Stratospheric age of air computed with trajectories based on various 3D-Var and 4D-Var data sets

[摘要] The age of stratospheric air is computed with a trajectory model, usingECMWF ERA-40 3D-Var and operational 4D-Var winds. Analysis as well asforecast data are used. In the latter case successive forecast segments areput together to get a time series of the wind fields. This is done fordifferent forecast segment lengths. The sensitivity of the computed age tothe forecast segment length and assimilation method are studied, and theresults are compared with observations and with results from a chemistrytransport model that uses the same data sets. A large number of backwardtrajectories are started in the stratosphere, and from the fraction of thesetrajectories that has passed the tropopause the age of air is computed.First, for ten different data sets 50-day backward trajectories starting inthe tropical lower stratosphere are computed. The results show that in thisregion the computed cross-tropopause transport decreases with increasingforecast segment length. Next, for three selected data sets (3D-Var 24-hand 4D-Var 72-h forecast segments, and 4D-Var analyses) 5-year backwardtrajectories 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 aresmaller than the observation-based age. For 4D-Var forecast series they areclosest 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 datasets, the difference in age between the trajectory and the chemistrytransport model results is small.

[学科分类] 大气科学