Synthesis report on initialization and ensemble generation

Météo-France is releasing the next generation of its seasonal forecasting system called System 9,based on the coupled climate model CNRM-CM (Voldoire et al, 2019). While the modeling optionsfor System 9 are described in the companion deliverable D3.1.2 of the present C3S2_370_MF con-tract (Recommendations for the System 9 model, Specq et al., 2024), this report describes thechoices relative to the seasonal forecasting setup that is applied to the climate model, justified bycomparisons between several options. The focus is set on the ensemble generation methodologyand on the initial conditions. The setup is defined as follows:– The ensemble size is 31 for the reforecasts, and 51 for the real-time forecasts.– Ensemble members are generated with:o A lagged initialization procedure, where ensemble members are initialized from 3 dif-ferent start dates (i.e forecast batches) before or on the first day of each month.o Online perturbations of the atmosphere dynamics using the Stochastic Dynamics ap-proach (Batté and Déqué, 2016).– Initial conditions come from:o The ERA5 reanalysis (Hersbach et al, 2020) for the atmosphere and land surface.o An in-house ocean-only simulation for ocean and sea-ice, that is forced at the surfaceby atmospheric fluxes from ERA5 and nudged towards the GLORYS12V1 ocean re-analysis in temperature and salinity.