ALERA2 is an experimental atmospheric reanalysis dataset from 1 Jan 2008 to 5 Jan 2013 produced on the Earth Simulator. This dataset is the second generation of ALERA. In ALERA2, the ensemble size is increased from 40 to 63 and the data assimilation system is updated from the previous one (see Enomoto et al. 2013).This dataset is produced by Japan Agency for Marine-Earth Science and Technology (JAMSTEC). ALERA2 may be used for research purposes for free under the terms and conditions.AFES (AGCM for the Earth Simulator) is run at a resolution of T119L48 (about 100 km in the horizontal and 48 layers in the vertical). The PREPBUFR complied by the National Centers for Environmental Prediction (NCEP) and archived at the University Corporation for Atmospheric Research (UCAR) is used for the observational data and assimilated using the LETKF (local ensemble transform Kalman filter).