Stratospheric age of air computed with trajectories based on various 3D-Var and 4D-Var data sets M. P. Scheele, P. C. Siegmund, and P. F. J. Velthoven Royal Netherlands Meteorological Institute (KNMI), P.O. Box 201, 3730AE, De Bilt, The Netherlands
Abstract. The age of stratospheric air is computed with a trajectory model, using
ECMWF ERA-40 3D-Var and operational 4D-Var winds. Analysis as well as
forecast data are used. In the latter case successive forecast segments are
put together to get a time series of the wind fields. This is done for
different forecast segment lengths. The sensitivity of the computed age to
the forecast segment length and assimilation method 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 are started in the stratosphere, and from the fraction of these
trajectories that has passed the tropopause the age of air is computed.
First, for ten different data sets 50-day backward trajectories starting in
the tropical lower stratosphere are computed. The results show that in this
region the computed cross-tropopause transport decreases with increasing
forecast segment length. Next, for three selected data sets (3D-Var 24-h
and 4D-Var 72-h forecast segments, and 4D-Var analyses) 5-year backward
trajectories are computed that start all over the globe at an altitude of 20km.
For all data sets the computed ages of air in the extratropics are
smaller than the observation-based age. For 4D-Var forecast series they are
closest to the observation-based values, 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 trajectory and the chemistry
transport model results is small.
Citation: Scheele, M. P., Siegmund, P. C., and Velthoven, P. F. J.: Stratospheric age of air computed with trajectories based on various 3D-Var and 4D-Var data sets, Atmos. Chem. Phys., 5, 1-7, doi:10.5194/acp-5-1-2005, 2005.