ERA 40 is a second generation reanalysis. It is the first reanalysis to directly assimilate satellite radiance data (TOVS, SSM/I, ERS and ATOVS). Cloud Motion Winds are also used. The result is better circulation over the tropics and southern hemisphere. However, unless comparing to previous ERA-40 based results, it is recommended that the 3rd generation reanalyses, ERA-Interim or MERRA, be used for new research.
National Center for Atmospheric Research Staff (Eds). Last modified 02 Mar 2016. "The Climate Data Guide: ERA40." Retrieved from https://climatedataguide.ucar.edu/climate-data/era40.