Several of the problems experienced in ERA-40 have been eliminated or significantly reduced in ERA-Interim: most notably a too-strong tropical oceanic precipitation that was marked from the early 1990s onwards and a too-strong Brewer-Dobson circulation in the stratosphere. Precipitation, which is higher in both ERA-Interim and ERA-40 than in the Global Precipitation Climatology Project (GPCP) over the tropical oceans, is closer to GPCP in ERA-Interim. ERA-40 and ERA-Interim are nevertheless in closer agreement with each other than either is to the GPCP estimate. A further indication of improvement of the hydrological cycle in ERA-Interim comes from diagnosis of the global balance of precipitation and evaporation. Comparison of the accuracy of 10-day forecasts from ERA-Interim and ERA-40 with those from operations provides further evidence of the improvement of forecasting systems over the years.
The results highlight the fact that instead of being viewed as largely independent "one-off" exercises, reanalysis has come to be seen more as an iterative process. Developments in modelling, data-analysis techniques and computing power together with data rescue efforts and experience from other reanalyses will produce a succession of reanalyses with increasing quality.