation of extreme events (heat waves) in Western Europe.