The NCEP-DOE AMIP-II Reanalysis (R-2) is an updated version of the NCEP-NCAR reanalysis (R-1), covering the period from 1979 to the present. R-2 addresses several human processing errors in R-1 and incorporates upgrades to the forecast model and data assimilation system. Key improvements include the use of observed pentad-average precipitation for soil wetness evolution, which significantly enhances the interannual variability of soil moisture fields. Other improvements involve corrections to the "spectral snow" problem, snow cover and snow depth, and changes in boundary layer and convective parameterizations. R-2 also features a new shortwave radiation scheme, updated cloud-top cooling, and more realistic cloud cover. These changes have improved precipitation, surface fluxes, radiation fluxes, and the land hydrology budget. However, some aspects, such as the tropical radiation budget and upper-level tropical moisture, may be less accurate than in R-1. R-2 is recommended for studies requiring high accuracy in soil wetness, near-surface temperature, and snow cover, but it is not considered a next-generation reanalysis due to its limitations in resolution and assimilation of remote observations.The NCEP-DOE AMIP-II Reanalysis (R-2) is an updated version of the NCEP-NCAR reanalysis (R-1), covering the period from 1979 to the present. R-2 addresses several human processing errors in R-1 and incorporates upgrades to the forecast model and data assimilation system. Key improvements include the use of observed pentad-average precipitation for soil wetness evolution, which significantly enhances the interannual variability of soil moisture fields. Other improvements involve corrections to the "spectral snow" problem, snow cover and snow depth, and changes in boundary layer and convective parameterizations. R-2 also features a new shortwave radiation scheme, updated cloud-top cooling, and more realistic cloud cover. These changes have improved precipitation, surface fluxes, radiation fluxes, and the land hydrology budget. However, some aspects, such as the tropical radiation budget and upper-level tropical moisture, may be less accurate than in R-1. R-2 is recommended for studies requiring high accuracy in soil wetness, near-surface temperature, and snow cover, but it is not considered a next-generation reanalysis due to its limitations in resolution and assimilation of remote observations.