Stratospheric age of air

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 5 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. A pilot study in the tropical stratosphere shows that the age increases with increasing length of the forecast series. Next, for three selected data sets (3-D-Var 1-day and 4-D-Var 3-day forecast series, 10 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 0.5–1.5 year too small. Compared to the difference in age between the results for the different data sets, the difference in age between the 15 trajectory and the chemistry transport model results is small.