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Improvements in the stratospheric transport achieved by a chemistry transport model with ECMWF (re)analyses: identifying effects and remaining challenges

Stratospheric simulations from a chemistry transport model (CTM) are used to evaluate the stratospheric Brewer–Dobson circulation and mixing processes in different datasets produced by the European Centre for Medium‐Range Weather Forecasts (ECMWF). A comparison between ERA‐40 and ERA‐Interim shows that the recent ECMWF reanalysis greatly improves on past problems detected for ERA‐40. CTM simulations with ERA‐Interim provide improved age‐of‐air distributions, in very good agreement with observations in the lower stratosphere. In a multiannual model run (1990–2009), we find a small but statistically significant increasing trend in age over Northern Hemisphere midlatitudes above 24 km. This is in contrast with most model results in existing literature, but in agreement with recent studies based on SF6 and CO2 observations. Lagrangian calculations also show that the constraint of vertical and horizontal mixing has been very significantly improved in the new ECMWF dataset. Causes of the improvements achieved by the new reanalysis are explored, revealing the roles that the assimilation technique and the analysis frequency play in the successful description of stratospheric transport. The implementation of correction techniques such as the use of forecasts is shown to have a smaller effect in CTM simulations with ERA‐Interim than in those with ERA‐40. The effect of using winds from the ECMWF data assimilation system (DAS) or from the underlying general circulation model (GCM) is also contrasted. For this, the CTM uses a set of ECMWF GCM winds from a free‐running model version equivalent to the system employed to produce ERA‐40. The GCM winds were archived every 12 h, which also allows an evaluation of the CTM analysis read‐in frequency effects on stratospheric transport. Some of the problems attributed in the past to data assimilation, such as the excess of vertical mixing in the lower stratosphere, are here shown to be caused also by too low read‐in frequencies in the CTM simulations. Copyright © 2012 Royal Meteorological Society