Thursday, 13 January 2000: 3:00 PM
We extend the concepts of the age spectrum, mean-age,
and the age-of-air in the middle atmosphere by specifying
the tracer source gases uniformly in the lower troposphere.
The concepts are illustrated by use of both a steady
one-dimensional diffusive model and an improved globally
balanced two-dimensional middle atmosphere model where
the time integration of geopotential tendency is performed
explicitly. We quantitatively examine several factors
that may cause the difference between the age-of-air
derived from the time lag and the mean-age derived from
the age spectrum. It is found that the age-of-air
derived from a finite time of model integration is
less than the mean-age. The age-of-air at the
stratopause is about 5 to 6 years in a globally balanced
two-dimensional middle atmosphere model. In the
mesosphere, the age-of-air changes with season significantly
due to the reversal of the mesospheric meridional circulation
between the summer and the winter. It is also demonstrated
that stratospheric mean-age is sensitive to the horizontal
mixing in the troposphere. Weak horizontal mixing in the
troposphere will reduce the mean-age in the upper stratosphere
but increase the mean-age in the high-latitude lower stratosphere
for a source tracer located at the equatorial lower troposphere.
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