ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-4-4489-2004 Stratospheric age of air computed with trajectories based on various 3-D-Var and 4-D-Var data sets Scheele M. P. 1 Siegmund P. C. 1 van Velthoven P. F. J. 1 Royal Netherlands Meteorological Institute (KNMI), P.O. Box 201, 3730AE, De Bilt, The Netherlands 20 08 2004 4 4 4489 4506 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/4/4489/2004/acpd-4-4489-2004.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/4/4489/2004/acpd-4-4489-2004.pdf

The age of stratospheric air is computed with a trajectory model, using ECMWF ERA-40 3-D-Var and operational 4-D-Var winds. The sensitivity to the forecast period and assimilation technique are studied, and the results are compared with observations and with results from a chemistry transport model that uses the same data sets. A large number of backward trajectories is started in the stratosphere, and from the fraction of these trajectories that has reached the troposphere, the age of air is computed. <br><br> A pilot study in the tropical stratosphere shows that the age increases with increasing length of the forecast series. <br><br> Next, for three selected data sets (3-D-Var 1-day and 4-D-Var 3-day forecast series, and 4-D-Var analyses) 5-year backward trajectories are computed that start all over the globe at an altitude of 20 km. For all data sets the computed ages of air in the extratropics are smaller than observed. For 4-D-Var forecast series they are closest to the observations, but still 0.5&ndash;1.5 year too small. Compared to the difference in age between the results for the different data sets, the difference in age between the trajectory and the chemistry transport model results is small.