Atmospheric mass‐transport inconsistencies in the ERA‐40 reanalysis

The ERA‐40 reanalysis from the European Centre for Medium‐Range Weather Forecasts (ECMWF) is an atmospheric data‐set based on a comprehensive collection of observations and a state‐of‐the‐art system for assimilating data. High‐quality data, encompassing more than four decades, have successfully been used in a wide range of applications, including climate change studies. However, even though the ERA‐40 data‐set benefits from improvements in the assimilation procedures since the release of earlier reanalyses, ERA‐40 exhibits unrealistic behaviour as regards some atmospheric quantities, including the water budget and the Brewer–Dobson Circulation. In addition, the results presented here show significant mass‐budget inconsistencies. Over inter‐annual timescales (greater than one year), the vertically‐averaged meridional mass transport is unrealistic and cannot be explained on the basis of naturally occurring physical processes. This mass inconsistency also yields spurious signals in meridional fluxes of other atmospheric quantities, such as energy. The total atmospheric mass content, on the other hand, shows a realistic evolution on daily timescales during the satellite era, from about 1979 onwards. Through this period, the variability of the total mass on intra‐annual timescales (less than one year) and annual timescales is consistent with differences between evaporation and precipitation. Here it is demonstrated that unrealistic mass fluxes are present in ERA‐40 because of inherent properties of the data assimilation process. In assimilating model forecasts, between analysis time‐steps, systematic and unrealistic changes of the local surface‐pressure field in the assimilating model are encountered. These local mass changes are associated with mass fluxes. During the assimilation procedure, observations reset the surface‐pressure field, whereas the mass fluxes are not adjusted properly. This is because surface‐pressure observations are plentiful and accurate, in contrast to wind observations. Consequently, the analysed mass fluxes show a spurious development, even though the mass field itself is well captured. A correction method can be applied in order to eliminate the spurious fluxes. For trend calculations it is demonstrated that this method yields more realistic results than those obtained from the original ERA‐40 data. Copyright © 2007 Royal Meteorological Society