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Model results of mean age and comparison with observations

Inert tracer

To give a general impression of the characteristics of the model transport, Figure 2 shows the zonal mean age for the months of December, March, June and September, averaged over the years of 1990 - 1998 for the inert tracer. The figure shows the typical features observed for inert tracers: for a specific height level the younger mean age values are found near the equator, such showing the mean ascent within the tropical pipe; at high latitudes, the season strongly affects the mean age distribution by ascent above the summer pole and descent above the winter pole. The mean age values are in the range of other model studies (Waugh et al., 1997, see e.g.). Figure 3 gives a time-height cross-section at the equator of mean age. The 2-year's isochrone clearly shows the effect of the QBO as an oscillation of mean age with an approximate period of 2 years. In addition, the upwelling expected during the QBO phase when the equatorial zonal winds at 30-50 hPa are easterly (Gray, 2000), e.g. spring 1992, is reflected by younger mean age values at 30 hPa.

In Figure 4 mean age observations are compared with the pure inert tracer profiles of the mean age of the model. The exact locations as well as the exact time of the balloon observations during the flights may deviate from the nominal values and errors may have been introduced in the comparisons. Therefore, the corresponding model results are shown for nine gridpoints (longitude, latitude) lying absolutely nearest to the observations. This also gives some indication of the variation within the model. If original mixing ratio data were available, mean age was reevaluated from these data using the same function as in the model. The correction for age spectral width yields somewhat higher age values than those given by Harnisch et al. (1996).

The main properties found can be summarized as follows: In general, we find a satisfactory agreement between the observations and model results of the inert tracer. Some polar model profiles have the tendency to show higher age values than the observations above about 25 km, especially in the year 1997. The sharp onset of a plateau in the age values evident in the observations often seems to be smeared out or shifted in height for the model. Exceptionally, profile P9 shows a constant shift to higher values observed over all altitude levels. The profiles observed in northern late spring 1997 show a rather high variation of the data (P14, P15). Here, also measurements exist. Both tracer observations are consistent with the assumption of highly filamentary air masses of different origin (polar vortex - midlatitudes). This conclusion is supported by the fact that our model profiles show a break in the vertical profile (P14) or crossing profiles in the environment (P15). In-situ measurements are expected to randomly detect even very small spatial inhomogeneities. However, the model resolution is about 500 km in both horizontal dimensions, so differences are expected in such cases. The very low age values of the tropical profile P5 are not reproduced in the model, whereas the situation of March-92 (profile P3, at E, N), where the observations show younger airmasses above older ones, shows up to some extent in the model.